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1
Executive Summary
New knowledge about disinfection and disinfectant by-products has led
to changes in procedures for disinfecting drinking water in the 6 years
since the Safe Drinking Water Committee last reviewed the potential health
effects of these practices (Volumes 2 and 3 of Drinking Water and Health).
This report, prepared by the Safe Drinking Water Committee's Subcom-
mittee on Disinfectants and Disinfectant By-Products, examines these
innovations and assesses their implications for human health.
The predominant method of drinking water disinfection practiced in the
United States today is chlorination. Studies of the toxicity of the by-
products of disinfectants have focused on the trihalomethanes (THMs),
which are formed during chlorination and for which considerable data on
carcinogenicity have been developed. The level of total THMs in finished
drinking water, currently regulated at 100 micrograms (fig) per liter,
should be reduced. Noting that chloroform is the principal THM produced
by chlorination, the subcommittee found this level to be unsupportable
on the basis of the risk values for chloroform developed in this review.
Other, nonvolatile by-products of chlorination may be important in
contributing mutagenic properties to drinking water, especially when the
natural water being treated contains high levels of organic matter. Short-
term animal skin tests, although not conclusive, provide indications 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. The subcommittee has developed risk
assessments and recommended SNARLs for some of these by-products
based on the available data.
1
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2 DR! N K! NO WATER AND H EALTH
Unfortunately, many by-products of chlorination and other disinfection
practices have not been identified. Consequently, the risks of ingesting
these by-products cannot be quantified at present, but are potentially high
enough to warrant continued efforts to analyze them. Further studies of
reaction mechanisms, controlling factors, and by-product identification
are needed. Improved methods for characterizing the nonvolatile products
are also needed to support such studies. Methods should be sought to
follow the risk associated with multiple chlorination by-products even in
the absence of individually quantifiable compound risks. The fact that
THM levels may also indicate the presence of unidentified by-products of
chlorination is further reason to reduce the total THMs in finished drinking
water whenever possible.
The use of alternative methods of drinking water disinfection is in-
creasing, largely due to health and regulatory concerns about trihalometh-
anes. Thus, the nature and toxicity of the by-products of some other widely
used water treatments (chloramination, ozonation, and chlorine dioxide)
are also evaluated in the report to the extent allowed by available data.
Research is also needed to improve understanding of their relative efficacy
in eliminating the currently most resistant viruses and protozoan cysts and
the major factors affecting such efficacy under treatment plant operating
conditions. To prevent overestimation of the degree of disinfection achieved
by alternative practices (especially chloramination, which is becoming
widely used), methods must be developed for fully quantifying both or-
ganic nitrogen precursors of toxic by-products and an organic chloramine
fraction in the presence of inorganic monochloramine. Recognizing the
paucity of information on these alternative practices, the subcommittee
urges that the direct and indirect implications of their potential widespread
use be investigated more thoroughly.
A major health concern is the chronic ingestion of low levels of dis-
infection by-products. In some epidemiological studies of the effects of
chlorination, investigators have found increased rates of bladder cancer
associated with trends in the levels of certain contaminants in water sup-
plies. Interpretation of these studies is hampered by a lack of control for
confounding variables (e.g., age, sex, individual health, smoking history,
other exposures). Nevertheless, the subcommittee recommended that ep-
idemiologists continue to improve protocols and conduct such studies,
particularly of drinking water and bladder cancer, wherever exposure data
can be obtained directly from individuals rather than by estimation from
exposure models.
Humans may also be exposed to disinfectants and their by-products
from sources other than drinking water and routes other than ingestion.
For example, cooking, showers, bathing, swimming, and other activities
could provide additional toxic exposures through inhalation or skin ab
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Executive Summary 3
sorption. Given the absence of data on these noningestion routes of ex-
posure and the lack of methods for estimating the magnitude of such
exposures, the subcommittee declined to include them in its risk estimates.
Recognizing their potential for producing toxic effects, however, it rec-
ommends that methods be developed for estimating both noningestion
exposures to contaminants in drinking water and exposures to the same
contaminants in other media, such as food, air, and dust, so that total
exposure can be considered by regulators in setting acceptable levels of
contaminants in water.
The subcommittee calculated quantitative risk assessment for disinfec-
tants or their by-products when there were sufficient data. These assess-
ments include four distinct components: hazard identification, exposure
assessment, dose-response assessment, and characterization of human risk
at projected levels and patterns of exposure. The first six volumes of this
series provided such assessments for approximately 220 drinking water
contaminants. Some of those were disinfectants that are reevaluated in
this volume along with other compounds not reviewed previously.
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Representative terms from entire chapter:
finished drinking
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