Abstract
Environmental health is often considered the oldest public health practice; yet, in many respects, particularly in relation to chemical exposure and impact, we know very little about the span of consequences. According to the World Health Organization, environmental hazards are responsible for 23% of the total burden of disease worldwide. Environmental and occupational diseases frequently manifest as common medical problems or have nonspecific symptoms; yet consideration of environmental factors rarely enters into the clinician’s history taking or diagnosis.
‘In its broadest definition, environmental health encompasses chemical, biological, physical, social, and psychosocial factors that affect health.’ If you want to learn about the health of a population, look at the air they breathe, the water they drink, and the places where they live.
Environmental health is often considered the oldest public health practice; yet in many respects, particularly in relation to chemical exposure and impact, we know very little about the span of consequences. In its broadest definition, environmental health encompasses chemical, biological, physical, social, and psychosocial factors that affect health. Water, sanitation, hygiene, smoking, respiratory infections, birth defects, cancer, and so on, are illustrative of the range of topics captured in environmental health. Environmental hazards are identified as any source of danger and thus run the gamut from trip/fall hazards to chemicals to feces-contaminated drinking water. According to the World Health Organization, environmental hazards are responsible for 23% of the total burden of disease worldwide. As many as 13 million deaths can be prevented every year by addressing environmental problems such as air and water pollution and through public health measures such as access to clean water, sanitation and hygiene, and the use of cleaner fuels. 1
Disease Burden due to Environmental Factors
In 1981, Doll and Peto 2 published a study on the estimated percentage of avoidable cancer deaths attributable to different risk factors, in the United States. This represented one of the earliest attempts to quantify the relationship between risk factors and health outcomes. Similarly, in 1996, Murray and Lopez 3 published the first global efforts to evaluate premature mortality and disability from a large number of diseases and injuries due to population exposures.
These studies identified environmental conditions having significant direct and indirect impacts on human health. According to a 2004 World Health Organization report, exposure to environmental risk factors was partially responsible for 85 of the 102 reported major diseases. 4 Addressing these environmental risk factors could potentially result in 40% fewer deaths from malaria, 41% fewer deaths due to lower respiratory infections, and 94% fewer deaths from diarrheal disease. 5 In the United States, injuries (a topic area frequently found within environmental health) are the leading cause of death among Americans aged 1 to 44 years. 6
In children younger than 5 years, one third of all disease is caused by environmental factors such as unsafe water and air pollution. 7 Some 2.5 million people die every year from cardiovascular disease attributable to environmental factors, including work-related stress and chemical, air pollution, and environmental tobacco smoke exposures. 4 Globally, more than 1.5 million deaths annually from respiratory infections are attributable to the environment, including at least 42% of lower respiratory infections and 24% of upper respiratory infections in developing countries. 8
In total, 4.9 million deaths (8.3% of total) and 86 million disability-adjusted life years (5.7% of total) were attributable to environmental exposure and management of selected chemicals in 2004. The largest contributors include indoor smoke from solid fuel use, outdoor air pollution, and secondhand smoke, with 2.0, 1.2, and 0.6 million deaths, respectively, annually. These are followed by occupational particulates, chemicals involved in acute poisonings, and pesticides involved in self-poisonings, with 375 000, 240 000, and 186 000 annual deaths, respectively. 9
Risk Assessment
Recognizing illnesses associated with environmental chemical exposures is often difficult. With the exception of poisonings, directly associating health outcomes to low-dose environmental chemical exposure is uncommon. There is a famous phrase in toxicology “the dose makes the poison” and although it is an oversimplification of how a chemical causes harm, certainly dose is an important factor in exposure assessment. But, one must also consider the developmental window of exposure, the length of exposure, the person’s genetic predisposition, race/ethnicity, preexisting medical conditions, and whether exposure to other chemicals or medications occurred. In medical practice, after reviewing an exposure history, one might engage in a risk assessment exercise to determine a patient’s risk for developing a particular disease. It is important to note, however, that in the practice of environmental health, risk assessment is a defined and systematic process for characterizing the nature and magnitude of the risks associated with environmental hazards. It is generally chemical specific, complex, and involves a great deal of uncertainty. Risk assessments are frequently used by regulators in the cleanup of hazardous waste sites, but they are also used by food and drug administrations to determine appropriate and safe drug use.
Intuitive risk assessment and risk management have been fundamental for human survival and evolution. Those who appreciated risk were more likely to survive and reproduce whereas those who could not were more likely to perish from environmental hazards. 10 However, as chemical industries expand and technology gives us the tools to detect and monitor chemicals at lower and lower levels and exposures to those are deemed hazardous, intuitive risk becomes obsolete as the public frequently has little to no knowledge of the low-level (in some cases, “background-level”) exposure or knowledge of the mixtures of chemicals to which they are exposed.
Populations at Risk
Consequences of chemical exposures are different for different people. Children, teenagers, adults, workers may react very differently to the same chemical exposure. Except in a few rare instances, everyone has the genes coding for DNA repair enzymes and metabolizing enzymes in the liver, which afford humans protection against effects of chemical insults. However, there is tremendous variability in these genes and some variants are more effective than others. Even under the same exposure conditions, there can be wide variation in how people’s bodies react to environmental agents.
Age and timing of exposures greatly influence both sensitivity to an environmental agent and the type of health effect it will cause. Infants and children can be particularly vulnerable and sustain lifelong damage from exposures that have no impact on adults. Lead is a famous example of a neurotoxicant that at high exposures (≥80 µg/dL in the bloodstream) causes epilepsy, mental retardation, and blindness. More than 50 years ago, an acceptable level of 60 µg/dL was set because immediate neurological symptoms did not occur in adults at this level. The “concern” threshold for lead dropped to 10 µg/dL because studies showed that 2- to 3-point IQ deficits occurred with every 10 µg/dL of lead in the bloodstream of children. Currently, it is recognized that there is no threshold and that blood levels below 10 µg/dL also lead to IQ deficits and learning disabilities. 11 These decrements translate into decreased attention spans, poor school performance, and increased aggression in juveniles. Lead is now banned in household paint and automotive gasoline: examples of data-driven policy.
Puberty is another vulnerable life stage. Exposure to hormonally active agents such as DDT (dichlorodiphenyl trichloroethane), PCBs (polychlorinated biphenyls), dioxins, and certain classes of plasticizers has been associated with adverse effects on hormonally sensitive tissue, such as breast tissue. 12
Workers constitute another population at risk. Typically, occupational exposures to chemicals are much higher than those found in the general environment: A noted “healthy worker” effect is factored into those exposures and the wearing of personal protective equipment when calculating risk. Even so, disease related to occupational exposure continues to be a public health priority.
As Hippocrates admonished, it is important to examine where people live to learn about their health. While communities living near highly contaminated hazardous waste sites can be at risk for health effects, there are also many instances of naturally occurring elements found at levels high enough to cause harm to those living nearby. Some examples include radon found in basements across the United States, arsenic in drinking water in southwest United States or Bangladesh, and asbestos in California hillsides. Determining who is at risk requires careful consideration of exposure history.
Communicating Risk
Risk is frequently simplified to magnitude and probability. However, the public generally equates risk to hazard and outrage (including fear). Public risk perception is strongly influenced by factors such as control, fairness, and voluntariness. 13 The “outrage” component is the very reason many people fear dying in an airplane crash whereas they do not give any thought to dying in an automobile crash (even though the odds of the second are much better than the odds of the first occurring). Breakdowns in risk communication generally occur because the primary provider will tend to address the hazard aspect of a diagnosis, while the patient focuses on the outrage (or fear).
There are specific environmental hazards that lend themselves to simple risk communication: encouraging people to purchase, install, and upkeep carbon monoxide detectors in their homes; teaching children not to eat lead-based paint (be they paint chips or chewing on painted jewelry or toys); checking radon levels in basements; having well-water tested for water quality and presence of metals or volatile organics. These are activities and exposures over which the public has a large degree of control. The difficulty comes in communicating risk, particularly for cancer, when there is very little control; for example, people who live in communities around hazardous waste sites or highly industrialized areas where air quality or water quality is poor.
Although cancer risk estimates based on personal exposures can be useful in guiding prioritization of research and indicators of potential hazards to guide regulatory actions, from a clinical treatment perspective, there is still a great deal of missing mechanistic data to be deemed uniquely useful. However, drawing comparisons may help give perspective to patients or clients who cannot comprehend risk. For example, exposures to radon can result in cancer risks of about 1000 per million, and the cancer risk associated with passive smoking has been estimated to be about 2000 per million. 14 Frequently, in discussing chemical risk, the public is more enraged by chemical exposure that they view they have no control over than those they do. Smoking is the number one cause of lung cancer in the United States, whereas radon exposure is second. The public has control over both. People can choose to not smoke or to quit and home owners can test their homes for radon and then, if necessary, make repairs to reduce levels and thus reduce exposure.
Biomonitoring
The previously discussed example of lead and biomonitoring in blood is a somewhat unique example of the ability to measure a chemical in body fluid and to interpret those measurements into health effects. Biomonitoring—a method for measuring amounts of toxic chemicals in human tissues—is a valuable tool for studying potentially harmful environmental chemicals. 15 The data collected through biomonitoring have been used to confirm exposures to chemicals and validate public health policies. While finding chemicals in bodily fluids or tissues is evidence of contact with them through inhalation, dermal exposure, or ingestion, it is much more difficult to determine if the biomonitoring result indicates a health risk. More generally, the results can be compared to a range to determine if it is typical of the general, nonoccupationally exposed population. As is commonly done in a clinical test, the 95th percentile of the distribution can be used to determine the upper limit value of this test result. Using a reference range approach, the Centers for Disease Control and Prevention analyzes biomonitoring results from its National Health and Nutrition Examination Survey (NHANES) and releases the data through their National Reports on Human Exposure to Environmental Chemicals. Blood and urine samples are collected from a nationally representative sample in 2-year cycles. 16 Although using the NHANES data to compare with smaller community samples presents its own set of challenges (nonrepresentative populations, lack of statistical power, etc), it is the best start point for comparison that we have. In general, most communities do not have the funding or political will for support of community-driven research, particularly when the exposures are difficult to quantify and likely caused by local industry that provides jobs to those same community members.
Workplace biologic reference values are another descriptive option for interpreting biomonitoring results in the general population. Values such as the biological exposure index (BEI) of the American Conference of Governmental Industrial Hygienists are workplace standards used to evaluate whether individual workers have received exposures that exceed a workplace air standard, such as a threshold limit value. A blood or urinary biomarker is a better indication of personal exposure than an area air sample. BEIs have been used as points of reference for biomonitoring results in the general public. BEIs, however, do not take into account the differing exposure patterns (continuous vs 8-hour workshift exposure) and vulnerability of the general public (including children, pregnant women, the elderly, and the ill) compared with healthy workers, thus using BEIs to judge community exposure and risk raises numerous interpretive issues. 17
Policy Implications
Environmental regulation and pollution control will remain an important cornerstone of public health policy in the 21st century. Because the focus is on prevention, rather than disease treatment, pollution control is a highly cost-effective means of ensuring public health. So, too, are primary seat-belt laws and helmet regulations. By enforcing safety laws and regulations through tickets and fines, the public and industry are more likely to comply.
The control of bacterial contamination of drinking water sources in the 20th century represents one of the greatest triumphs in public health. The chlorination of public drinking water to prevent waterborne diseases began in the early 1900s and is responsible for the virtual elimination of cholera, typhoid, dysentery, and hepatitis A in the United States. 18
Removing lead from gasoline; redesigning gasoline pumps to reduce benzene exposure and reduce cancer risk; advising pregnant women to avoid eating certain large ocean fish, such as shark and swordfish, which accumulate mercury; improving air quality and emission regulations; banning smoking from most public establishments are all 21st-century policies that have led to improved health outcomes. While protection of public health through policy making is crucial, it generally takes years for those policies to become enacted and enforced. Primary care providers can protect their patients or clients by taking complete exposure histories and recommending interventions to reduce or remove exposures. Educating patients is a great first step.
Tools for Primary Care Providers
Environmental and occupational diseases frequently manifest as common medical problems or have nonspecific symptoms; yet consideration of environmental factors rarely enters into the clinician’s history taking or diagnosis. 19 A chart review of 2922 histories taken by 137 third-year medical students showed that smoking status was documented in 91% of cases, occupation in 70%, and specific occupational exposures in 8.4%. Patients younger than 40 years and women were significantly less likely than older patients or men to have their occupation and industry noted.19,20
The Agency for Toxic Substances and Disease Registry, a federal agency within the Department of Health and Human Services, develops Case Studies in Environmental Medicine to increase primary care providers’ knowledge of hazardous substances in the environment and to aid in the evaluation of potentially exposed patients. These are self-instructional, continuing-education primers which come with additional companion products such as Grand Rounds in Environmental Medicine and Patient Education/Care Instruction Sheets. There are currently 16 topics available for continuing education credits. They cover topics, including chemical-specific cases (asbestos, radon, etc) and asthma and how to take an exposure history. 21
Primary care providers may seek additional counsel from Poison Control Centers or the Association of Occupational and Environmental Clinics (AOEC). Poison control centers are crucial to the detection of and response to illness or potential illness from intentional and unintentional exposures to chemical agents. The 57 centers help the public and health care workers manage chemical or potential chemical exposures. A national toll-free number (800-222-1222) connects a caller 24 hours a day to the poison control center covering the calling area. While providing triage and case management recommendations, specialists in poison information at each poison control center collect and electronically document call data. The specialists in poison information follow up approximately 44% of human exposure calls to document the clinical course and the final outcome. 22
Additionally, the AOEC has a network of more than 60 clinics that provide a means for occupational/environmental health clinics to share information to enable them to diagnose and treat occupational/environmental diseases. Their Web site (http://www.aoec.org/index.htm) provides a clinic directory apart from links to educational resources and epidemiologic tools and provides access to the Pediatric Environmental Health Specialty Units (PEHSUs), which is a network of experts in children’s environmental health. The PEHSUs ensure that children and communities have access to (generally no cost) special medical knowledge and resources for children faced with health risks due to exposures to natural or human-made environmental hazards. Parents or those providing health care to children with persistent, nonresponsive conditions or recognized chemical exposures are able to call the unit assigned to their state for assistance. The units are academically based and can offer advice on prevention, diagnosis, and treatment of environmentally related health effects in children. 23
Environmental health and environmental medicine are dynamic fields. By accessing the tools discussed herein, practitioners will be better equipped to treat their patients and educate them on environmental hazards and risks and ways to reduce exposure.