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OCR for page 190
5
Conclusions and
Recommendations
Dl Sl N FECTION M ETHODS AN D EFFICACY
Waterborne disease outbreaks continue to occur, not only in developing
nations, but also in the United States, where almost 70,000 cases were
reported in 235 disease outbreaks during the period 1978-1983. The etiol-
ogy of waterborne disease in the United States has changed dramatically
since the early 1900s. Recent outbreaks have generally been dominated
by gastrointestinal illness associated with viruses and protozoan cysts.
These pathogens are generally more resistant to disinfection than the kinds
of pathogenic bacteria that formerly caused most outbreaks. Problems also
continue to occur in cases of consumption of untreated water, errors of
insufficient or interrupted disinfection, failures to maintain adequate levels
of residual disinfectant in potable water distribution systems, and breaches
in the systems.
Although chlorination continues to be the predominant method of drink-
ing water disinfection in the United States, the use of alternative methods
is increasing. Treatment facilities in several states have recently increased
and/or switched to chloramination for primary disinfection, largely in
response to the maximum contaminant level of 100 log of total trihalo-
methanes per liter set by the Environmental Protection Agency under the
Safe Drinking Water Act. Kansas, which formerly prohibited chlorami-
nation, now requires the use of ammonia to convert all free chlorine
residual to chloramines following 30 minutes of chlorination. The Met-
ropolitan Water District of Southern California now uses chloramination
for distribution system residual maintenance. Other alternatives, such as
190
OCR for page 191
Conclusions and Recommendations 19 ~
ozone disinfection, are being used increasingly in Europe and Canada.
Potentially substantial increases in ozone use in the United States also
appear possible in view of recent improvements in the reliability and
efficiency of ozone disinfection technology, together with the higher ef-
ficacy of ozone (compared with chlorine) against resistant protozoan cysts
and viruses.
Research is needed to improve understanding of the comparative effi-
cacy of major drinking water disinfection practices (especially chlorina-
tion, chloramination, ozone, and chlorine dioxide) against the currently
most important, resistant protozoan cysts and viruses. Studies of the major
factors affecting such efficacy under treatment plant operating conditions
are also important. Without such studies, it is possible that many future
drinking water treatment operations, decisions about alternative methods,
and trade-offs regarding toxic by-products of chlorination may be inor-
dinately influenced by the preponderance of existing knowledge about
types of bacterial pathogens that pose less of a public health problem in
the United States today than several decades ago.
"Life-cycle" studies of disinfectants are also needed for comprehensive
examination of the direct and indirect implications of potential widespread
local conversions to alternative disinfection practices. For example, the
reliability of ozone disinfection technology and the tropospheric impact
of potentially large increases in ozone generation that might result from
widespread application should be investigated more thoroughly, before
national and local decisions regarding conversion become a matter of fact.
CH EM ISTRY AN D TOXICITY OF DIS I N FECTION
Reactions and By-Products
Studies of chlorination of model compounds and isolated humic and
fulvic acid precursors during the past few years have improved under-
standing of the reaction mechanisms and types of by-products formed
during chlorine disinfection of drinking water. Although many of the
specific by-products are not yet well characterized, they appear to vary
according to the structures of the humic and fulvic acid molecules being
chlorinated, the chlorine-to-carbon (CI/C) ratio, the pH, the time of re-
action, and other factors. The principal by-products, especially at high
Cl/C molar ratios of 3:1 or 4:1, are volatile, hydrophobic compounds
(mainly chloroform). However, a large variety of nonvolatile, hydrophilic
compounds are also produced. These nonvolatile products include both
chlorinated and unchlorinated aromatic and aliphatic compounds. The
production of these smaller, hydrophilic molecules appears to increase at
lower pH and Cl/C molar ratios (less than 1:1), while higher ratios favor
the formation of chloroform and other volatile by-products. At the lower
OCR for page 192
192 DRINKING WATER AND HEALTH
CT/C ratios, which more closely approximate typical drinking water dis-
infection conditions, the humic acid precursors appear to support the
formation of unchlorinated by-products (e.g., monobasic and dibasic al-
iphatic acids) to a greater extent than do fulvic acid precursors. Increasing
the Cl/C ratio appears to drive both types of precursors toward chloroform
production and a larger fraction of identifiable products, which neverthe-
less represent only a small fraction of the initial organic material.
It is clear from the studies described in Chapter 4 that the importance
of trihalomethanes may be overestimated from experiments involving Cl/
C ratios and chlorination times that greatly exceed typical drinking water
disinfection conditions. Nonvolatile by-products of humic and fulvic acid
chlorination may be more important than previously believed. Further
studies of reaction mechanisms, controlling factors, and by-product iden-
tification are needed. Improved methods for characterizing the nonvolatile
products are also needed to support such studies.
Toxicity
Both chlorinated and untreated drinking water contain genotoxic com-
pounds identified in concentrated residues by short-term assays. Chlori-
nation tends to destroy some of these compounds, as well as produce new
ones. Short-term animal skin tests, although not conclusive, provide in-
dications that organic concentrates from chlorinated water are tumorigenic
under some experimental conditions. Studies by routes other than dermal
application have not shown such an effect. Based on the available data,
the recommended SNARLs and risk assessments developed by the com-
mittee are shown in Table 5-1.
One finding common to most studies performed throughout the world
is that chlorination as a means of disinfection introduces mutagens that
are not present, or that are present in lower amounts, in raw, untreated
water. Recent studies indicate that most of the mutagenic activity in treated
water may be due to nonvolatile (rather than volatile) compounds that are
produced from chlorination of humic and fulvic acids.
The upper 95% lifetime cancer risk for humans based on drinking water
studies in laboratory animals show a risk of 8.9 x 10-8, or approximately
1 chance in 10 million of cancer for the consumption of 1 ~g/liter of
chloroform in water. This and other risk assessment calculations are shown
in the section on trihalomethanes. The committee in reviewing the results
of these calculations recommends that the current level of total trihalo-
methanes (THMs), regulated at 100 ~g/liter in finished drinking water,
be reduced. This level is unsupportable on the basis of the risk values for
chloroform developed in this review, noting that chloroform is the principal
THM. Nonetheless, the committee is concerned about the toxicity of the
OCR for page 193
Conclusions and Recommendations 193
TABLE 5-1 Summation of Suggested No-Adverse-Response Levels
(SNARLs) and Risk Estimates for Chemicals Reviewed in This Volume
Estimated SNARLs Upper 95%
Confidence
Estimate of Lifetime
CompoundAdultChild Cancer Risk
Disinfectants
Chlorineaa
Ozoneaa
Chlorine dioxide210 ~g/liter60 ~g/liter
Chloramine581 ~g/liter166 ~g/liter
Disinfectant by-products
Chlorate24 ~g/liter7 ~g/liter
Chlorite24 ~g/liter7 ~g/liter
Trihalomethanes
Chloroform 8.9 x 10-8e
1.9 x 10-6
Chlorodibromomethane 8.3 x 10 7
Haloacids
Dichloroacetic acid420 ~g/liter175 ~g/liter
Trichloroacetic acid120 ~g/liter50 ~g/liter
Haloaldehydes
Chloroacetaldehydebb
Dichloroacetaldehydebb
Trichloroacetaldehydebb
Haloketones
1, 1,1-Trichloroacetonebb
1,1,3,3-Tetrachloroacetonebb
Hexachloroacetonebb
Haloacetonitriles
Dichloroacetonitrile56 ~g/literC
Dibromoacetonitrile161 ~g/liter23 ~g/liter
Bromochloroacetonitrilebb
Trichloroacetonitrilebb
Chloropicrin40 ~g/liter12 ~g/liter
Chlorophenols
2,4-Dichlorophenol7,000 ~g/liter2,000 ~g/liter
2,4,6-Trichlorophenolbb
2-Hydroxylchlorophenolbb
aNot calculated.
bInsufficient data for calculation.
CNot calculated; the adult value was calculated for comparison purposes; it is not recommended
by the committee.
~Tumor data for risk assessment calculation from drinking water animal study.
eTumor data for risk assessment calculation from corn oil gavage animal study.
OCR for page 194
194 DRINKING WATER AND HEALTH
other individual by-products produced by the reactions of alternative chem-
ical disinfectants in common use in water supplies because their toxicity
is essentially unknown and their potential health impacts cannot be ade-
quately assessed.
There is a larger risk associated with the unidentifiable by-products of
water disinfection, especially with chlorine. The magnitude of this risk is
not quantifiable by the studies done to date, but it is high enough to
warrant additional effort to determine its qualitative source and quantitative
magnitude. Associated with this effort, methods should be sought to follow
the risk associated with chlorination by-products even in the absence of
individually quantifiable compound risks. Correlation of health risks with
surrogate parameters is a classic method, but unfortunately the parameters
measured to date, those of THMs and total organic halogen, do not appear
to correlate well. Also many of the risk studies with humic material, the
principal source of precursor, have been done with commercial humic
acid. It is recommended that future studies focus on humic material con-
centrated from aquatic sources. A large variety of such materials has been
collected by the USCS laboratory in Denver, Colorado, under the auspices
of the International Humic Substances Society, and methods have been
developed for the concentration of large quantities of aquatic humic ma-
terials from waters rich in such materials.
Chloramination is becoming widely practiced as a method of disinfec-
tion because of the regulation of THM levels and the low capacity of
chloramine to form THM. Because it is a much weaker disinfectant than
chlorine, chloramine must be used at higher concentrations and for longer
periods of contact to achieve sufficient disinfection. Even with extended
contact time and higher concentrations, however, chloramination is not
recommended as a primary disinfectant, especially where virus or parasitic
cyst contamination is potentially present. Preformed monochloramine is
undesirable as a primary disinfectant. The use of marginal chlorination as
a method of introducing chloramines into a water supply system is spe-
cifically not recommended because, along with the depletion of chlorine
to produce inorganic monochloramine, organic chloramines that have even
lower efficacy as disinfectants are formed. Organic chloramines have also
been implicated as major contributors to the mutagenicity of chlorinated
drinking and natural waters. For these reasons, chloramine treatment is
used to minimize by-product formation. When free chlorine is used as the
primary disinfectant, an amount should be used that is sufficient to produce
a slight residual of free chlorine above that required to oxidize nitrogen,
followed by the addition of ammonia to form monochloramine and limit
THM formation.
Chloramine, as inorganic monochloramine, is only weakly mutagenic.
Organic concentrates from chloraminated water exhibited half the muta
OCR for page 195
Conclusions and Recommendations 195
genie response of those from chlorinated water. Even though it does not
form significant levels of THM, chloramine is nevertheless capable of
producing halogen substitution to form organic compounds, and thus may
produce important levels of total organic halogen (TOX).
Little is known about the oxidant residuals formed in drinking water
because of the general lack of accuracy in measuring them. There is also
a great need to define more accurately the nature of the oxidant residuals
referred to as total chlorine, by which chloramines are measured by dif-
ference. The determination of adequate water disinfection has traditionally
relied on accurate measurement of the concentration of residual disinfec-
tant. There are currently no suitable methods for fully quantifying the
organic chloramine fraction in the presence of inorganic monochloramine.
Until such methods are developed, utilities that handle water supplies
containing high concentrations of organic nitrogen run the risk of over-
estimating the ability of their systems to maintain adequate disinfection.
Additional work is required on organic base precursor fractions, pri-
marily on the organic nitrogen precursors. Neither the toxicity nor products
formed from these precursors have been well documented. This is espe-
cially true for the organic chloramine portion of the chlorine residual.
These compounds appear to be candidates for significant health concern
because of their potential to interfere with judgments made on the basis
of chlorine residual as to the degree of disinfection achieved, as well as
their potential for increasing mutagenicity.
When possible, organic precursors should be removed prior to the dis-
infection process. This can be achieved by changing the order of the
procedures of conventional treatment. A better approach, however, is to
improve specific conventional water treatment processes to remove organic
compounds and to add processes such as carbon absorption and preoxi-
dation. Initial removal of organic by-product precursors precludes the need
for reducing contact time, thus improving the efficacy of the disinfection
processes and minimizing formation of organic chlorine by-products.
The use of alternative oxidants, especially ozone and chlorine dioxide,
will increase in the United States in the coming decades. Little is known
about the types of by-products produced by ozonation of natural organics.
Well-conceived studies need to be conducted that will focus on the stable
compounds expected from ozone reactions with humic material. But these
studies must recognize that the analytical methods used for nonpolar
chloroorganics (extraction, GC/MS) may not be successful for the more
polar, more labile compounds expected from ozonation processes. Par-
ticular attention should be given to the search for unsaturated aldehydes
and the hydroxy-hydroperoxides.
Following these studies, further health effect studies are needed to
determine whether ozone by-products are mutagenic or carcinogenic or
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196 DRINKING WATER AND HEALTH
produce other adverse effects. These studies should take into account
variations that are likely to occur when the oxidation process is carried
out in different matrices (pH, O3/TOC ratio, alkalinity).
Notwithstanding the fact that these studies need to be carried out, drink-
ing water suppliers should not dismiss the possibility of using ozone as
an alternative to chlorine and chloramines in water treatment. Ozone is
an excellent disinfectant (although it must be used in combination with a
secondary disinfectant to maintain a residual in the distribution system);
ozone is an excellent oxidant for the various needs of water treatment; it
does not form chlorinated by-products; and the admittedly inadequate
studies now available point to lower toxicities of ozonated water than of
chlorinated water.
EPIDEMIOLOGY
The studies reviewed in this volume present progressive improvement
in design and in suggestive evidence that by-products induced by chlo-
rination, or some other water parameter, may be causally related to some
internal cancers of the epithelial tissue of the digestive organs and the
lower urinary tract. Confidence in the demonstration of causal relationships
would be increased if well-designed studies could be replicated in other
populations.
There have been many differences in research design among studies by
various investigators. These differences can greatly influence observed
risk ratios. No epidemiological study has measured actual levels of THMs
or other potentially carcinogenic materials over periods of time in drinking
water. Many have relied on dichotomous coding of chlorination as a yes-
or-no variable. Few have considered population migration.
It is possible that true geographic differences are involved in exposure
to chlorination by-products, with rural areas having generally higher lev-
els. The National Bladder Cancer Study (see Chapter 3) demonstrated
distinct geographic variations in risk with the use of a common method-
ology. Geographic differences in available substrate may lead to varying
amounts of chlorination by-products, or there may be other carcinogens,
such as pesticides, in rural drinking water. This latter factor may be
important for two reasons: (1) the presence of other causal factors in the
drinking water that are randomly distributed with respect to chlorination
but unaccounted for in the analysis would produce a lower observed risk
ratio than the true risk ratio for chlorination; and (2) other unknown causal
variables whose distribution parallels that of chlorination could act as
classical confounders. Surface waters are the most frequently chlorinated
source of drinking water, followed by shallow alluvial wells; the least
frequently chlorinated sources are deep wells. The same distributional
OCR for page 197
Conclusions and Recommendations 197
pattern would be expected for many runoff contaminants. A major fault
of virtually all previous studies has been the failure to obtain exposure
data on carcinogens unrelated to chlorination in finished water supplies.
In addition, by-products of chlorination other than the trihalomethanes
may be of importance in the genesis of human cancer.
Even in the studies showing statistically significant risk ratios, the mag-
nitude of the ratios has been relatively small but of major public health
importance. For example, if the risk ratio for rectal cancer associated with
drinking chlorinated water is 1.5 and 50% of the persons in the United
States drink chlorinated water, about 6,400 new cases of rectal cancer
might be caused each year by chlorinated drinking water. Given the fre-
quency of water consumption, even a small excess risk (less than 10%)
may account for a lot of disease. The entire issue of drinking water and
cancer deserves continued investigation.
The following are recommendations of approaches to the assessment
of human health risks to water disinfectants. One role of epidemiological
studies is to provide qualitative descriptions of the relation of disease to
the various methods of disinfection. Current evidence suggests that the
relative risk to human health from exposure to disinfectants is probably
small in settings with effective control of treatment levels in distribution
systems. Routine monitoring of disease occurrence in populations with
different treatment methods may provide crude qualitative descriptions of
the patterns of disease in populations using different treatment strategies.
The expense of such studies can be kept relatively low, as they can use
existing data bases maintained by state health departments, cancer regis-
tries, and lists of residents. A major limitation to this approach is that it
is difficult to control for important confounders, such as smoking history.
The recommended design for descriptive studies, aimed toward gen-
erating hypotheses about the health effects of drinking water disinfectants,
is a retrospective cohort study. The health outcomes typically would be
prevalent cases of disease (mortality is an exception), and exposure clas-
sification would be the type of disinfectant in drinking water in the resi-
dence at the time of ascertainment of health status (for example, if cause
of death is the outcome of interest, then residence at death would determine
the exposure classification). Public health department statistics routinely
include information on some important covariates of disease that are po-
tential confounders, such as socioeconomic status, last occupation, race,
age, and gender. These qualitative descriptions would be based on a range
of point estimates (usually rate ratios of standardized mortality ratios) that
offer evidence about the nature of an association. There is often little
quantitative meaning to these estimates. To suggest that rate ratios ranging
from 1.13 to 1.39, which represent low to high doses of chlorine, re-
spectively, are evidence of a dose-response relationship of chlorine to a
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i98 DRINKING WATER AND HEALTH
particular cancer is an inappropriate interpretation of results based on crude
and indirect measures of human exposure. The implication of such crude
analyses is that one must report results that are consistent with the accuracy
of the exposure measures. For descriptive studies such as these, estimates
of effect (e.g., rate ratios) should not be reported beyond two significant
digits. Point estimates would best be described within a range of confidence
intervals, rather than a p-value, since it is easy to achieve "statistical
significance" levels when there is a lot of information. Statistical differ-
ences may not have bearing on biological differences.
After studies have been undertaken to generate hypotheses about the
health effects of water disinfectants, the role for epidemiological studies
is to quantify associations when data are available. For these studies, data
on confounding factors and accurate historical information on the duration
and type of disinfectant exposures should be required. Information on the
timing of the onset of disease should also be confirmed if there truly were
a health hazard associated with a particular disinfectant.
Design options for these analytical studies depend on the frequency of
occurrence of the health effects of interest and the true relationship of a
particular disinfectant to the occurrence of these health effects. One ap-
proach is a prospective cohort study, in which members of a healthy
population are chosen as samples according to the type of disinfectant in
their drinking water supply. Over time, the cohort is monitored for disease
occurrence, and at the termination of follow-up, the rates of disease are
compared. The advantages of this design are (1) direct measures of in-
dividual water consumption can be obtained; (2) by-products can be mea-
sured from periodic tap samples; (3) incident cases of disease are measured
(as opposed to prevalent cases); and (4) a variety of outcomes can be
investigated. The major disadvantages of this design are the expense and
administrative tedium in carrying out these studies. However, if large
cohorts have already been assembled for other investigations, then it may
be feasible to add a component to the study objectives to investigate
potential adverse human health effects of water disinfectants. Usually a
prospective cohort design is not feasible, particularly if the etiologic period
is many years or decades, as it may well be for the chemicals under
discussion, and if the disease occurrence due to disinfection by-products
is rare. The occasion of a change in water disinfection treatment is an
ideal time to begin such a study.
A more suitable approach is to sample people according to their health
status and historically assess their usual source of drinking water. This
design is a case-control sampling strategy. The advantages to this design
are (1) a large series can be identified; (2) the cost, usually, is lower than
a prospective sample; and (3) if data are available on lifetime drinking
water history, it may be possible to identify the etiologic period during
OCR for page 199
Conclusions and Recommendations 199
which these by-products have an effect on human health. To this end,
both the age of exposure and duration, as well as the lag time from exposure
to disease onset, could be investigated. The disadvantages are (1) infor-
mation on the duration and type of disinfectant is difficult to obtain, and
direct measures of chemical levels are not possible unless routine testing
of water has occurred in the past; (2) the study cost can increase dra-
matically if data collection includes personal interviews to learn about
exposure to potential confounders. Despite these advantages, case-control
designs are the most effective in meeting the objective of quantifying the
risk of a particular disinfectant strategy on human health. Incident cases
of disease can be identified from existing registries (e.g., cancer or birth
defects records) or pathology logs.
A third role for epidemiological studies of the health effects of drinking
water disinfectant methods is that these studies can contribute to experi-
mental models of chemical carcinogenesis, atherogenesis, and teratoge-
nesis. As noted above in the list of advantages of case-control studies, it
is possible, although difficult, to identify the relevant etiologic period
when exposure has the greatest impact on disease occurrence. Usual life-
time exposure will dilute the measure of effect, if there truly is an adverse
effect of a particular treatment strategy. In addition, exposure measured
only at the time of disease onset or death may not accurately reflect the
relevant exposures that occurred or began to occur 10, 20, or 30 years
earlier, during childhood or early adulthood.
Finally, a fourth contribution is to describe the effect of disinfectant
by-products in conjunction with other risk factors for disease. For example,
epidemiological studies can evaluate interaction between smoking and
drinking water or concurrent chemical contamination and disinfectant con-
tent on the risk to human health. Epidemiological studies undertaken to
learn about effect modification require stratified analysis or a sampling
approach that selects individuals according to their exposure to a particular
covariate so that the distribution of exposure is balanced for efficient
contrasts of the disease rates according to categories of the covariates.
For example, if the interest were to study the interaction of smoking and
chlorination on bladder cancer risk, then the study population should be
selected expressly in terms of their smoking experience. A 50% split
between current smokers and nonsmokers would provide an efficient ex-
perience to examine the additional risk of chlorination in the presence of
smoking relative to chlorination exposure in the absence of smoking and
no exposure to either smoking or chlorination.
Clearly there are limitations to epidemiological research in clarifying
the risks of environmental exposures to human health. Human studies are
unlikely to contribute knowledge about dose effects because the levels of
exposure to hazardous by-products of chlorination hardly vary. In addition,
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200 DRINKING WATER AND HEALTH
the weaker the effect of a particular chemical on disease occurrence, the
more difficult (in terms of measurement accuracy, cost, and administration
of the study) it is to detect this association in an epidemiological study.
In light of current knowledge, it seems probable that the by-products of
concern incur a relatively low risk on human health, regardless of the
treatment strategy. One would therefore want to be particularly conscious
of locating a study base that is potentially informative about the relation
under study. In principle, the population experience should be quite ho-
mogenous with respect to covariates of the diseases of interest and, in
addition, at low risk for the same diseases so that a small excess risk could
be detected.
Four potential contributions from epidemiological observational studies
have been discussed: (1) descriptive research, primarily aimed at gener-
ating hypotheses; (2) analytic research, aimed at quantifying risk with
proper control for confounding; (3) evidence for improving models of
chemical carcinogenesis, teratogenesis, and atherogenesis; and (4) effect
modification by covariates of disease, such as smoking and occupation.
Results from epidemiological studies that are properly conceptualized and
employ valid methodologies can offer important information for policy-
makers on disinfectant treatment strategies.
Representative terms from entire chapter:
water disinfection
