When reading any information about health risks of drinking water
contaminants, it is important to understand that for many of
these contaminants, it is often difficult to assign an absolute health risk. You
can't just go out and experiment on a group of people
by taking a
few thousand men, women, and children, dividing them into
random groups, dosing them with varying amounts of
potentially harmful
chemicals, and recording the results on their
health. Experiments may be performed on rats and mice, but
then
there is the argument that chemicals which
cause health problems in other animals will not behave the same way in
humans.
As a consequence, much of the information that is known about the health
risks of various drinking water contaminants (as well as
other harmful compounds, disease causing
organisms, and risky behaviors) is determined from what are called epidemiological
studies rather than experimental studies.
Epidemiology is the branch of medical science that studies various factors which
influence the incidence (rate of occurrence), distribution, and dynamics of diseases, injury
or other health-related problems in
populations. Epidemiology can be thought of in terms of who, where, when, what, and why.
That is, who has the infection (or
disease or injury), where are they located geographically and in relation to each other, when is
the infection (or disease or injury)
occurring, what is the cause, and why did it occur.
In epidemiological studies, health and disease data are collected from the health
records and or surveys of two or more different
populations of people over
some period of time. The populations are chosen to be as similar
as possible to each other except for
the risk factor (chemical
exposure, pathogen, behavior, etc.) that is being studied. The group which
does not have the risk factor
is the control population for the
study. The goal is to try and determine what risk factors contribute to
a particular health
outcome, usually a disease, injury,
and/or death.
Examples of possible study populations and outcomes might be:
male smokers 40-70 vs. male non-smokers 40-70 - outcome, lung
cancer;
people in a community that drink hard water vs. people in another
community that drink soft water - outcome, heart disease;
people in a water district that has high levels of disinfection
byproducts vs. those in another with low levels of disinfection
byproducts
- outcome, bladder cancer;
children in a town where fluoride is added to the water vs. children in
a nearby town who drink water containing low fluoride
levels -
outcome, number of cavities per year.
When populations are similar, except for the risk factor being studied,
any observed differences in health, death rate, or other
outcome, will lead researchers to
conclude that there is reason to suspect a possible association (or correlation)
between
the risk factor and the observed
differences in outcome. The larger the difference in outcomes
the greater the possible
association between risk factor and
outcome. A term that is used to describe the results of an
epidemiological study and
the difference between the possible risk
factor and the outcomes measured in the two populations is Relative
Risk.
Relative Risk is the number of outcomes
in the population with the risk factor divided by the number of outcomes
in the
population that does not have the risk
factor. For example, in a study quoted in one of the links below,
in two populations,
similar except that one group smoked and
the other did not, there were 397 lung cancer deaths in the smoker group
compared
with 37 lung cancer deaths in
non-smokers. The relative risk of smoking in this study was 397/37
= 10.73 (that is, smokers were
10.73 times more likely to die from lung
cancer than non-smokers).
It is important to realize, however, that epidemiological studies can never prove
causation; that is, they cannot prove that a
specific risk factor actually causes the disease being studied.
Epidemiological evidence can only show that this risk factor
is associated (correlated) with a higher incidence of disease in the population exposed to that risk factor. The higher the
correlation the more certain the
association, and the more probable it might cause the disease, but it cannot prove the
causation. For instance, one of the
articles listed below indicates a correlation between heavy smoking and liver
cirrhosis.
That correlation, however, is probably
caused by the fact that many heavy smokers are also heavy
drinkers. The heavy
drinking is more probably the cause of
cirrhosis than smoking.
In an epidemiological study which considered
the likely causes of cirrhosis, smoking would be a confounding
variable, or
in the jargon of the mystery genre, a red
herring. It would be important when studying two populations to
determine the possible
effects of heavy drinking on cirrhosis,
that both study groups had a similar percentage of smokers. It
would also be
important to select the groups so other
risk variables like age, gender, economic status, diet, etc. were as
similar as possible
in both groups. Since the two
populations that are being compared are never identical, there are
various statistical methods
that are used to help determine how risk
factors interact with each other and contribute to the observed outcome.
In many epidemiological studies, particularly where it is
difficult to isolate risk factors, difficult to find populations
that are
similar except for the risk factor of
interest, or where there is little relative risk between the two
populations, it may be
challenging to demonstrate a clear
correlation between a specific risk factor and the health problem it is
suspected of
causing. That is the case with many
of the drinking water studies of the health effects of organic compounds
described
below.
Several issues that contribute to
potential problems and limitations interpreting epidemiological drinking
water
studies:
The
low concentrations of the compounds suspected of causing health problems
in most water supplies. If someone
drinks an ounce
of chloroform (one of the disinfection byproducts of adding chlorine to municipal
water) the health effects
will be
sudden, obvious, and deadly. The health effects of drinking
chloroform that is diluted in water to a few parts per
billion are
neither sudden or obvious. Usually nothing obvious happens even
over a lifetime of exposure. For the few
people
who eventually develop cancer because of the increased risk of a
lifetime's exposure to this compound, however,
it can
still be deadly.
The
difficulty in ruling out all other possible risk factors which may
contribute to the observed outcomes. This is well
illustrated
by the ongoing debate about the reported health benefits of drinking
hard water over soft water. The proposed
risk
factors of soft water are a lack of the obvious hardness minerals
(calcium and magnesium) which appear to be correlated
with an
increase in heart disease in many epidemiological
studies, even in areas where dietary sources of calcium and
magnesium
and other essential minerals appear to be adequate. There are also
studies that do not find a health risk of
soft
water. When an apparent increased risk of heart disease is
observed in populations that drink soft water instead of
hard water,
there are no obvious reasons to explain this observation if the body's
needs for calcium, magnesium, and other
essential
minerals are met by the diet. Consequently, there is considerable
speculation and debate about as yet unidentified
or
unrecognized confounding variables which are causing
the reported findings.
Related to the above issue is the difficulty
of finding populations that are quite similar in all characteristics
except the one risk
factor of
interest. The populations which are compared are typically from
different towns or different locations within a town,
so that,
for example, if one population is drinking hard water and the other soft
water, there are probably a number of other
things
about the two groups (the people's lifestyle, their diet, work
environment, etc.) that are also different and must be
accounted
for in the analysis of the results.
The
relative risk that is measured is often very small between the
population that drinks water containing the risk factor
which is
being studied (disinfection byproducts, for example) and the group that
is not drinking the risk factor. Instead of
having a
relative risk of over 10 for smokers vs. non-smokers, the relative risk
of many of the compounds in drinking water
that are
considered to be possible causes of health problems may be in the range
of 1.3 to 2.
Epidemiological drinking water studies are
complicated by the often long period of time
before drinking water containing low
levels of
suspected risk factors begin to have an observable effect on health.
Much of the existing research on long-term health
effects of
water contaminants examines historical health and water quality records
to find patterns of health problems over time
that correlate
with recorded drinking water quality issues over the same period of
time.
The difficulty in determining the actual
amount of exposure of a population to a particular risk
factor. Think about how, if you
were
researching the health effects of disinfection byproducts, you would go
about trying to determine the amount of
disinfection byproducts the members of your study population had
consumed over a 30 to 40 year period
The
different ways an epidemiological study can be set up and conducted
influences the results. The locations and
characteristics of the populations that are selected for the study, the
data the researcher decides to collect, the time period
used for
the study, the historical records and materials that are collected and evaluated,
the methods that are used to analyze
the data, and
yes, even the researcher's expectations and biases, can all have an effect on
the conclusions reached.
Many of the epidemiological studies read like a mystery novel; there is
the killer, a list of suspects, several red herrings, the
suspense that if the killer is not apprehended
more innocents will die, and finally, the steadfast detective collecting
and
analyzing evidence, making deductions,
and relentlessly pursuing his or her quarry. The evidence
collected in an
epidemiological study is always
circumstantial, and there is seldom a 'smoking gun' that leads
conclusively to the culprit.
There are many cases where the evidence
has been sufficiently convincing to overcome political and bureaucratic
inertia
and provide the incentive to regulate or
ban certain contaminants (lead, E. coli, cryptosporidium, mercury,
trihalomethanes,
MTBE, and arsenic to name a few).
There are also many cases where the evidence is not as conclusive, and
the jury is still
out on what the health risk of a chemical
is, what levels of exposure are harmful, and what regulatory actions (if
any) to take.
Even if there
is a fairly clear indication that a contaminant causes a health problem,
there is frequently debate about the levels
at which the contaminant is
harmful. There are, of course, also special interest groups
promoting a particular conclusion about
the seriousness of the health threat
which can influence the decisions that are made about banning or
regulating products and
setting maximum contaminant levels for
drinking water contaminants.
The issues discussed above are presented to help explain why, even in
careful reading of a number of epidemiological studies
on the health effects of a specific
drinking water contaminant, you will frequently find differing results
and perhaps even
contradictory conclusions.
After a number of studies have been conducted, reviewed, and published
on the health effects of a specific contaminant, a
consensus
will usually begin to emerge in the scientific community about the 'actual'
health risks of the contaminant.
The strength
of the consensus is based, in part, on the number of studies published
that report similar results and the perceived
quality of
those studies.
The level of
concern in the scientific community about the contaminant is based, in
part, on the reported relative risks (or odds
ratios -
mentioned below), the severity of the health effect (death vs. skin
rash), the level of exposure in typical water supplies,
and the
length of time it takes someone drinking the water to experience the
harmful effects.
If many studies show a clear correlation of a risk factor with a
disease, and/or if the relative risk (or odds ratio) of disease
from the contaminant is high, and/or if the the disease is severe,
and/or if the onset of disease symptoms is rapid, there will
be a fairly rapid consensus on the harmful health effects of the
contaminant and a call to take regulatory or preventive
action.
-
E. coli in drinking water causing rapid onset of disease and death is an
example of this situation.
There is also now a strong consensus among scientists for concern about
the harmful effects of the disinfection byproducts,
particularly their apparent contribution to the increased risk of
getting several cancers after many years of drinking water
containing these products. The epidemiological evidence for
increased risk is strong and consistent but not nearly as
conclusive as it is for E. coli.
There are, however, many contaminants for which a consensus on their potential
harm has not been reached - often because
few
studies have yet been conducted, study results may be inconsistent, the
perceived health risk may low, the calculated
relative risk (or odds ratio) may not be high
enough to raise alarm, and so forth.
Many proposed negative health
effects of the disinfection byproducts, in addition to increased cancer rates,
for example, fall
into this category. Increased rates
of miscarriage, low birth weight, chromosomal abnormalities, neural tube
defects, and
cardiac defects have all been
attributed within the last few years to drinking water containing
various levels of disinfection
byproducts.
In conclusion, the jury is still out on whether many of the other proposed risks
of the disinfection byproducts - or the risks of many
hundreds of other compounds that can be found in
drinking water - need to be taken seriously by the
scientific community, the
government officials in charge of regulating
drinking water supplies, the drinking water professionals whose job it is to provide
safe water to their customers,
and the public that consumes the water. The decision on whether to
fund epidemiological studies
to uncover harmful drinking
water contaminants or whether the costs of regulation or cleanup of
contaminants (even those identified
as posing some risk) is based, in part on another
branch of the sciences, Risk Assessment and Management, but that's a
story
for another time.
A final thought and story. Dr. John Snow (1813-1858)
is often recognized as the first modern epidemiologist. During the
1854
Cholera epidemic in London, the city's
water was supplied by two water companies. One drew its water out of the Thames River
upstream from the main city while the
other, Southwark and Vauxhall, drew its water from the lower Thames,
downstream from
the city, where it had become contaminated with sewage.
Dr. Snow, during his investigation of the epidemic, plotted
on a city
map some 500 Cholera deaths that occurred
within a 10 day period. He found that most of the deaths were
within a 200 yard
radius of a pump located at the intersection of Cambridge and Broad
Streets. After he persuaded officials to remove the handle
of the Broad Street Pump that supplied the water to
that neighborhood, the number of cholera cases and deaths in London was
dramatically reduced. At the time
the actual infectious agent, the bacterium, Vibro cholerae, was
unknown.
Abstracts of journal articles covering: Disinfection Byproducts
and Cancer, Disinfection Byproducts and Pregnancy,
and Calcium & Magnesium in Drinking Water
If you do read these abstracts, you will find that comparisons and
results are frequently reported as Odds Ratios (OR) instead
of relative risk.
Although an epidemiologist or statistician will violently disagree, you
can more-or-less interpret an odds ratio in
the same way you would
think of relative risk. A study result might be reported as "Those
exposed to chlorinated surface water
for 35 or more years
had an increased risk of bladder cancer compared with those exposed for
less than 10 years (OR = 1.41,
95 percent confidence
interval [CI] = 1.10-1.81)." An odds ratio above 1.0
means the risk factor had an increased risk on the
study outcome - the
higher the OR value the greater the risk, and the confidence interval
typically reported with an odds ratio (or
relative risk)
indicates whether the risk is statistically significant or not.
More information on odds
ratios vs. relative risk.
Some
resources: Epidemiology
- Trying to Establish Cause, Epidemiology,
Interpreting Health
Reports
A good, one-page
description of Epidemiology
A remarkable series of on-line college-level
lectures on epidemiology
Toxicologic Epidemiology
by Dr. Michael H. Dong
Principles of Epidemiology by Dona Schneider, PhD, MPH, FACE
A Brief Introduction to Epidemiology by Betty C. Jung RN MPH CHES
Principles of Epidemiology by Kevin E. Kip, Ph.D
Many have seen the movie, Erin Brockovich, the true story of Ms.
Brockovich, a legal assistant, who in 1993 lined up some 650
prospective plaintiffs from the tiny desert town of Hinkley, Calif., to sue Pacific Gas & Electric.
PG&E's nearby plant was leaching
chromium 6, a rust inhibitor, into Hinkley's water supply, and the suit blamed the chemical for dozens of symptoms, ranging from
nosebleeds to breast cancer, Hodgkin's disease, miscarriages and spinal deterioration. In 1996 PG&E settled the case for $333
million. If you read this
article (and the associated links, and another
report), you will get a 'picture' of some of the complexity of
trying to figure out the cause of specific
diseases and the effects of suspected harmful chemicals. The
epidemiological and
scientific studies help to provide the
understanding of the "actual" causes and effects in disease
processes. They are just one
piece of the overall puzzle of how the public
perceives the problem, however.
Drinking
Water Disinfection Byproducts: Review and Approach to Toxicity
Evaluation - There is widespread potential for human
exposure to disinfection byproducts (DBPs)
in drinking water because everyone drinks, bathes, cooks, and cleans
with water. The
need for clean and safe water led the
U.S. Congress to pass the Safe Drinking Water Act more than 20 years ago
in 1974. In 1976,
chloroform, a trihalomethane (THM) and a
principal DBP, was shown to be carcinogenic in rodents. This prompted
the U.S.
Environmental Protection Agency (U.S.
EPA) in 1979 to develop a drinking water rule that would provide
guidance on the levels of
THMs allowed in drinking water. Further
concern was raised by epidemiology studies suggesting a weak association
between the
consumption of chlorinated drinking water
and the occurrence of bladder, colon, and rectal cancer...
{The article goes on to describe
disinfection products of various drinking water treatment methods and
the "balancing act" described above between providing
safe water and creating treatment
byproducts that have some health risk.}
How
are the risks of water contaminants determined?