|
Items in cart [0] |
Questions? Call 888-624-8373 |
PAPERBACK PDF BOOK Free Resources |
||||||||||||||||
|
||||||||||||||||
|
|
|||||||||||||||
| ||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
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 9
l
Approach to the Study
INTRODUCTION
In this chapter the general approach, principles, and criteria adopted in
the study are discussed in outline. Considerations that entered into
evaluations of the ejects on health of the various contaminants of
drinking water are described, together with the reasons for selecting the
subjects that were studied. The findings of the study are not summarized
comprehensively in this section; each succeeding chapter includes a
summary of the relevant conclusions and recommendations. A short
summary of the principal conclusions of the study is given in Appendix
C.
The study was undertaken by the NAS-NRC to meet the needs
expressed in the Safe Drinking Water Act (PL 93-523), which requires the
Environmental Protection Agency to promulgate national drinking water
standards and, for the first time, regulations for enforcing them. The Act
also directs the Administrator of the Environmental Protection Agency
to arrange with the National Academy of Sciences, or other appropriate
organization, to study the adverse ejects on health attributable to
contaminants in drinking water. Although the high quality of drinking
water in the United States is recognized throughout the world, the law is
an expression by the Congress of the concern of many citizens about
maintaining the quality of public water supplies in this country.
The reader should not equate the size of this report or that of any of its
chapters with the Committee's assessment of the magnitude of the
challenge to public health that may be due to the presence of particular
9
OCR for page 10
10 DRINKING WATER AND HEALTH
constituents in drinking water in the United States. Several factors have
contributed to the length of this report: The Safe Drinking Water Act
defined the scope of the study in encyclopedic terms and consequently
the length of some of the chapters reflects the large number of topics and
substances that it was necessary to consider. Other chapters deal with
subjects that are complex and about which there are uncertainties,
conflicting opinions, and inconclusive or incomplete data. The relevant
studies, assumptions, methodologies, health elects, and research recom-
mendations for each group of constituents required detailed consider-
ation from several points of view before balanced judgments could be
achieved. In some cases brevity had to be sacrificed to reach this
objective within a reasonable time.
The primary purpose of the study was to assess the significance of the
adverse elects that the constituents of drinking water may have on public
health. The economic or technological feasibility of controlling the
concentration of these constituents was outside the scope of the study.
The health elects associated with some methods of disinfection were
noted, but the relative effectiveness and potential hazards associated with
the various methods of water disinfection were not evaluated.
Application of analytical methods of great sensitivity has, in recent
years, expanded our knowledge of the occurrence and diversity of
impurities in drinking water. However, information about the biological
results of chronic ingestion, at low dose rates, of most of these substances
is acquired slowly because the bioassays that are usually required may
take two or more years to complete. Although new approaches to the
problem of estimating chronic adverse health elects may, in the future,
ease this difficulty, the current knowledge on which this study is based is
insufficient to assess all the contaminants of drinking water. The results
reported here must therefore be considered as a contribution to an effort
that should be continued.
Besides the known constituents of drinking water, some were also
considered that it would be plausible to expect to be present, even though
they have not yet been detected in water. (Certain pesticides used in large
quantities fall into this category.)
In our review of water constituents, we have attempted to take into
account not only their identities, concentrations, and toxicities, but also
to consider other questions, such as:
1. What reason is there for concern about the material? What risks are
associated with its presence in water?
2. How does the material get into water?
3. What sources are there other than water?
OCR for page 11
Approach to the Study 11
4. What contaminants need to be controlled?
5. Are there special places or persons at higher than average risk?
6. Are there essential nutritional requirements for this material?
7. In view of the data at hand, can one say that this is a material that
causes temporary ill effects? Permanent ill-e~ects? Reversible ejects?
8. In view of these effects and their reversibility (or lack of it) is it
possible to set "no-observed-adverse-health-effects" levels?
9. For materials with special health benefits, what concentrations will
maximize these benefits, while keeping the health risk associated with
them at an acceptably low level?
10. What additional information is required to resolve the outstanding
problems?
Many of the constituents of drinking water are natural materials, and
enter water from the rocks and the soil and the air. Some are the natural
waste products of men or animals. Others are artificial or synthetic
materials, made and used for special purposes, that inadvertently find
their way into water. Yet others occur naturally, but have become more
widely distributed in populated areas as a result of industrial and
agricultural activity.
WATER CONSUMPTION
In this study, a quantity of 2 liters per day has been taken to be the
average amount of water consumed per person. This is also the amount
used by EPA to calculate the current interim standards. Daily consump-
tion of water is a function of temperature, humidity, physical activity, and
other factors that vary widely. The average per capita water (liquid)
consumption per day as calculated from a survey of nine different
literature sources was 1.63 liters (NAS, 1974; McNall and Schlegal, 1968;
Wolf, 1958; Guyton, 1968; Evans, 1941; Bourne and Kidder, 1953;
Walker et al., 1957; Randall, 1973; Pike and Brown, 1975~. However, the
larger volume of 2 liters/day was adopted as representing the intake of
the majority of water consumers. We estimate that most-of those who
consume more than 2 liters per day still are afforded adequate protection,
because the margin of safety estimated for the contaminants is sufficient
to offset excess water consumption. Nevertheless, consideratioin should
be given to establishing some standards on a regional or occupational
basis, to take extremes of water consumption into account.
~, .
J
OCR for page 12
12 DRINKING WATER AND HEALTH
RISK AND SAFETY
The hazards of ingesting chemical pollutants in drinking water have been
assessed in two general ways: with laboratory toxicity studies and
epidemiological studies. The aim of studies of both types is to provide
information about the risk to man. Risk constitutes only half of the
equation; the other half is benefit to the exposed population. It is not
possible to guarantee a risk-free society. The scientific methods and
criteria we have used for evaluating long-term ejects and risks in man are
described in Chapter II, "Chemical Contaminants: Safety and Risk
Assessment" and in the chapters concerning each group of contaminants.
Most of the experimental results on which the current knowledge of
toxicity rests are based on observed effects on man and animals of doses
and dose rates that are much larger than those that correspond to the
usual concentrations of harmful materials in drinking water. There is,
consequently, great uncertainty in estimating the magnitude of the risk to
health that ingestion of contaminants in water ~ may produce. An
Lou; iiQllal p1 Ill in tO LAKE ill LO account ine combined ejects of two or
more contaminants.
The theoretical and experimental bases for extrapolating estimations of
risk to low levels of dose have been reviewed, and some principles are
proposed to guide the conduct of this and similar studies.
~ _ . . ~ ~ . . . . ~ . . ..
MICROBIOLOGICAL CONTAMINANTS
Outbreaks of waterborne disease are reported to the National Center for
Disease Control (CDC) by state health departments. In addition, EPA
obtains information about additional outbreaks from state water-supply
agencies. Both CDC and EPA are aware that data on waterborne
outbreaks have limitations and must be interpreted with caution. The
data collected represent only a small part of a larger public health
problem. The number and kind of reported outbreaks and of some
etiologies may depend upon the interest or capabilities of a particular
state health department or individual. They do not reflect the actual
number of outbreaks, cases, or etiologies of disease associated with
drinking water.
Many small outbreaks are not reported to state health departments.
There is no law or regulation requiring state authorities to report all
gastroenteritis cases to CDC. In 1975, CDC reported 24 waterborne
disease outbreaks involving 10,879 cases. No etiologic agent was found
for the two largest outbreaks (Sewickley, Pa., 5,000 cases and Sellersburg,
OCR for page 13
Approach to the Study 13
Ind., 1,400 cases). In fact, no etiologic agent was identified in 17 of the 24
outbreaks. These 17 outbreaks accounted for 9,760 cases, or 89%, of the
total reported in 1975.
Conclusions in the microbiology chapter, based on epidemiological
data, are subject to the limitations of the reporting system and to our
limited ability to identify etiologic agents in outbreaks known to be
associated with drinking water.
The microbiological contaminants selected for consideration in this
report are those for which there is epidemiological or clinical evidence of
transmission by drinking water. They include a variety of bacteria,
viruses, and protozoa. Methods of detecting these contaminants of
drinking water were reviewed, and the quantitative relationships between
dose levels and infectivity were examined. Because current drinking
water standards place major emphasis on detection of microbiological
contaminants, attention was devoted to the validity and health sig-
nificance of microbiological standards.
PARTICULATE CONTAMINANTS
Finely divided solid particles of mineral and organic composition are
commonly found suspended in some drinking water, particularly those
supplies that do not practice coagulation and filtration. To discover
whether or not the long-term ingestion of these materials in water is likely
to produce adverse ejects on human health, their occurrence, composi-
tion, and properties were reviewed.
This review indicated that many kinds of particulate matter may
indirectly, through adsorption, facilitate the transport of toxic substances
and pathogenic organisms and affect the efficiency of disinfection.
Particles of organic composition also may indirectly give rise to
chlorinated compounds by reaction with chlorine in water treatment.
Only in the case of particles derived from asbestos minerals, however,
are there grounds for suspecting that direct ejects on human health could
be involved. Fibrous particles of asbestos minerals are known to be
associated with increased incidence of cancer, including gastrointestinal
cancer, among workers who inhale asbestos-laden air. Experiments on
the inhalation of asbestos mineral fibers by animals have-also demon-
strated a carcinogenic eject. The particulate matter in drinking water
often includes similar particles.
Although epidemiological studies have not indicated an increase with
time in cancer death rates that can be ascribed to fibrous contamination
of the drinking water, these negative findings do not exclude the
OCR for page 14
14 DRINKING WATER AND HEALTH
possibility that such an increase may be detected in the future, because
many cancers have long induction periods.
For these and other reasons, detailed elsewhere, it is believed to be
important that research on the analysis of fibrous mineral particles in
water, and on the toxicity of these materials when ingested, should be
strongly pursued.
INORGANIC SOLUTES
The Interim Primary Drinking Water Regulations list maximum allow-
able concentrations for six metallic elements barium, cadmium, chro-
mium, lead, mercury, and silver. Ten additional metals were reviewed in
this study beryllium, cobalt, copper, magnesium, manganese, molybde-
num, nickel, tin, vanadium, and zinc. Sodium, which is also a metal, was
considered separately, because the problems it poses are quite distinct
from those associated with the other metallic substances. In addition, the
ejects on health of several other inorganic constituents of drinking water
were studied. These include arsenic, selenium, fluoride, nitrate, and
sulfate. The relationship between water hardness and health also received
attention.
The sources of inorganic ions in groundwater, surface water, water-
treatment chemicals, and from the storage and distribution system are
considered along with the health ejects resulting from the total intake
from food, air, and water.
ORGANIC SOLUTES
Of the 298 volatile organic compounds so far identified in drinking water,
74 were selected for detailed study along with 55 pesticides. A compound
was selected for consideration if any of the following criteria applied:
Experimental evidence of toxicity in man or animals, including
carcinogenicity, mutagenicity, and teratogenicity.
2. Identified in drinking water at relatively high concentration.
3. Molecular structure closely related to that of another compound of
known toxicity.
4. Pesticide in heavy use; potential contaminant of drinking water
supplies.
5. Listed in the Safe Drinking Water Act or National Interim Primary
Drinking Water Regulations.
OCR for page 15
Approach to the Study 15
The evaluation of toxicity was based on a critical review of the
scientific literature. The available data were of variable quality and
quantity and, in some instances, inadequate for proper assessment of
toxicity. In those cases where sufficient data were available, professional
judgment was used to determine which compounds are carcinogenic,
mutagenic, teratogenic, and noncarcinogenic.
The limitations that are inherent in the extrapolation of high-dose
animal bioassay data to low-dose human exposure and the difficulty of
making predictions for species that may have different metabolic rates
and pathways for handling carcinogens, or different target-organ
responses, are well known. Such risk assessment and extrapolation
procedures are further compromised by the limited information that is
available concerning the mechanisms by which these agents act (such as
initiators, promoters, and modifiers) and the almost total lack of data
regarding the potential synergistic and antagonistic interactions of these
agents with each other and with other environmental agents. The risk of
ingesting known or suspected carcinogens was estimated by the methods
described in Chapter II. These methods are based on an assumption that
there is no threshold in the dose-response relationship. The risk-estimate
approach may provide unique advantages for other areas of toxicologic
evaluation.
The more traditional approach of combining the maximum no-
observed-adverse-effect level with an uncertainty (safety) factor to
calculate an acceptable daily intake (ADI) was used for agents that were
not considered to be known or suspected carcinogens and for which there
was adequate toxicity data from prolonged ingestion studies in man or
animals. Several alternative terms other than ADI were considered, but it
was concluded that the introduction of new terms might lead to
confusion and that the use of a widely recognized and generally
acceptable term would be preferable for this report. Although the ADI
has been used previously as an internationally established standard for
the toxicological evaluation of food additives and contaminants, the
concept is applicable to other cases of exposure by ingestion. The ADI is
an empirically derived value that reflects a particular combination of
knowledge and uncertainty concerning the relative safety of a chemical.
The uncertainty factors used to calculate ADI values in this report
represent the level of confidence that can be justified on the basis of the
animal and human toxicity data. ADI values were not calculated for
agents where the data were considered to be inadequate.
Since the calculation of the ADI values is based on the total amount of
OCR for page 16
16 DRINKING WATER AND HEALTH
a chemical ingested, the ADI values calculated in this report do not
represent the safe level for drinking water.
Little or no data are available on the toxicity of many organic
compounds identified in drinking water. There is a need to determine
which of these compounds should be subjected to extensive toxicity
testing. Some of the criteria used for developing the order in which
compounds should be tested are:
1. The relative concentrations of the compounds and the number of
people likely to be exposed.
2. The number of supplies in which they occur.
3. Positive responses in in vitro mutagen screening systems.
4. Positive responses in in vitro prescreening systems for potential
carcinogens (mammalian cell transformations).
5. Similarity of the chemical structure of the test compound with that
of other compounds having defined toxic properties (i.e., structure-
activity relationships).
6. Relationships of dose from water to total body burden.
RADIOACTIVE CONTAMINANTS
Because the presence of ionizing radiation is one of the standard features
of the earth's surface, the adverse effects on health that may be ascribed
to radioactive contaminants of drinking water were assessed in relation to
the average background radiation dose, from all sources, of 100 mrem per
year.
Previous estimations of the biological effects of the background
radiation on human health were reviewed in the light of more recent
scientific knowledge and used to calculate the magnitude of three kinds of
adverse health ejects that radiation can produce; namely, developmental
and teratogenic ejects on the fetus, genetic disease, and somatic
(principally carcinogenic) ejects.
When these estimates are related to the concentrations of radionuclides
that are commonly found in drinking water, it is seen that consumption of
2 liters of water per day contributes such a small fraction to the total
radiation background that the incidence of developmental, teratogenic,
and genetic disorders is not increased enough for the change to be
detectable.
Where somatic ejects are concerned, it is estimated that the radionu-
clides in drinking water typically account for less than 1% of the
incidence of cancers that may be attributed to the natural background of
OCR for page 17
Approach to the Study 17
radiation. Only certain bone-seeking radionuclides (chiefly radium), in a
few regions, reach concentrations in drinking water that are high enough
to cause a significant increase in the incidence of bone cancer.
SUSCEPTIBLE SUBGROUPS AND OTHER CONSIDERATIONS
Groups that are more susceptible than the normal population are
considered in the chapters on various classes of contaminants.
This report is concerned only with water used for drinking. Although
all contaminants may cause problems when present in water used in
health care facilities, the health hazards associated with such diverse uses
of water as in humidifiers, kidney dialysis units, laundries, heating and
cooling equipment, or many special uses that require further treatment of
tap water, have not been considered. References and summaries of the
scientific literature in this field have been published by DeRoos et al.
(19741.
REFERENCES
Bourne, G.H., and G.W. Kidder, eds. 1953. Biochemistry and Physiology of Nutrition, vol.
1. Academic Press, New York.
DeRoos, R.L., V.R. Oviatt, A.G. DuChene, and N.J. Vick. 1974. Water use in biomedical
research and health care failities-A presentation of article summaries. National
Institutes of Health, Department of Health, Education, and Welfare, Contract no. NIH-
ORS-72-2 1 1 1.
Evans, C.L. ed. Starling's Principles of Human Physiology, 8th ed. Lea and Febiger,
Philadelphia.
Federal Register, Wednesday, December 24, 1975, vol. 40, no. 248.
Guyton, A.C. 1968. Textbook of Medical Physiology, ad ed. W.B. Saunders Co.,
Philadelphia.
McNall, P.E., and J.C. Schlegel. 1968. Practical thermal environmental limits for young
adult males working in hot, humid environments. ASlIRAE Transactions 74:225-235.
National Academy of Sciences-National Research Council. 1974. Recommended Dietary
Allowances, 8th ed. Washington, D.C.
Pike, R.L., and M. Brown. 1975. Minerals and Water in Nutrition-An Integrated
Approach. 2d Ed. John Wiley, New York.
Randall, H.T. 1973. Water, electrolytes and acid base balance. In R.S. Goodhart and M.E.
Shils, eds. Modern Nutrition in Health and Disease. Lea and Febiger, Philadelphia.
Walker, B.S., W.C. Boyd, and I. Asimov. 1957. Biochemistry and Human Metabolism, 2d
ed. Williams & Wilkins Co., Baltimore.
Wolf, A.V. 1958. Body water. Sci. Am. 99:12S.
OCR for page 18
Representative terms from entire chapter:
state health
