Environmental epidemiology is a branch of epidemiology concerned with determining how environmental exposures impact human health.
This field seeks to understand how various external risk factors may
predispose to or protect against disease, illness, injury, developmental
abnormalities, or death. These factors may be naturally occurring or
may be introduced into environments where people live, work, and play.
Scope
The World Health Organization
European Centre for Environment and Health (WHO-ECEH) claims that 1.4
million deaths per year in Europe alone are due to avoidable
environmental exposures.
Environmental exposures can be broadly categorized into those that are
proximate (e.g., directly leading to a health condition), including
chemicals, physical agents, and microbiological pathogens, and those that are distal (e.g., indirectly leading to a health condition), such as socioeconomic conditions, climate change, and other broad-scale environmental changes.
Proximate exposures occur through air, food, water, and skin contact.
Distal exposures cause adverse health conditions directly by altering
proximate exposures, and indirectly through changes in ecosystems and
other support systems for human health.
Environmental epidemiology research can inform government policy
change, risk management activities, and development of environmental
standards. Vulnerability
is the summation of all risk and protective factors that ultimately
determine whether an individual or subpopulation experiences adverse
health outcomes when an exposure to an environmental agent occurs.
Sensitivity is an individual’s or subpopulation’s increased
responsiveness, primarily for biological reasons, to that exposure. Biological sensitivity may be related to developmental stage, pre-existing medical conditions, acquired factors, and genetic factors.
Socioeconomic factors also play a critical role in altering
vulnerability and sensitivity to environmentally mediated factors by
increasing the likelihood of exposure to harmful agents, interacting
with biological factors that mediate risk, and/or leading to differences
in the ability to prepare for or cope with exposures or early phases of
illness. Populations living in certain regions may be at increased risk
due to location and the environmental characteristics of a region.
History
Acknowledgement that the environment impacts human health can be found as far back as 460 B.C. in Hippocrates' essay On Airs, Waters, and Places.
In it, he urges physicians to contemplate how factors such as drinking
water can impact the health of their patients. Another famous example of
environment-health interaction is the lead poisoning experienced by the ancient Romans, who used lead in their water pipes and kitchen pottery. Vitruvius, a roman architect, wrote to discourage the use of lead pipes, citing health concerns:
"Water conducted through earthen pipes is more wholesome than that through lead; indeed that conveyed in lead must be injurious, because from it white lead is obtained, and this is said to be injurious to the human system. Hence, if what is generated from it is pernicious, there can be no doubt that itself cannot be a wholesome body. This may be verified by observing the workers in lead, who are of a pallid colour; for in casting lead, the fumes from it fixing on the different members, and daily burning them, destroy the vigour of the blood; water should therefore on no account be conducted in leaden pipes if we are desirous that it should be wholesome. That the flavour of that conveyed in earthen pipes is better, is shewn at our daily meals, for all those whose tables are furnished with silver vessels, nevertheless use those made of earth, from the purity of the flavour being preserved in them"
Generally considered to be the father of epidemiology, John Snow
conducted perhaps the first environmental epidemiology study in 1854.
He showed that London residents who drank sewage-contaminated water were
more likely to develop cholera than those who drank clean water.
U.S. government regulation
Throughout
the 20th century, the United States Government passed legislation and
regulations to address environmental health concerns. A partial list is
below.
Law | Year | Brief Description |
---|---|---|
Federal Food, Drug, and Cosmetic Act | 1938 | Created the U.S. Food and Drug Administration (FDA) |
Federal Insecticide, Fungicide, and Rodenticide Act | 1947 | Pesticide users and manufacturers must register with the EPA; pesticides must not cause unreasonable harm |
Federal Water Pollution Control Act | 1948 1977 | Also known as the Clean Water Act, set water quality standards |
Clean Air Act | 1955,
1977
|
Created National Ambient Air Quality Standards (NAAQS) for environmental and public health protection |
Solid Waste Disposal Act | 1965 | Specified standards for municipal and industrial waste disposal |
Occupational Safety and Health Act | 1970 | Created worker protections standards and established the National Institute for Occupational Safety and Health (NIOSH) |
Toxic Substances Control Act | 1976 | Allowed the EPA to regulate chemicals, including the ability to ban substances that are shown to harm humans |
Comprehensive Environmental Response, Compensation, and Liability Act | 1980 1986 | Also known as Superfund, it taxes chemical and petroleum industries to fund clean-up of hazardous waste sites |
Indoor Radon Abatement Act | 1988 | Funded radon clean-up and research programs |
Food Quality Protection Act | 1996 | Amended the Federal Insecticide, Fungicide, and Rodenticide Act to include a requirement that pesticides have reasonable certainty that they do not cause human harm |
Precautionary principle
The precautionary principle
is a concept in the environmental sciences that if an activity is
suspected to cause harm, we should not wait until sufficient evidence of
that harm is collected to take action.
It has its roots in German environmental policy, and was adopted in
1990 by the participants of the North-Sea Conferences in The Hague by
declaration. In 2000, the European Union began to formally adopt the precautionary principle into its laws as a Communication from the European Commission.
The United States has resisted adoption of this principle, citing
concerns that unfounded science could lead to obligations for expensive
control measures, especially as related to greenhouse gas emissions.
Investigations
Observational studies
Environmental epidemiology studies are most frequently observational in nature, meaning researchers look at people's exposures to environmental factors without intervening and then observe the patterns that emerge. This is due to the fact that it is often unethical or unfeasible to conduct an experimental study of environmental factors in humans. For example, a researcher cannot ask some of their study subjects to smoke cigarettes
to see if they have poorer health outcomes than subjects who are asked
not to smoke. The study types most often employed in environmental
epidemiology are:
Estimating risk
Epidemiologic studies that assess how an environmental exposure and a health outcome may be connected use a variety of biostatistical approaches to attempt to quantify the relationship. Risk assessment
tries to answer questions such as "How does an individual's risk for
disease A change when they are exposed to substance B?," and "How many
excess cases of disease A can we prevent if exposure to substance B is
lowered by X amount?"
Some statistics and approaches used to estimate risk are:
Bradford Hill criteria
To differentiate between correlation and causation,
epidemiologists often apply a set of criteria to determine the
likelihood that an observed relationship between an environmental
exposure and health consequence is truly causal. In 1965, Austin Bradford Hill
devised a set of postulates to help him determine if there was
sufficient evidence to conclude that cigarette smoking causes lung
cancer.
The Bradford Hill criteria are:
- Strength of association
- Consistency of evidence
- Specificity
- Temporality
- Biological gradient
- Plausibility
- Coherence
- Experiment
- Analogy
These criteria are generally considered to be a guide to scientists,
and it is not necessary that all of the criteria be met for a consensus
to be reached.