Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
E· ~ xecullve Summary EPIDEMIOLOGY A review of 12 epidemiological studies failed either to support or to refute the results of positive animal bioassays suggesting that certain trihalomethanes (THM's), e.g., chloroform, may cause cancer in humans. Any association between THM's and bladder cancer "was small and had a large margin of error"-not only because of statistical variances but also, much more importantly, because of the very nature of the studies. All of the epidemiological studies were handicapped by the extreme difficulty of identifying a very small effect in a population. The methodological complexities inherent in epidemiological studies of human populations exposed to multiple contaminants at low concentra- tions (ppb) in drinking water make it virtually impossible to establish a causal link between THM's and an increase in cancer of the bladder or of any other site. Small differences in cigarette consumption between two population groups could account for the observed associations. Any causal relationships between THM's and bladder cancer are weakened by imprecise exposure data. In most of the studies, THM concentrations in different water sources were only inferred, rather than actually measured. In addition there are difficulties in controlling for a multitude of factors that are known to affect cancer incidence: cigarette smoking, diet, occupation, use of alcohol and drugs, socioeconomic status, ethnicity, and nonaqueous sources of THM's. Data regarding drinking water hardness and cardiovascular disease 1
OCR for page 2
2 DRINKING WATER AND H"LTH (CVD) were also examined. Current knowledge is derived largely from "ecological" epidemiological studies, in which individual exposures or risk factors have generally not been considered. In general, when studies encompass large geographical areas, hard water is correlated with low cardiovascular disease rates This correlation breaks down when smaller areas are considered or when the studied populations are grouped by altitude or the proximity of a seacoast. Some noncardiovascular ailments are also associated occasionally with soft water areas, raising the possibility that soft water may merely be a concomitant of some more basic risk factor. Given the current status of knowledge regarding water hardness and the incidence of cardiovascular disease, it is not appropriate at this time to recommend a national policy to modify the hardness or softness of public water supplies. The data do not indicate clearly which (if any) additions to soft water would benefit human health. RISK ESTIMATION Chapter III, Problems of Risk Estimation, examines the prediction of risks to human health using acute and chronic toxicity data from laboratory animals. The concentration of most potentially toxic chemicals in drinking water is usually so low that it is difficult to predict potentially adverse ejects from drinking the water. In cases of noncarcinogenic toxicity, the preferred procedure would be to make a risk estimate based on extrapolation to low dose levels from experimental curves obtained from much larger doses for which ejects can be readily measured. In most instances, such data are not available, and the acceptable daily intake (ADI) approach should be used until better data are obtained. In the ADI approach, "safety factors" based on the quality of the data are applied to the highest no-observable-e~ect dose found in animal studies. The Subcommittee on Risk Assessment believes that the ADI approach is not applicable to carcinogenic toxicity and that high dose to low dose extrapolation methods should be used for known or suspected carcinogens. Six models were evaluated for low dose carcinogenic risk estimation. They were the dichotomous response model; linear, no- threshold model; tolerance distribution model; logistic model; "hitness" model; and time-to-tumor-occurrence model. Because of the uncertain ties involved in the true shapes of the dose-response curves that are used for extrapolation, a multistage model was judged to be the most useful. Such a model has more biological meaning than other models, e.g., the
OCR for page 3
Executive Summary 3 probit or logistic model. Moreover, it tends to be conservative in that at low doses it will give higher estimates of the unknown risk than will many others. More confidence would be placed in mathematical models for extrapolation if they incorporated biological characteristics such as pharmacokinetic data and time-to-occurrence of tumors. Until such data are available, the extrapolation from animals to humans should be done on the basis of surface area. Epidemiological risk assessment is also evaluated in this report. The subcommittee considered descriptive and analytic (prospective cohort studies and case-control studies) epidemiological approaches. TOXICOLOGY Chapter IV, Toxicity of Selected Drinking Water Contaminants, is an evaluation of the health effects associated with the products of water disinfection that were identified by the Subcommittee on Chemistry of Disinfectants and Products. A second group of compounds evaluated was selected for one or more of the following reasons: They were judged to be of concern because of potential spill situations. · They have been identified in drinking water subsequent to the 1977 report (300 volatile organic compounds were known then; 700 are known at present). · Newly available data justify reexamination of several chemicals dealt with in 1977 in Drinking Water and Health (:National Academy of Sciences, 19771. In addition to providing information on chronic toxicity, the subcom- mittee evaluated the potential acute toxicity insofar as justified by the available data. These data provide a basis for making judgments of possible health effects resulting from accidental spills of chemicals into drinking water. To this end, the subcommittee provided a suggested no- adverse-response level (SNARL) for acute exposures of 24 hr and 7 days as well as for chronic exposures to single compounds. The safety (uncertainty) factor used in the calculations reflects the degree of confidence in the data. No estimates were given for mixtures of contaminants because of unknown interactions. The information provided on each compound includes its metabolism, health effects (acute and chronic) on humans and other species,
OCR for page 4
4 DRINKING WATER AND H"LTH mutagenicity, estimate. NUTRITION carcinogenicity, teratogenicity, and carcinogenic risk The Subcommittee on Nutrition has examined the contribution of selected inorganic elements in drinking water to the optimal nutrition of human populations. It studied the benefits of the presence of selected inorganic elements in water and their adverse effects in those cases in which symptoms of both deficiency and toxicity are known. Previous studies by the Safe Drinking Water Committee are limited to an evaluation of the adverse ejects. Recommended dietary allowances (RDA's) for various nutrients are discussed in the report. Estimates are made of adequate and safe intakes of nutrients for which a RDA has not been established. The contribution that drinking water can make to the requirement for these nutrients is calculated. At their typical levels in drinking water, the nutrients reviewed in this report usually make a small, but by no means negligible, contribution to the mineral nutrition of humans. When the intake of a particular nutrient by the general population or a particular group is marginal, the contribution by water may be important in preventing deficiency and ill health. This may be the case for magnesium, fluoride, iron, copper, zinc, vanadium, and chromium. For the overwhelming majority of the nutrients studied, the risk of toxicity to normal individuals from typical levels in drinking water is negligible. When the level of a nutrient in drinking water is typical and reduction of total intake is prudent (e.g., for iodine or sodium), reduction from sources other than water are likely to be the option by which the largest initial reduction can be made. The accurate assessment of the contribution of drinking water to nutrition is hampered by a lack of information on the speciation and bioavailability of elements. Additionally, water treatment practices, e.g., the addition of phosphates for corrosion control, as well as the use of chelators in food preparation, may alter nutrient composition of water. REFERENCE National Academy of Sciences. 1977. Drinking Water and Health. Safe Drinking Water Committee, National Academy of Sciences, Washington, D.C. 939 pp.
Representative terms from entire chapter: