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OCR for page 579
Health Effects of Alclehydes
d Al h 1 M b 1
Source Emissions
LAWRENCE J. MARNETT
Wayne State University
Ambient Levels and Production by Mobile Sources / 580
Health Effects / 582
Aldehydes / 582 Methanol / 587 Phenols and
Catechols / 587 Cocarcinogenic Effects of Aldehydes, Alcohols, and
Phenols / 588
Metabolism / 589
Methanol and Formaldehyde / 589 Acrolein / 590 Measurement of
Inspired Methanol and Formaldehyde / 590 Chemical
Reactions / 591
Quantification of Exposure and Estimation of Human Risk / 595
Summary / 597
Summary of Research Recommendations / 598
Exposure / 598 Health Effects / 598 Cellular
Effects / 599 Molecular Dosimetry / 599
Air Pollution, the Automobile, and Public Health. (a) 1988 by the Health Effects
Institute. National Academy Press, Washington, D.C.
579
OCR for page 580
580
Aldehydes and Alcohols in Mobile Source Emissions
Aldehydes are oxidation products of alco-
hols; phenols contain an alcohol function-
ality attached to an aromatic ring. Al-
though they are structurally related, the
chemistry and toxicology of the three
classes of compounds are different. From a
toxicologic standpoint, aldehydes have
been more extensively investigated than
alcohols and phenols and constitute the
most important health hazard. Thus, most
of the emphasis of this chapter is placed on
them, but the literature on exposure, health
effects, metabolism, and chemistry of al-
dehydes, alcohols, and phenols are also
examined. Recommendations are made for
research necessary to fill important gaps in
our knowledge of these subjects. The liter-
ature review highlights key experiments but
is not comprehensive; recent review articles
and monographs are cited and can be con-
sulted for more complete information.
Ambient Levels and
Production by Mobile
Sources
Estimates of the atmospheric levels of some
common pollutants present in mobile
source emissions and the series of com-
pounds that are discussed in this chapter are
presented in table 1. Formaldehyde and
acetaldehyde have been monitored more
extensively than acrolein, alcohols, and
phenols. The data for the latter three com-
pounds represent episodic reports rather
than averages of extensive compilations.
Nevertheless, they are useful for assigning
an order of magnitude to the levels likely to
be found in urban air. Formaldehyde is
usually the most abundant of the com-
pounds of interest and acetaldehyde is next
most abundant. Their concentrations are
between 100 and 1,000 times lower than
carbon monoxide (CO), the principal pol-
lutant in auto exhaust. Although ambient
formaldehyde levels range from 4 to 86
parts per billion (ppb), occasional levels in
excess of 1,000 ppb 1 part per million
(ppm) have been reported (Goldsmith
and Friberg 1977~. Acrolein is usually de-
tected at levels below 10 ppb. Most of the
aldeh.ydes in urban air are present as gases.
Estimates suggest the percentage of al-
dehydes bound to particles is approxi-
mately 1 percent of the concentration in the
gas phase (Grosjean 1982~. Acrolein is also
produced In fires, but by far the highest
concentrations are present in cigarette
smoke (12 ppm) (Treitman et al. 1980;
Carson et al. 1981~. Acetaldehyde is also
present in very high concentrations in cig-
arette smoke (1,65~2,500 ppm) (Elmen-
horst and Schultz 1968~; the formaldehyde
concentration in cigarette smoke is approx-
imately equal to that of acrolein (Newsome
et al. 1965~.
Alcohol concentrations in the atmo-
sphere have only been determined in a
cursory fashion. In a recent study in urban
and rural sampling stations in Arizona,
Snider and Dawson (1985) reported values
of 7.9 ppb for methanol in Tucson and 2.6
Table 1. Atmospheric Levels of Compounds Present in Mobile Source Emissions
Compound Level % from Mobile Source Reference
Carbon monoxide 5-13 ppm 70 Graedel, this volume
Nitrogen oxides 20-30 ppb 50 Graedel, this volume
Nonmethane hydrocarbons 1-2 ppmC 37 Graedel, this volume
Sulfur dioxide 9 ppb <5 Graedel, this volume
Formaldehyde 086 ppba 55-75 Grosjean (1982)
Acetaldehyde 2-39 ppba 55-75 Hoshika (1977)
Grosjean (1982)
Acrolein 2-7 ppb NA Beauchamp et al. (1985)
Alcohols 8-100 ppb NA Snider and Dawson (1985)
Phenols NA NA
a Los Angeles.
NOTE: NA = not available.
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Lawrence I. Marnett
581
Table 2. Concentrations of Formaldehyde and Acetaldehyde in Exhaust Diluted Gases of Internal
Combustion Engines Fueled by Gasoline or Ethanol"
Gasoline-Fueledb
Ethanol-Fueled
Formaldehyde Acetaldehyde Formaldehyde Acetaldehyde
Cold starts 540 <8 67019,800
Hot starts 97 <8 100550
40-mph cruise 48 <8 360840
a Concentrations are given in ppb, and dilution factor is 10:1.
b Equipped with a catalyst.
c Federal Test Procedure driving cycle.
SOURCE: Based on data from Swarin and Lipari 1983.
ppb at a location 50 km from Tucson.
Atmospheric sampling was by a condensa-
tion procedure. The concentrations of eth-
anol at the two stations were reported to be
3.3 and 0.4 ppb, respectively. The origin of
the alcohols was not clear. The authors
were unable to detect propanol, butanol, or
acrolein. Phenol, as well as o- and m-cresol,
have been reported in undiluted auto ex-
haust at levels of 1.4, 0.2, and 0.3 ppm,
respectively (Kuwata et al. 1981~.
The ambient levels of aldehydes reported
to exist in Los Angeles appear to be higher
than levels in other cities in the United
States and Japan. Therefore, upper limits
can be approximated by studies performed
in the Los Angeles metropolitan area. Data
gathered at a reporting station in Clare-
mont (~50 km east of Los Angeles) indi-
cate that diurnal patterns exist for formal-
dehyde and acetaldehyde levels (Grosjean
1982~. There is a close correlation between
variations in the aldehyde levels and the
diurnal fluctuations of ozone (O3) as well as
the movement of smog banks. Other re-
ports establish a correlation of fluctuations
in aldehyde levels to diurnal variations in
CO levels (Cleveland et al. 19774. Diurnal
and seasonal fluctuations of formaldehyde
and acetaldehyde levels have been observed
at a monitoring station on Long Island
(Tanner and Meng 1984~. These data sug-
gest that mobile sources contribute sign111-
cantly to ambient concentrations. Esti-
mates of the percentage contribution made
by mobile emission sources to the levels of
the various compounds are presented in
table 1. Such estimates have not been made
for alcohols and phenols.
Analysis of the levels of aldehydes in
exhaust gases provides evidence for a de-
pendence on the type of engine and fuel
(Swarin and Lipari 1983~. Table 2 lists the
concentrations of formaldehyde and acetal-
dehyde detected in a 10:1-diluted sample of
exhaust gas from an internal combustion
engine fueled by gasoline or ethanol. Dra-
matic increases in acetaldehyde concentra-
tions are detected in ethanol-fueled engine
exhaust, as might be expected since acetal-
dehyde is the two-electron oxidation pro-
duct of ethanol. Presumably a similar in-
crease in formaldehyde concentrations
would be observed in exhaust from engines
fueled by methanol.
Diesel engines produce significantly
higher amounts of aldehydes than do gas-
oline engines (table 3~. Ratios of acetal-
dehyde to formaldehyde also increase if
catalysts are placed in the exhaust stream
(Swarin and Lipari 1983~. Taken together,
these findings suggest that the character of
automotive emissions varies dramatically
with the type of engine and fuel. This
Table 3. Concentrations of Selected
Aldehydes in Diluted Diesel Exhausta
Aldehyde Cold Startb
Formaldehyde
Acetaldehyde
Propionaldehyde
Acrolein
Crotonaldehyde
Benzaldehyde
Hot Startb
428
80
24
24
6
NA
539
115
57
57
11
9
a Concentrations are given in ppb, and dilution factor
is 10:1.
b Federal Test Procedure driving cycle.
NOTE: NA = not available.
SOURCE: Based on data from Swarin and Lipari
1983.
OCR for page 582
582
Aldehydes and Alcohols in Mobile Source Emissions
implies that a major shift to, for example,
alcohol-containing fuels would have a sig-
nificant effect on the concentration of cer-
tain aidehydes and alcohols in urban air.
The levels of alcohols in urban air are
rather low, and it is difficult to imagine that
they are high enough to exert any health
effects. Levels of aldehydes are normally
well below levels at which they induce
hazardous effects, although they can occa-
sionally reach high ambient concentrations
(Grosjean 1982; Beauchamp et al. 1985~.
Results of test burns indicate that dramati-
cally increased levels of aldehydes are
produced from alcohol-containing fuels
(Swarin and Lipari 1983). These higher
levels could be well within the range that
induces adverse health effects. If methanol-
and ethanol-based fuels are widely adapted,
it will be important to be able to determine
the levels of atmospheric aldehydes and to
have a baseline value against which to
compare them. Thus, routine monitoring
should be initiated now.
· Recommendation 1. Routine moni-
toring of atmospheric alcohol and aldehyde
levels should be performed in regions
where alcohol-based fuels are or will be in
heavy use.
Reports of the detection of phenols and
catechols from mobile source emissions are
extremely limited and usually do not pro-
vide quantitative information. As a result,
knowledge of their levels in urban air and
the contribution made by mobile sources is
totally inadequate. No realistic risk assess-
ment can be undertaken for this class of
compounds without such information.
~ Recommendation 2. Methods should
be developed to routinely analyze phenols
and catechols in urban air.
Health Effects
Aldehydes
The literature on the health effects of al-
dehydes and alcohols is enormous, but
several recent reviews by Consensus Work
Table 4. Acute Effects of Acrolein on Human
Volunteers
Concentration Effects
(ppb)
30-34
14~150
300-500
800-900
1,200
Odor threshold for most acrolein
sensitive people.
Some eye irritation in 2 mini in-
creased annoyance and almost no
eye or nose irritation during re
peated exposures.
Slight eye and nose irritation; no ef-
fect on respiratory frequency or
amplitude; odor perceived.
Changes in amplitude of respiratory
movements; slightly increased res-
piratory frequency; decreased eye
sensitivity to light.
Extremely irritating to all mucous
membranes in 5 mini lacrimation.
1,000-23,000 Medium to severe eye irritation in 5
man.
SOURCE: Based on data from Beauchamp et al.
1985.
shop on Formaldehyde (1984), Beauchamp
et al. (1985), and Tephly (1985) are partic-
ularly appropriate. The present discussion
is restricted primarily to inhalation toxicol-
ogy.
Acute Elects in Humans. Acute irritant
effects of aldehydes on human volunteers
have been documented. In general, acrolein
is the most potent of the series acrolein,
formaldehyde, acetaldehyde, crotonalde-
hyde. For example, acrolein is approxi-
mately two to three times more potent than
formaldehyde as an irritant (Beauchamp et
al. 1985). Table 4 is a compilation of the
doses at which various short-term re-
sponses to acrolein have occurred in human
volunteers (Carson et al. 1981). Ocular and
olfactory irritation is the first detectable
response and occurs at doses that are 1 (}20
times higher than the ambient levels in
urban air (table 1). Extreme irritation to
mucous membranes and alteration in respi-
ration occur at doses approximately 100
times ambient. At such levels, it is likely
that irreversible epithelial damage occurs
on chronic exposure (see Acute Effects in
Rodents, below). A similar profile of ef-
fects is observed for formaldehyde in hu
OCR for page 583
Lawrence]. Marnett
583
mans at somewhat higher doses. Irritation
occurs at 0.1-3.0 ppm, and respiratory
difficulties are evident at 10-20 ppm (Fas-
sett 1963~. Acetaldehyde and crotonal-
dehyde are 10-100 times less active than
acrolein and formaldehyde.
Allergic responses to aldehydes have
been reported. Hendrick and Lane (1975)
documented a case of asthma induced by
exposure of a hospital staff member to
formalin vapor. A pronounced decrease in
respiratory performance was observed after
exposure to a 25 percent solution for 15
mini however, the ambient levels of form-
aldehyde present were not measured.
Symptoms were prevented by pretreat-
ment of the patient with betamethasone.
Skin allergies have been induced by topical
application of solutions of formaldehyde
but the dose responses have not been ex-
tensively determined (Maibach 1983~. Der-
mal but not respiratory sensitivity has been
observed in guinea pigs exposed to 10 ppm
formaldehyde for 6 or 8 hr/day for 5 con-
secutive days. (Lee et al. 1984~.
Chronic Effects in Humans. Numerous
groups of individuals are occupationally
exposed to formaldehyde, acetaldehyde,
acrolein, and crotonaldehyde. Epidemio-
logic studies of the chronic effects of form-
aldehyde have been conducted with several
of these groups, but the results are incon-
clusive. Despite the ability to identify ex-
posed individuals, there is little information
on their smoking and drinking habits,
which confounds the interpretation of any
detected alterations in disease incidence.
The Consensus Workshop on Formalde-
hyde (1984) evaluated several epidemio-
logic studies of professional and industrial
workers exposed to formaldehyde and con-
cluded that there are insufficient data to
establish whether or not it is a human
carcinogen. The level of atmospheric expo-
sure in those workers was approximately
0.1-1.0 ppm. In the same groups, there
appeared to be no excess mortality associ-
ated with formaldehyde exposure. Chronic
exposure of humans to high levels of acro-
lein is considered unlikely because of its
extreme irritation. At levels below those
that cause olfactory or respiratory damage
(~1 ppm), prolonged exposure to acrolein
is intolerable, causing individuals to leave
the contaminated environment. Conse-
quently, there is no information on human
carcinogenicity or other chronic effects of
acrolein, nor are data on potential human
carcinogenicity of acetaldehyde or croton-
aldehyde available.
Acute and Chronic Effects in Rodents.
Pathological changes occur in the upper
respiratory, especially nasal, epithelium of
rodents exposed to aldehydes. The site and
severity are dose dependent (Kutzman et al.
1985~. Acute effects have also been noted,
and the lesions observed include exfolia-
tion, ciliastasis, cell erosion, ulceration, ne-
crosis, squamous metaplasia, and inflam-
mation (Dalhamn and Rosengren 1971;
Buckley et al. 1984~. Most of the damage is
reversible but some is irreversible. Signifi-
cantly, these effects are detected when ro-
dents are exposed to the RD50s for formal-
dehyde and acrolein (Buckley et al. 1984~.
(The RD50 is defined as the level at which a
50 percent reduction in respiratory rate
occurs. This level reflects the stimulation of
sensory receptors that attempt to limit ex-
posure to irritants.) The RD50s for formal-
dehyde and acrolein are 3.1 and 1.7 ppm in
mice (Buckley et al. 1984~; the RD50 for
acrolein is 6.0 ppm in rats (Babiuk et al.
1985~. Since pathological changes occur in
experimental animals as a result of expo-
sure to the RD50 levels, proposals have
been made that the RD50 be used to esti-
mate "safe" exposure levels for humans
(possibly 0.01-0.1 x RED (Kane et al.
1979; Alarie 1981~. Whether damage occurs
in response to exposure to these much
lower levels is unknown.
a, ~
Recommendation 3. Chronic low-
dose inhalation toxicology studies should
be undertaken to determine if tissue dam-
age occurs in response to exposure to levels
of formaldehyde, acetaldehyde, and acro-
lein that are 10-100 times lower than their
RDsos.
Although pure formaldehyde and acro-
lein do not cause neutrophil recruitment, an
inflammatory response has been observed
OCR for page 584
584
Aldehydes and Alcohols in Mobile Source Emissions
in response to carbon particles coated with
either compound (Kilburn and McKenzie
1978~. Paradoxically, exposure of suspen-
sions of neutrophils to formaldehyde and
acrolein results in lowered responsiveness
to soluble stimuli, such as phorbol esters,
and decreased generation of superoxide an-
ion (Witz et al. 1985~. This may be respon-
sible for the decreased in vivo killing of
bacteria by mice treated with either com-
pound Jakab 1977~.
Carcinogenicity. Exposure of 232 Fischer
344 rats to 14.3 ppm formaldehyde (6
hr/day, 5 days/week for 24 months fol-
lowed by 6 months of nonexposure) in-
duced squamous cell carcinoma in the nasal
epithelium of 103 animals (Kerns et al.
1983~. Exposure to 5.6 ppm formaldehyde
induced squamous cell carcinoma in only 2
of 235 animals, and at 2.0 ppm no response
was observed in 236 animals. In mice
(B6C3F~), exposure to 14.3 ppm induced
nasal tumors in only 2 of 215 animals
(Kerns et al. 1983~. This figure did not
represent a statistically significant increase
but is notable because of the rarity of nasal
tumors in mice. Exposure of Syrian golden
hamsters to 10 ppm formaldehyde (5 hr./
day, 5 days/week for 120 weeks) did not
induce any airway tumors (Dalbey 1982~.
Mixtures of formaldehyde and hydrochlo-
ric acid induced nasal cancer in Sprague-
Dawley rats that was entirely due to the
formaldehyde; no enhancing effect of hy-
drochloric acid was seen (Albert et al. 1982;
Sellakumar et al. 1985~. In all of these
chronic exposure studies, clearcut evidence
was acquired for reversible as well as irre-
versible damage to respiratory epithelium.
Chronic exposure of groups of Wistar
rats (110 animals/group) to acetaldehyde at
initial doses of 750, 1,500, or 3,000 ppm (6
hr/day, 5 days/week for 27 months) in-
duced 14, 34, and 38 nasal tumors, respec-
tively, compared to 1 in controls (Wou-
tersen et al. 1984~. Severe irreversible
degenerative changes of the upper respira-
tory tract were observed in the high-dose
group so the acetaldehyde concentration
had to be reduced repeatedly throughout
the course of the experiment. The tumors
observed at the low and moderate doses of
acetaldehyde occurred in the olfactory epi-
thelium (Woutersen et al. 1984), whereas
nasal tumors induced by low levels of
formaldehyde occur in the respiratory epi-
thelium (Kerns et al. 1983~. Nasal tumors
induced by high-level exposure to acetal-
dehyde and formaldehyde occur in the ol-
factory and respiratory epithelium. The
results at low levels suggest that acetal-
dehyde is better able to penetrate to remote
anatomic locations than formaldehyde.
Further evidence for differential effects of
aldehydes is provided by the observation
that acetaldehyde at levels of 1,65~2,500
ppm (7 hr/day, 5 days/week for 52 weeks)
induces tracheal, but not nasal, tumors in
Syrian golden hamsters (Feron et al. 1982~.
Exposure of Syrian golden hamsters to 4
ppm acrolein (7 hr/day, 5 days/week for 52
weeks) induced a number of pathological
changes in the upper respiratory tract, par-
ticularly the nasal epithelium, but no tu-
mors were observed in any organs (Feron
and Kruysse 1977~. Acrolein exhibits po-
tent teratogenic and embryolethal effects
when it is administered to rats intraamni-
otically but not by inhalation (Slots and
Hales 1985~.
Neither acrolein nor formaldehyde was
carcinogenic in Syrian golden hamsters
(Feron et al. 1982~. Acrolein is similar to
formaldehyde in chemical reactivity, irri-
tant activity, and retention in the respira-
tory tract, so it should be tested in the same
species in which formaldehyde has been
detected as a carcinogen the rat (Kerns et
al. 1983~. Attention should be paid to the
development of nasal tumors.
~ Recommendation 4. A chronic inha-
lation toxicology study of acrolein should
be undertaken in rats, with emphasis on
. . .
carclnogenlclty.
The species and organ specificities of
different aldehydes with respect to their
ability to induce respiratory tumors on
inhalation exposure is fascinating and has
been discussed (Kerns et al. 1983; Swen-
berg et al. 1983~. Stimulation of nasal re-
ceptors may play a key role in the difference
in the higher sensitivity of rats relative to
mice. Rodents attempt to restrict their in
OCR for page 585
Lawrence I. Marnett
585
take of irritants by reducing their respira-
tory minute volume (Chang et al. 1981~.
This response is more pronounced in mice
than in rats so, for example, at the same
level of exposure to formaldehyde, rats
breathe approximately twice as much
formaldehyde as mice (Chang et al. 1981~.
Indeed, the tumorigenic response of mice
to the effects of 14.3 ppm formaldehyde is
roughly the same as the response of rats to
6 ppm formaldehyde (Kerns et al. 1983~.
The importance of effective dose on tissue
specificity is further emphasized by the
observation that no tumors have been de-
tected outside of the respiratory tract with
any aldehyde. Studies of the retention (that
is, the amount of compound bound to
tissue) of various aldehydes in the respira-
tory tract of dogs indicate that formalde-
hyde is completely retained and acrolein is
nearly completely retained in the upper
tract whereas propionaldehyde is much less
retained (Egle 1972b). Acetaldehyde is the
least retained of all the aldehydes tested in
the upper respiratory tract, which is con-
sistent with its ability to induce tumors in
hamster trachea (Egle 1972a).
Decreases in minute volume cannot ex-
plain the sharpness of the dose response of
rats to formaldehyde. However, Swenberg
and colleagues (1983) proposed that effects
on mucociliary activity may play a role.
The nasal respiratory epithelium is nor-
mally covered by a dynamic protective
layer of mucus. The carbohydrate and pro-
tein in this layer may react with molecules
such as formaldehyde, preventing their ac-
cess to epithelial tissue. Interruption of
mucous flow might saturate the capacity of
these macromolecules to react with form-
aldehyde over certain anatomic locations,
thereby increasing the effective dose.
Formaldehyde increases mucous flow at
low exposure levels but reduces it at high
levels (Swenberg et al. 1983~; this may
result from the ciliastatic activity exhibited
by formaldehyde (Morgan 1983; Morgan et
al. 1983~. Inhibition of mucociliary clear-
ance introduces an additional step in the
carcinogenic process, suggesting that short
exposures to high concentrations would be
more effective for compound delivery than
long exposures to low doses. This is consist
ent with the nonlinear dose response ob-
served for formaldehyde carcinogenicity in
rats. It also suggests that occasional high
levels of exposure might exert biological
effects not expected from extrapolation of
dose responses obtained by chronic low-
level exposure (Swenberg et al. 1983~. Fur-
ther support for nonlinearity of formalde-
hyde action on respiratory epithelium is
provided by the dose dependence for in-
duction of squamous metaplasia in the nasal
cavity of Fischer 344 rats and B6C3F~ mice
(Kerns et al. 1983~. Formaldehyde at 2 ppm
only induces squamous metaplasia in the
anterior-most regions of the nasal cavity in
rats. Extensive metaplasia in midlevel and
posterior portions of the cavity are ob-
served with 5.6 and 14.3 ppm, respectively.
By comparing the extent of squamous meta-
plasia in mice and rats, it is possible to
approximate doses that exert similar patho-
logical effects. Using this criterion, the extent
of penetration by formaldehyde appears
equivalent in rats and mice at doses of 5.6 and
14.3 ppm, respectively. This result correlates
well to the difference in sensitivity of the two
species to the carcinogenic action of formal-
dehyde in the respiratory tract.
Studies indicate that alterations of mu-
cous flow and ciliatoxicity are important
components of the nonlinear dose response
for the carcinogenic action of formaldehyde
(Swenberg et al. 1983~. Acrolein exhibits
the most potent ciliatoxic activity of any
volatile aldehydes (Beauchamp et al. 1985~.
This may enhance the carcinogenic re-
sponse to other less ciliatoxic aldehydes
such as formaldehyde or acetaldehyde. The
most likely combination to test first is
acrolein and formaldehyde because they are
the most potent ciliatoxins and carcino-
gens, respectively, in mobile source emis-
s~ons.
~ Recommendation 5. A chronic inha-
lation toxicology study of mixtures of
formaldehyde and acrolein should be un-
dertaken in rats and hamsters, with empha-
s~s on carc~nogen~c~ty.
How one extrapolates the results of car-
cinogenicity studies in rodents to human
exposure is uncertain. Humans are rou
OCR for page 586
586
Aldehydes and Alcohols in Mobile Source Emissions
finely exposed to atmospheric levels of
formaldehyde that are 10~1, 000 times
lower than the doses that induce nasal
tumors in rodents. However, individuals in
certain cities are intermittently exposed to
much higher levels. Whether lon~-term
damage results from these episodic expo-
sures is uncertain, although there is little
doubt that acute effects, such as irritation,
occur. High intermittent exposure might
serve as an initiating event that provides a
focus of transformed cells sensitive to pro-
motion by other pollutants or environmen-
tal agents. Epidemiologic data do not pro-
vide evidence for a significant contribution
of air pollution to human cancer but one
might suggest that the combination of ex-
posure to aldehydes in automotive emis-
sions with other environmental agents is
important in some individuals such as
smokers (Doll and Peto 1981~. This seems
reasonable enough, but the concentrations
of aldehydes in cigarette smoke are several
orders of magnitude higher than their con-
centrations in urban air. Therefore, the
significance of the contribution of al-
dehydes in mobile source emissions to
health effects in smokers is uncertain. The
other complication of extrapolating results
from rodent bioassays to humans is the
difference in anatomy and physiology of
the two species. Rodents are obligate nose
breathers whereas humans are not. This has
obvious implications for the amounts of
toxic agents that reach respiratory tissues
by inhalation.
Cultured Cells. Formaldehyde and other
aldehydes exert numerous effects on iso-
lated cells in culture. They are toxic to
normal as well as tumor cells and, in fact,
certain cr,,~unsaturated aldehydes were
used in human clinical trials as potential
chemotherapeutic agents (Schauenstein et
al. 1977; Krokan et al. 1985~. The genotox-
ic effects of formaldehyde have long been
recognized (Auerbach et al. 1977~. It in-
duces single-strand breaks, DNA-protein
cross-links, sister chromatic exchanges,
and chromosome aberrations (Ross and
Shipley 1980; Bedford and Fox 1981; Ross
et al. 1981; Fornace 1982; Fornace et al.
1982; Levy et al. 1983~. It is mutagenic in a
variety of prokaryotic and eukaryotic cells
including human fibroblasts (Chanet and
van Borstel 1979; Boreiko et al. 1982;
Goldmacher and Thilly 1983; Szabad et al.
1983), transforms rodent cells (Ragan and
Boreiko 1981), and enhances viral transfor-
mation of Syrian hamster embryo cells
(Hatch et al. 1983~. Formaldehyde-induced
DNA lesions appear to be repaired, but
formaldehyde itself inhibits the ability of
human bronchial epithelial cells and fibro-
blasts to repair damage by x rays and
methylating agents (GraEstrom et al. 1983,
1984~. A similar constellation of events
occurs in response to treatment of cells
with acrolein (Schauenstein et al. 1977;
Beauchamp et al. 1985~.
Despite the extensive documentation of
the cellular effects of formaldehyde and
other aldehydes, the understanding of their
actions at the molecular level is incomplete.
For example, the critical targets that lead to
various cellular pathologies are, for the
most part, unknown. Evidence exists link-
ing the toxicity of cr,,~unsaturated al-
dehydes to modification of a critical sulf-
hydryl protein,
., , , ~.
but its identity is
unspec~ea t~chauenstein et al. 1977~. Cer-
tain DNA polymerases contain important
sulfhydryl groups that are sensitive to
modification, so these are likely candidates
(Kornberg 1980~. Sulfhydryl reactivity
may also contribute to the inhibition of
DNA repair by methyl transferases caused
by formaldehyde (Krokan et al. 1985~.
Recommendation 6. Experiments
should be undertaken in cells cultured from
various segments of the upper respiratory
tract to determine the mechanisms by
which aldehydes exert pathological changes
such as toxicity, hyperplasia, ciliatoxicity,
and so on.
Such experiments should concentrate on
identifying the critical targets for modifica-
tion by each compound and the extent of
modification that triggers the response. For
example, despite the extensive literature on
killing of prokaryotic and eukaryotic cells
by a,,l3 unsaturated aldehydes, the precise
mechanism of killing and the macromol-
ecules involved are uncertain. Does modi
OCR for page 587
Lawrence.~. Marnett
587
fication of DNA polymerases lead to tox-
icity or does toxicity result from inhibition
of enzymes of ATP generation? At what
level of modification does toxicity result?
Such knowledge will be important for basic
biology as well as for risk assessment based
on molecular dosimetry (see below).
A major unresolved question is how one
extrapolates the results of experiments
demonstrating pathological effects of al-
dehydes on cultured cells to risk assessment
for human exposure. For most of the in
vitro experiments, aldehydes are added in
solution, whereas in animal exposure ex-
periments they are administered by inhala-
tion. How one relates molar concentrations
of liquids to dosages of a gas that may
accumulate in a target cell is unknown.
Methanol
Methanol is rapidly absorbed following
oral, cutaneous, or respiratory exposure
and undergoes general distribution to body
water (Yant and Schrenck 1937; Haggard
and Greenberg 1939~. Its biological half-life
is 1.5-2 hr (Sedivec et al. 1981), which
means that many of the toxicologic effects
triggered by inhalation exposure may be
similar to those observed following oral
administration. Methanol's oral toxicity to
humans has been known for over 100 years
(Tephly 1985~. Considerable variability is
observed in the dose at which toxicity
results but best estimates of a dose required
for severe intoxication and death are
around 1 g/kg (Roe 1982~. A lag phase of
12-24 hr is observed before any symptoms
of toxicity are seen, which implies that a
metabolite is involved in the toxicity
(Tephly 1985~. Metabolic acidosis occurs
followed by visual effects that can lead to
blindness. Ocular toxicity is occasionally
followed by coma, other central nervous
system effects, and death. Ethanol antago-
nizes the effects of methanol and it may be
that varying amounts of ethanol contami-
nation account for the variability in dose at
which methanol is toxic to individuals (Roe
1955~. Rodents are not susceptible to the
toxic effects of methanol but nonhuman
primates are; for example, methanol exhib-
its ocular toxicity in monkeys (Roe 1982)
(see Metabolism, Methanol and Formalde-
hyde). Exposure of human volunteers to an
atmosphere containing 200 ppm methanol
results in accumulation of 750 mg of which
50 60 percent is retained in the lung (Se-
divec et al. 1981~. Considering the dose of
methanol estimated to be toxic to humans
(1 g/kg), it is unlikely that a normal human
being could ever be exposed to enough of it
by inhalation to experience acute toxicity.
This author was unable to find carcino-
genicity studies of methanol by inhalation
exposure. Methanol is metabolized slowly
during systemic circulation to formalde-
hyde, which is quickly metabolized to
formic acid (Tephly 1985~. A remote possi-
bility is that methanol is oxidized to form-
aldehyde in the respiratory epithelium
which is sensitive to its carcinogenic action.
If so, methanol may act as a latent form of
formaldehyde leading to accumulation in
tissues that formaldehyde is ordinarily in-
accessible to. Similar considerations hold
for ethanol with respect to acetaldehyde.
Taken with the potential importance of
methanol and ethanol as alternate fuels, it
seems important to test them thoroughly
for carcinogenicity via the inhalation route.
It is less important to test ethanol in inha-
lation studies because its oxidation product,
acetaldehyde, is 100 times less active as a
carcinogen than formaldehyde.
· Recommendation 7. A chronic inha-
lation toxicology study of methanol should
be undertaken in rats and hamsters, with
. . . . .
emphasis on carclnogenlclty.
Phenols and Catechols
No information is available on the inhala-
tion toxicology of phenols or catechols.
Most toxicologic studies have been per-
formed by oral or intravenous administra-
tion, so the concentrations used are difficult
to relate to inhalation exposure. Phenols are
not strongly toxic, and substituted phenols
such as butylated hydroxy toluene and
butylated hydroxy anisole are used as pre-
servatives in food. There is some specula-
tion that the presence of phenolic antioxi-
dants in food accounts for the steady
decrease in stomach cancer in developed
OCR for page 588
588
Aldehydes and Alcohols in Mobile Source Emissions
countries since 1945 (Doll and Peto 1981~.
Indeed, phenolic antioxidants such as buty-
lated hydroxy anisole inhibit chemical car-
cinogenesis and appear to act at the promo-
tion stage (Slaga et al. 1983~. However,
high doses of phenolic antioxidants actually
appear to be tumor promoters themselves
(Ito et al. 1982~.
Cocarcinogenic Effects of Aldehydes,
Alcohols, and Phenols
Cocarcinogenic effects have been reported
for formaldehyde and acetaldehyde (Dal-
bey 1982; Feron et al. 1982~. Lifetime ex-
posure of Syrian golden hamsters to 30
ppm formaldehyde concomitant with sub-
cutaneous administration of 0.5 mg dieth-
ylnitrosamine resulted in an enhancement
of the number of tracheal tumors over
treatment with diethylnitrosamine alone
(Dalbey 1982~. As mentioned above, form-
aldehyde does not induce tracheal tumors
in hamsters. No enhancement of tumori-
genesis was seen in the larynx or lung, and
the effect on the trachea was only observed
when formaldehyde exposure began before
diethylnitrosamine administration. En-
hancement did not result when formalde-
hyde exposure began after diethylnitrosa-
mine injections were completed. A similar
experiment was performed in Syrian
golden hamsters with acetaldehyde (1,65()
2,500 ppm) and benzota~pyrene adminis-
tered by intratracheal instillation (Feron et
al. 1982~. At a dose of 36.4 mg but not 18.2
mg benzota~pyrene, enhancement of tra-
cheal and bronchial tumorigenesis was ob-
served after 52 weeks. When a similar
experiment was performed with injection
of diethylnitrosamine no enhancement of
tracheal tumorigenesis was observed. In
fact, there appeared to be a decrease over
controls but this was considered a casual
association. Formaldehyde has been re-
norted to exhibit "initiating" and "pro-
moc~ng achy in the C3H/lOT1/2 in
vitro transformation system (Ragan and
Boreiko 1981; Frazelle et al. 1983~.
Methanol has not been tested for cocar-
cinogenicity by the inhalation route. An
epidemiologic association has been estab-
lished between consumption of alcoholic
beverages and esophageal cancer in smok-
ers, but there is no evidence for direct
carcinogenicity of ethanol (Doll and Peto
1981~. Its role in enhancing the carcinogen-
icity of cigarette smoke is uncertain. Cat-
echol has been identified as the major co-
carcinogenic component of cigarette smoke
(Van Duuren and Goldschmidt 1976; Hecht
et al. 1981~. However, bioassays were per-
formed using the initiation-promotion
model on mouse skin so the importance of
catechol as an inhalation cocarcinogen is
uncerta~n.
· Recommendation 8. Attempts should
be made to develop an initiation-promo-
tion protocol for carcinogenesis testing of
aldehydes and other components of mobile
. .
source em1sslons.
The two-stage mouse skin model has
been very useful for detection of potential
carcinogens, tumor initiators, and tumor
promoters. There is no analogous model
that can be used to screen compounds for
their effects on respiratory tissues. When
aldehydes were administered to rodents
simultaneously or after administration of
benzota~pyrene or diethylnitrosamine (Dal-
bey 1982; Feron et al. 1982), some stimu-
latory and inhibitory effects were noted but
they were not dramatic, and it was diff~cult
to speculate whether the aldehydes were
acting as cocarcinogens or promoters based
on the design of the experiments. A repro-
ducible initiation-promotion model would
enable rapid testing of mixtures of mobile
source emission components by the inhala-
tion route and would provide useful mech-
anistic information. Considering that
formaldehyde and acrolein exert most of
their effects on the respiratory epithelium
of the nasal tract of rats and that acetal-
debyde is a nasal carcinogen in rats and a
tracheal carcinogen in hamsters (Feron et
al. 1982; Woutersen et al. 1984), efforts
should be directed toward developing a
model in which the biological effects are
monitored in the upper respiratory tract. It
appears that most of an inspired dose of
these compounds does not reach the bron-
chi and lungs, so the model should be
designed with this in mind. In other words,
OCR for page 589
Lawrence J. Marnett
589
it would not seem prudent to perform
developmental experiments using com-
pounds that exert possible initiating or pro-
moting effects in the lungs.
Metabolism
Although some adverse effects of aldehydes
and alcohols have been described in hu-
mans, experimental animals, and cell sys-
tems, quantification of risk, especially at
ambient concentrations, is difficult with the
current data base. Additional research is
necessary to better estimate the potential
toxicity of these compounds. The design
and interpretation of experiments will be
aided by understanding their metabolism
and chemical reactions.
Methanol and Formaldehyde
All of the alcohols and aldehydes consid-
ered here are soluble in aqueous and or-
ganic solutions, which means they distrib-
ute rapidly throughout the body and within
cells (Beauchamp et al. 1985~. The major
pathway of metabolism is oxidative with
alcohols oxidized to aldehydes and al-
dehydes oxidized to acids. For example,
methanol is oxidized to formaldehyde,
which is oxidized to formic acid:
OH O
C ~
// \
H H H H H
Methanol Formaldehyde
o
11 11
C ~C
/ \ /
H OH
Formic Acid ( 1 )
Metabolism of alcohols and aldehydes can
result in either detoxification or metabolic
activation. The fact that a lag phase is
observed before the onset of clinical symp-
toms of methanol toxicity, coupled with
the findings that ethanol and alcohol dehy-
drogenase inhibitors antagonize methanol
toxicity, suggests that a metabolite of
methanol is responsible for its observed
toxicologic effects. Alcohol dehydrogenase
appears to be primarily responsible for the
oxidation of methanol (McMartin et al.
1975~. Its binding constant for methanol is
approximately six times lower than its
binding constant for ethanol, which ac-
counts for the ability of ethanol to antago-
nize methanol's effects (Maker and Tephly
1975~. Catalase is important for methanol
metabolism in rats but not in monkeys
(Mannering and Parks 1957~.
The major metabolite of methanol in
monkeys is formic acid (eq. 1~. Formate
also exhibits ocular toxicity in monkeys
(Martin-Amat et al. 1978~. It accumulates
in monkeys following methanol adminis-
tration, thereby resulting in metabolic aci-
dosis, but does not accumulate in rats. This
is consistent with the differential sensitivity
of these species to methanol toxicity and
implies a role for formate as a toxic metab-
olite. Methanol oxidation by alcohol dehy-
drogenase is the rate-limiting step of me-
tabolism and appears to be equally rapid in
rats and monkeys (Watkins et al. 1970;
Clay et al. 1975~. The accumulation of
formate in monkeys relative to rats appears
to be due to a decreased rate of its oxidation
to carbon dioxide (CO2) in monkeys (Mc-
Martin et al. 1977~. Formate metabolism
occurs by a folic acid-dependent pathway,
so folate deficiency renders monkeys ex-
tremely sensitive to methanol toxicity
(McMartin et al. 1977~. Conversely, folate
supplementation lowers their sensitivity
(Noker et al. 1980~. It appears that folic acid
levels are rate-limiting for formate metab-
olism to CO2 in monkeys.
The toxic effects of methanol may be
enhanced by simultaneous exposure to
other compounds. For example, antifolates
are used clinically for treatment of psoriasis
and cancer, and it is conceivable that indi-
viduals undergoing treatment could exhibit
enhanced sensitivity to methanol. Acute
methotrexate treatment of monkeys does
not decrease their rate of formate oxida-
tion, but the effects of chronic treatment are
unknown (Noker et al. 1980~. Perhaps
more important is the observation that
nitrous oxide (N2O) is an inhibitor of an
enzyme of folic acid metabolism and leads
to folate depletion (Eells et al. 1982~. En-
hanced sensitivity to methanol toxicity is
observed following exposure of monkeys
to N2O, and metabolic acidosis is induced
in rats, a species normally resistant to
OCR for page 594
594
Aldehydes and Alcohols in Mobile Source Emissions
oxides and hypochlorous acid, both
products of activated neutrophils. It is also
oxidized by free radicals such as the one
present in cigarette smoke (Pryor 1984~.
The latter reaction may be especially im-
portant in the genesis of diseases associated
with chronic cigarette smoking such as
emphysema. Although it has not been
tested, it seems quite likely that acyl per-
oxyl radicals formed by autoxidation of
aldehydes inactivate cz-1-proteinase inhibi-
tor by oxidizing its critical sulfide to a
sulfoxide. This provides a mechanism by
which one-electron oxidation of aldehydes
could lead to pulmonary emphysema.
Alcohols. Alcohols are relatively unreac-
tive chemically with nucleophiles and elec-
trophiles. They can be converted to more
reactive derivatives by conjugation with
functionalities (for example, glucuronate,
sulfate) that render the hydroxyl groups
more reactive Jakoby et al. 1980; Kasper
and Henton 1980~.
H3C-OH + ·OH , H~C OH + HERO (14)
These conjugates could conceivably act as
electrophiles and alkylate nucleophiles, but
there is no evidence that the toxicity exhib-
ited by, for example, methanol is a result of
such reactions. Furthermore, short-chain
alcohols are highly water-soluble, which
removes much of the driving force for their
conjugation with polar moieties. Alcohols
are oxidized to aldehydes and acids by
dehydrogenases, which is a reaction of pri-
mary importance in the metabolism of al-
cohols. Alcohols are not oxidized by one
electron to free radicals very readily, al-
though they will react with hydroxyl rad-
ical to form hydroxylmethyl radicals.
Phenols and Catechols. Attachment of a
hydroxyl group to an aromatic ring greatly
enhances the reactivity of the O H bond.
By comparison to aliphatic alcohols, the
chemistry of phenols is rich. Phenols are
more acidic than alcohols and possess sig-
nificant nucleophilicity toward reactive elec-
trophiles. For this reason they can serve a
protective role as scavengers of metaboli-
cally generated electrophiles. They are also
conjugated readily, which effects a marked
change in their polarity. The most important
reaction of phenols is probably with one-
electron oxidants (Simic and Hunter 1983~.
They readily donate electrons of hydrogen
atoms, thereby generating phenoxyl radicals.
OH
H9C4_~h'C4Hg
R.
+ ROO
O
H9C4_~4H9
+ ROOH
R.
(15)
Phenoxyl radicals are much more stable
than aliphatic alkoxyl radicals because of
conjugation with the aromatic ring. Phe-
nols with alkyl substitutents ortho to the
hydroxyl group are widely used as chain-
breaking antioxidants (Howard 1973). These
compounds donate a hydrogen atom to per-
oxyl radicals that are formed during the
propagation step of autoxidation. The stabil-
ity and steric hindrance of phenoxyI radicals
prevent them from abstracting hydrogen at-
oms from donors that will react with chain-
carrying peroxyl radicals. Phenoxyl radicals
couple to second molecules of peroxyl radical
to form peroxycyclohexadienones.
o
HgC4_:C4H9
+ ROO
R1
R
o
HgC4_~
O R.
-o
~C4Hg
(16)
OCR for page 595
Lawrence I. Marnett
595
As a result, every phenol molecule removes
two peroxyl radicals from autoxidation JO
mixtures, which interrupts the radical chain ~ - ,
and inhibits the autoxidation process. ~ I' + Nu H
Phenols that lack alkyl groups ortho to ~~<
the hydroxyl group react with one-electron t'
oxidants to form phenoxyl radicals that are
quite reactive. An important reaction of
phenoxyl radicals is coupling to 02, which
forms peroxyl radicals.
O
b+o~
o
to
11 rH
\' (17)
As discussed above, peroxyl radicals ab-
stract hydrogen atoms from reactive mol-
ecules such as unsaturated fatty acids to
initiate and propagate radical-chain oxida-
tions. This leads to the paradox that phenols,
which are generally thought to be antioxi-
dants, can actually stimulate free-radical au-
toxidation. The reactions of phenoxyl radi-
cals are especially important because phenols
can be oxidized to phenoxyl radicals by high-
valence metals as well as alkoxyl and peroxyl
free radicals. This provides a mechanism for
metal-catalyzed initiation of free-radical oxi-
dations. Free radicals are believed to enhance
carcinogenesis, particularly the promotion
phase, and free-radical initiators are promot-
ers in the two-stage assay in mouse skin
(Slaga et al. 1981~. This may contribute to the
reported carcinogenicity of certain phenols in
mouse forestomach (Ito et al. 1982) and to
the cocarcinogenicity of catechols on mouse
skin (Van Duuren and Goldschmidt 1976;
Hecht et al. 1981~.
Unhindered phenols are oxidized to hydro-
quinones that are extremely air-sensitive and
oxidize to quinones (Irons and Sawahata 1985~.
OH OH
~ ,.,
o
OH ° (18)
Quinones are strong electrophiles that un-
dergo addition of nucleophiles to the dou-
ble bonds of the ring (Irons and Sawahata
1985~.
o
o
AH
Nu
o
OH
I;>,
Nu
OH (1 9)
This can result in the formation of protein
and nucleic acid adducts and to depletion
of glutathione. Polycyclic quinones are
readily reduced to semiquinones that are
either further reduced to hydroquinones or
are oxidized by O2 to regenerate the quinone
and form O2 (Smith et al. 1985~. The con-
tinuous reduction and oxidation of quinones
is called redox cycling and can lead to copi-
ous superoxide formation (Smith et al. 1985~.
This causes DNA strand scission, mutage-
nicity, and toxicity, all of which probably
require metals. Redox cycling may well ac-
count for some of the pathophysiological
effects of phenols and catechols.
Quantification of
Exposure and Estimation
of Human Risk
One of the fundamental unsolved problems
of toxicology is how to extrapolate dose/
response data obtained in animal testing
(usually in rodents) to risk assessment in
humans. An approach to crossing this spe-
cies barrier is to quantitate the dose that
reaches the target organ at a series of expo-
sure levels. This is called molecular dosim-
etry. By knowing the amount of com-
pound that must reach a target cell to exert
an effect in, for example, rats, it should be
possible to more intelligently estimate the
risk of a given amount of the same com-
pound reaching the same cell type in hu-
mans. The effective dose that reaches target
tissues is more relevant to risk assessment
OCR for page 596
596
Aldehydes and Alcohols in Mobile Source Emissions
than are the atmospheric levels. For exam-
ple, at an ambient concentration of 10
ppb, the amount of formaldehyde in-
spired in 24 hr by an average human is 7
,umole. This does not seem a significant
amount until one realizes that it is almost
exclusively localized in the mucus and epi-
thelial cells lining the upper respiratory
tract.
What is the starting point to be if one is
to quantitate binding of aldehydes, alco-
hols, and phenols to critical intracellular
targets for their toxic and carcinogenic ef-
fects? Most of the toxicity exhibited by
these agents is probably due to covalent
binding to protein in which the aldehyde or
quinone reacts as an electrophile. There is
no doubt that protein binding to these
compounds or their metabolites occurs and
that it can cause toxicity. Saturated and
a,,l3 unsaturated aldehydes as well as qui-
nones bind rapidly to sulfhydryl proteins
and inactivate them. Stable adducts also
form to lysine residues. Aldehyde/lysine
conjugates have been isolated from rat
urine that most likely arise from proteolysis
of aldehyde/protein conjugates (McGirr et
al. 1985~. This provides direct evidence for
covalent binding of aldehydes to proteins in
viva. A considerable amount of informa-
tion suggests that covalent binding to sulf-
hydryl groups of DNA polymerases is
responsible for the toxic effects of a,,l3
unsaturated aldehydes (Schauenstein et al.
1977). Evidence also suggests that the cv-
tostatic effects of quinones derives from
their ability to bind specifically and cova-
lently to tubulin (Irons et al. 1981~. A
similar reaction of tubulin or another com-
ponent of the flagellar system may also
account for the ciliatoxic activity of al-
dehydes.
Molecular dosimetry offers an approach
to the quantitation of physiologically rele-
vant damage resulting from exposure to
aldehydes and quinone metabolites of phe-
nols. Methods could be developed for the
analysis of covalent adducts to proteins that
are involved in pathological responses in
target tissues. Of course, this requires that
the key protein targets are known. Under-
standing the role of individual proteins in
. . .
tOXlCo. OglC responses represents a mad or
gap in our knowledge. This is why mech-
anistic toxicology studies in cell culture are
so important. Until an adequate knowledge
of key protein targets is available, methods
should be designed to quantitate adducts to
proteins that are not necessarily important
in the observed response but are abundant
in the cell or tissue in which the response is
observed. This approach is similar to the
use of hemoglobin for estimation of expo-
sure to methylating agents and carcino-
gens. Abundant proteins that contain reac-
tive amine groups would be especially
useful because amine/aldehyde conjugates
are more stable than thiol/aldehyde con-
jugates. It is likely that the target cells for
the effects of airborne aldehydes are in the
epithelial tissue of the nasal and respiratory
tracts. This is based on the types of effects
observed and the extreme reactivity of the
molecules. It is very unlikely that aldehydes
or quinones escape pulmonary epithelia to
reach peripheral tissues. If most of the
covalently bound material is localized in the
nasal or respiratory t'ract, it should be pos-
sible to sample these matrices in individual
human subjects by ravage techniques.
Molecular dosimetry of nucleic acid ad-
ducts in target tissue will be important
when attempting to relate inhaled dose to
carcinogenic response. Thus, method de-
velopment is recommended. However,
quantitation of nucleic acid adducts in viva
is of less value than quantitation of pro-
tein adducts for estimation of inhaled dose.
The levels of DNA adducts produced in
cells are several orders of magnitude lower
than protein adducts and are subject to
varying degrees of removal by repair en-
zymes, which further lowers the steady-
state concentration of the nucleic acid ad-
ducts.
~ Recommendation 9. Methods should
be developed for quantitation of the
amounts of aldehydes that reach target or-
gans or potential target organs in humans
and rodents.
Despite the long history of investigation
of the reaction of formaldehyde with nu
OCR for page 597
Lawrence I. Marnett
597
cleic acids and the knowledge that formal-
dehyde is a nasal carcinogen, the identity of
the abducts that it forms on reaction with
DNA in vivo is unknown. This may be due
to the hydrolytic instability of imine and
hydroxymethyl derivatives of purines and
pyrimidines, and it underscores the need
for development of novel methods of iso-
lation and analysis. Less is known of the
reaction of acetaldehyde and acrolein with
DNA although acrolein/deoxyguanosine
adducts have been recently identified and
detected following the reaction of acrolein
with DNA in vitro (Chung et al. 1984~.
Acrolein is metabolized by microsomal cy-
tochrome P-450 preparations to glycidal-
dehyde, which binds to DNA and is a
carcinogen (International Agency for Re-
search on Cancer 1976; Patel et al. 1983~.
The adduct that glycidaldehyde forms on
reaction-with deoxyguanosine is different
than the acrolein/deoxyguanosine adducts,
so isolation and quantitation of acro-
lein/DNA adducts following inhalation of
acrolein would determine the extent to
which metabolism plays a role in acro-
lein's genotoxic effects in viva. The avail-
ability of methods for detection and quan-
titation of aldehyde/DNA adducts formed
in viva might be important as part of a
molecular dosimetry approach to risk as-
sessment.
· Recommendation 10. Methods should
be developed to detect and quantitate DNA
adducts derived from formaldehyde, acet-
aldehyde, and acrolein. The techniques
should then be applied to the detection of
DNA adducts formed in target tissues
after administration of carcinogenic and
subcarcingenic doses of the inhaled com-
pounds. Detection of these adducts in
cultured target cells would be a helpful
intermediate step in adaption of the analyt-
ical methods to detection of in viva
adducts.
There is ample precedent for the im-
portance of electrophilic additions to pro-
teins and nucleic acids in aldehyde and
phenol biochemistry and toxicology, but
the possibility that free-radical reactions
contribute to their health effects has not
been rigorously established. Therefore, it
would be useful to conduct experiments to
probe for the involvement of free radicals
as mediators of aldehyde and phenol pa-
thology. This is not a trivial undertaking,
because free radicals are species with rela-
tively short half-lives that make them
nearly impossible to detect and quantitate
directly (usually <1 see). Nevertheless, it is
now possible to trap certain types of free
radicals that might be produced from al-
dehydes and phenols (Packer 1984~. In ad-
dition, products of in viva free-radical re-
actions can be detected and quantitated as
indirect evidence for free-radical formation
(Packer 1984~.
· Recommendation 11. Experiments
should be performed to determine if al-
dehydes exert toxicologic effects by gener-
ation of free radicals.
Summary
A review of the literature indicates that
aldehydes are the most potent biologically
active substances of the compounds under
consideration in mobile source emissions.
They exert toxicologic effects at concentra-
tions approximately 10-100 times their
ambient atmospheric levels. Variations in
ambient levels have been reported with
occasional toxicologically relevant concen-
trations reported in heavily polluted met-
ropolitan areas. Inhaled aldehydes exert
their toxicologic effects in the upper respi-
ratory tract, and there is no reason to
believe that they trigger systemic re-
sponses. This may be due to their high
reactivity or to the fact that they are rapidly
metabolized. Metabolism can result in de-
toxification or metabolic activation. The
formation of substantial amounts of meth-
anol and formaldehyde during normal
human metabolism precludes development
of analytical methods for their quantitation
based on "systemic" approaches, such as
plasma or urine analysis. These observa-
tions mandate novel approaches to quanti
OCR for page 598
598
Aldehydes and Alcohols in Mobile Source Emissions
ration of exposure and estimation of risk
to the human population. Aldehydes and
quinones are reactive electrophiles that
form adducts with DNA and proteins. The
structures of several aldehyde/nucleic acid
adducts have been elucidated, but the
~ . . . . .
extent ot t heir formation In veto Is un
known. Phenols are oxidized to free radi-
cals, which may play a role in tumor pro-
motion. Aldehydes are oxidized to very
reactive free radicals in chemical systems
but it is uncertain if they form free radi-
cals in biochemical systems in vitro or in
Volvo.
Summary of Research Recommendations
Exposure
HIGH PRIORITY
Recommendation 1 Routine monitoring of atmospheric alcohol and aldehyde levels
should be performed in regions where alcohol-based fuels are or
will be in heavy use.
LOW PRIORITY
Recommendation 2 Methods should be developed to routinely analyze phenols and
catechols in urban air.
Health Effects
HIGH PRIORITY
Recommendation4 A chronic inhalation toxicology study of acrolein should be
undertaken in rats, with emphasis on carcinogenicity.
Recommendation 5 A chronic inhalation toxicology study of mixtures of formalde
hyde and acrolein should be undertaken in rats and hamsters, with
emphasis on carcinogenicity.
MEDIUM PRIORITY
Recommendation 3 Chronic low-dose inhalation toxicology studies should be un
dertaken to determine if tissue damage occurs in response to
exposure to levels of formaldehyde, acetaldehyde, and acrolein that
are 10-100 times lower than their RD50s.
Recommendation 8 Attempts should be made to develop an initiation-promotion
protocol for carcinogenesis testing of aldehydes and other compo
nents of mobile source emissions.
LOW PRIORITY
Recommendation 7 A chronic inhalation toxicology study of methanol should be
undertaken in rats and hamsters, with emphasis on carcinogenic
ity.
OCR for page 599
Lawrence I. Marnett
599
Cellular Effects
HIGH PRIORITY
Recommendation 6 Experiments should be undertaken in cells cultured from various
segments of the upper respiratory tract to determine the mecha
nisms by which aldehydes exert pathological changes such as
toxicity, hyperplasia, ciliatoxicity, and so on.
Molecular Dosimetry
HIGH PRIORITY
Recommendation 9 Methods should be developed for quantitation of the amounts of
aldehydes that reach target organs or potential target organs in
humans and rodents.
Recommendation 10 Methods should be developed to detect and quantitate DNA
adducts derived from formaldehyde, acetaldehyde, and acrolein.
The techniques should then be applied to the detection of DNA
adducts formed in target tissues after administration of carcino
genic and subcarcingenic doses of the inhaled compounds. Detec
tion of these adducts in cultured target cells would be a helpful
intermediate step in adaption of the analytical methods to detection
of in viva adducts.
MEDIUM PRIORITY
Recommendation 11 Experiments should be performed to determine if aldehydes
exert toxicologic effects by generation of free radicals.
References
Alarie, Y. 1981. Bioassay for evaluating the potency
of airborne sensory irritants and predicting accept-
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Representative terms from entire chapter:
source emissions
