Environmental Pollution and Disease

Many studies in people have demonstrated an association between environmental exposure and certain diseases or other health problems. Examples include radon and lung cancer; arsenic and cancer in several organs; lead and nervous system disorders; disease-causing bacteria such as E. coli O157: H7 (e.g., in contaminated meat and water) and gastrointestinal illness and death; and particulate matter and aggravation of heart and respiratory diseases.

To understand the relationship between health and the environment, scientists study a series of events that begins with the release of a pollutant into the environment and may end with the development of disease in a person or a population. Exhibit 4-6 broadly illustrates these events: (1) release of pollution into the environment (air, water, food, soil, and dust), (2) exposure through a variety of activities (inhalation, skin contact, and ingestion of contaminated media), and (3) the development of disease or other health problems.

Elucidating the linkage between environmental pollution and disease is challenging. We understand this linkage fairly well for some pollutants, such as those listed above, but poorly for others. This section describes some of the challenges to elucidating those linkages, and uses examples to highlight the role that indicators can play in strengthening our understanding of that linkage and in supporting environmental management efforts.

Exhibit 4-6: Pathway from pollution to exposure to potential health effects.
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What is the role of the environment in disease?

Decades of research have provided the scientific foundation for understanding the role of the environment in disease. For many pollutants, scientists know with some certainty that exposure to these pollutants, at sufficiently high concentrations, can cause a variety of health effects. For other pollutants, where scientific evidence is less conclusive, scientists can only establish an “association” between exposure and health problems.

Some effects on health may be short-term and reversible, such as irritated eyes from smog. Other effects, such as emphysema, heart disease, and cancer are chronic or even fatal. Some effects may appear shortly after exposure. Others, such as cancer, may require a long lead time before the disease appears.

In many cases, pollution likely is just one of several factors—including diet, exercise, alcohol consumption, and genetic make-up—that influence whether an exposed person will ever become sick. Although exposure to ETS is associated with lung cancer, whether a person will develop cancer from that exposure depends on the amount, frequency, and length of exposure, exposure to other contaminants, and personal characteristics (genes) and behavior (diet and other lifestyle choices).³⁸ All these factors can be important in illness and premature death, but they are poorly understood, difficult to quantify, and not routinely tracked or reported. Because of these complexities, it is very difficult to establish causal relationships, and few diseases are known to be exclusively the result of exposure to an environmental pollutant. In many cases, only a small portion of the national incidence of a particular disease is likely to be attributed to a specific environmental factor.

Further complicating the picture is the fact that several segments of the population may be at higher risk for damage or disease from environmental pollutants. Potentially sensitive groups include children; older Americans; people with existing health problems such as diabetes, respiratory disease, or heart disease; and persons with compromised immune systems, including those who have HIV/AIDS or are undergoing cancer chemotherapy. Poor or other disadvantaged populations may live in more polluted environments that expose them to higher concentrations of pollutants. Understanding the impacts of pollutants on such sensitive groups is important for those people directly, as well as for the development of protective national health standards and policies.

Children may be more vulnerable to some environmental pollutants than adults for a number of reasons related to their size, growth, and behaviors. Further, children may become ill from exposures that would not affect adults.

Older Americans may also be especially vulnerable to harmful health effects associated with environmental pollutants, in part because some health problems take many years to develop. A long life span may provide the time needed for occupational or cumulative environmental exposures to induce illness or disease. Also, because of medical advances, many older Americans may be living with health conditions that previously shortened life spans. And, older Americans may have preexisting conditions—such as heart ailments, diabetes, or respiratory disease—that reduce their tolerance to pollutants. Even relatively healthy older people may, merely as a result of age, have a diminished capacity to fight infections, pollution, or other causes of stress to their systems that might have posed little risk when they were younger. Harmful substances may be processed and eliminated from the body more slowly in older people, which can prolong exposure to those substances and increase susceptibility to associated health problems. Older people are also more likely to become dehydrated and experience other serious consequences of gastrointestinal disease.

Sorting out the role of all these risk factors—including the environment—and their interactions is a major challenge of scientific research. In addition to the tools already available for elucidating the linkage between environmental exposure and disease, EPA is exploring the use of indicators to complement the traditional tools—exposure assessment, toxicology, and human studies—that are used to evaluate the potential impacts of environmental exposures. Three examples are presented below that illustrate how indicators can play a role in elucidating linkages between environmental pollution and health problems. In two of these examples (lead and waterborne diseases), indicators also play a key role in focusing the environmental protection decision and in evaluating the success of those decisions.

Health Effects of Exposure to Lead

Lead, a naturally occurring metal, has been used to produce gasoline, ceramic products, paints, and solder. In homes built before 1978, lead-based paint and lead-contaminated dust from paint are the primary sources of exposure to lead. Major initiatives have been implemented to reduce lead exposure by phasing lead out of gasoline, paint, solder, and plumbing fixtures.

Health problems from lead exposure are a major environmental health problem because exposure to lead is widespread and can cause health effects at relatively low levels. Substantial data are available to link lead exposure with health effects. Lead adversely affects the nervous system, can lower intelligence, and has been associated with behavioral and attention problems. It also affects the kidney and blood-forming organs.³⁹ Children and the developing fetus are more vulnerable to the effects of lead than adults.

The level of lead in blood has long been used as an indicator of exposure to lead. And, because the linkage between lead exposure and health effects is so strong, blood lead is also used as an indicator of adverse effects on the nervous system.

In the 1970s, lead poisoning occurred increasingly in children who did not live in dwellings with lead-based paint, suggesting that another source or sources of lead exposure were of even greater concern than lead paint. Research found that combustion of leaded gasoline was the primary source of lead in the environment. In the 1970s, EPA promulgated regulations to ban lead in gasoline. Since that time, concentrations of lead in blood samples and in ambient air have declined significantly (Exhibit 4-7). In young children, the median concentration of lead in blood decreased by 85 percent from 1976 to 1999-2000 based on nationwide surveys (Exhibit 4-8).⁴⁰

Exhibit 4-7: Lead used in gasoline production and NHANES blood level averages, 1976-1980.
Exhibit 4-8: Concentrations of lead in blood of children age 5 and under.
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But national averages of blood levels tell only part of the story. Between 1999 and 2000, approximately 430,000 children ages 1 to 5 (about 2 percent) had elevated blood lead levels (10 µg/dL or greater) from eating paint chips or inhaling lead-containing dust in older homes, primarily in urban areas.⁴¹ Even today, lead poisoning is considered to be a serious environmental hazard in young children in the U.S.⁴² Several major metropolitan areas, including Chicago, Detroit, Milwaukee, Palo Alto, and St. Louis, are evaluating blood lead levels of young children, focusing on areas at high risk (i.e., older housing and poorer neighborhoods), to study and address potential problems (see box, “Children’s Lead Levels Remain a Concern in Urban Hot Spots”). These blood lead screening programs, however, do not report in a systematic fashion to a central location where the data can be evaluated.

Health Effects of Air Pollution

Several outdoor air pollutants are associated with harmful health effects. These include the six “criteria” pollutants—particulate matter, ground-level ozone, nitrogen dioxide, carbon monoxide, sulfur dioxide, and lead—for which EPA has established standards to protect human health, including the health of sensitive populations such as asthmatics, children, and the elderly. The burning of fossil fuels is the principal source of these pollutants. Air pollutants can be transported long distances, so they can potentially have effects distant from their source. (See Chapter 1- Cleaner Air, for further discussion of the health effects related to air pollutants.)

Air pollution has been associated with several health problems, including reported symptoms (nose and throat irritation), acute onset or exacerbation of existing disease (e.g., asthma, hospitalizations due to cardiovascular disease), and premature deaths. The impact of air pollution on health was underscored in December 1952 when a slow-moving area of high pressure came to a halt over the city of London. Fog developed over the city, and particulate and sulfur pollution began accumulating in the stagnating air mass. Smoke and sulfur dioxide concentrations built up over 3 days. Mortality records showed that deaths increased in a pattern very similar to that of the pollution measurements. An estimated 4,000 extra deaths occurred over a 3- to 4-day period. This represents the first quantitative air pollution exposure data with a link to health.

While the London episode highlighted the hazard of extreme air pollution episodes, it was unclear whether health effects were associated with lower concentrations. By the 1970s, the association between respiratory disease and particulate and/or sulfur oxide air pollution had been well established.⁴³ Improvements in the measurement of air pollution and health endpoints, plus advances in analytical techniques, have made it possible to quantitatively evaluate air pollution and health. For example, research has shown that many air pollutants may contribute to the onset or aggravation of heart disease, especially carbon monoxide and fine particulate matter (PM_2.5).^{44, 45, 46}

Particulate Matter

Particulate air pollution is associated with increased daily mortality in many U.S. communities and other countries. The elderly and those with preexisting diseases are particularly vulnerable.⁴⁷ Exposure to ambient particulate matter has also been associated with an increased number of hospital admissions and visits to doctors due to cardiovascular problems and respiratory disease.⁴⁸ Some studies show that exposure to particulate matter exacerbates asthma. Long-term exposure to particulate matter has been associated with increased deaths from heart and lung diseases, increased respiratory disease and bronchitis and with decreased lung function in children.⁴⁹

Ozone

Repeated short-term exposures to ozone may damage children's developing lungs, which may lead to permanent reductions in lung function.⁵⁰ Controlled studies in healthy adults have demonstrated ozone-induced lung inflammation, decrements in lung function, and associated respiratory symptoms, such as cough and pain on deep inspiration.⁵¹ Ozone exposures have also been associated with an increased number of hospital admissions and visits to doctors.⁵²

Indicators

As noted in Chapter 1 - Cleaner Air, national average criteria pollutant levels, including particulate matter and ozone levels, have decreased over the past 20 years. As discussed earlier, however, there are limitations in using these national air pollution data to evaluate rates of asthma attacks occurring during acute exposure episodes. Possible future health indicators for air pollution include death due to respiratory and cardiovascular disease, increased hospital admissions for respiratory and cardiovascular disease, and subtle changes in the cardiovascular system that can increase people's risk of heart attacks and other cardiovascular effects. Use of these indicators is still challenged by limits in our understanding of how much air pollution contributes to the risk of cardiovascular and respiratory disease.

Waterborne Diseases

In the early 20th century, waterborne diseases such as cholera and typhoid fever were major health threats across the U.S. Deaths due to diarrhea-like illnesses, including typhoid, cholera, and dysentery, were the third largest cause of death in the nation. For instance, more than 150 in every 100,000 people died from typhoid fever each year.

Around that time, scientists began to understand the cause of these diseases. They had identified the bacteria responsible for most diarrheal deaths (typhoid, cholera, and dysentery) and elucidated how these bacteria were transmitted to and among humans. Infected and diseased individuals shed large quantities of microbes in their feces, which flowed into and contaminated major water supplies. This contaminated water was then distributed untreated to communities, which used the water for drinking and other purposes. This created a continuous transmission cycle.

Once treatment (filtration and chlorination) of drinking water was initiated to remove pathogens, the number of deaths due to diarrheal diseases dropped dramatically in communities with treated water. Deaths due to typhoid fever were tracked throughout the early 20th century, as drinking water treatment was implemented across the country, providing an indicator of the success of this environmental management strategy (Exhibit 4-10).

Exhibit 4-10: Percent of population with treated water versus typhoid deaths in the US, 1880-1980.
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Drinking water treatment is one of the great public health success stories of the 20th century. Not only did it dramatically and significantly reduce death rates from waterborne disease, it also increased life expectancy and reduced infant mortality. Today, public health is protected against new and emerging waterborne microbial contaminants by continual improvements to the drinking water treatment process.

This example illustrates how a link was made between gastrointestinal disease (an outcome indicator) and exposure to pathogens in drinking water. Based on this connection, officials were able to take effective action to protect public health. They also were able to use an outcome measure (deaths due to typhoid) to monitor the success of these protective actions.

Today, deaths due to typhoid, cholera, and dysentery are so rare in the U.S. that they cannot serve as indicators to evaluate drinking water management decisions. The actual number of cases of typhoid, cholera, and dysentery are tracked to some extent; however, the reporting of these cases is not federally required. The waterborne disease outbreak surveillance system is a passive system in that it relies on state health departments to voluntarily report their outbreaks to CDC. (For further information on waterborne diseases, see Chapter 2 - Purer Water.)