Unit 6: Risk, Exposure, and Health // Section 1: Introduction
Kayla is a normal teenager except she has asthma, a chronic condition of the airways that makes it difficult for her to breath at times. Allergens such as pollens, dust mites, cockroaches, and air pollution from cigarettes, gas stoves, and traffic make asthmatics' airways swell so that only limited amounts of air can pass through and respiration becomes a struggle akin to breathing through a tiny straw.
Growing up poor and black in Boston, Kayla is part of an epidemic that has seen the asthma prevalence rate for children rise from 3.6 percent in 1980 to 5.8 percent in 2005 (footnote 1). Asthma incidence has risen in many industrialized countries around the world (Fig 1), but it is much more common among children living in inner cities. Children like Kayla living in Roxbury and Dorchester, Massachusetts, are five times more likely to be hospitalized for asthma than children living in wealthier white sections of Boston.
Figure 1. Inner city ER admissions for pediatric asthmatics
See larger image
Source: Courtesy of the Environmental Health Office at the Boston Public Health Commission.
Starting in 2001, the Healthy Public Housing Initiative (HPHI), a collaboration between Harvard, Tufts, and Boston University, worked with the Boston housing authority and tenant organizations to conduct test interventions aimed at reducing the suffering of children with asthma. HPHI reduced allergen exposures by thoroughly cleaning apartments, educating mothers about pest controls, implementing integrated pest management (discussed in Unit 7, "Agriculture"), and providing dust-mite reducing mattresses. Symptoms decreased and quality of life measurements improved for Kayla and other asthmatic children living in three public housing developments during a year of follow-up assessments after the interventions (Fig. 2) (footnote 2).
Figure 2. Change in asthma symptoms among children participating in HPHI before and after intervention
See larger image
Source: Data courtesy of Jonathan I. Levy, Sc.D., Harvard School of Public Health.
We are exposed to environmental contaminants from conception to our last breath. Some of these materials are naturally-occurring substances such as dust, pollen, and mold, while others are manmade chemicals used for numerous industrial and commercial purposes. As of 2006, the U.S. Environmental Protection Agency (EPA) estimated that about there were 15,000 chemicals in commerce (footnote 3).
Some contaminants have been demonstrated to have harmful effects on various human organs, such as the reproductive or respiratory systems, or on functions such as fetal development. Based on evidence from toxicological, ecological, and epidemiological studies, health experts suspect many more contaminants of being possible risks to humans. The EPA screens chemicals that it believes are the greatest potential threats to human health and the environment, but most of the chemical compounds that are already in wide use today have been subject to little or no toxicological testing. Virtually none has been tested for potential as endocrine disruptors.
In complex modern societies, the most critical environmental health challenge is defining a balance between the social and economic benefits that materials and technologies provide on one hand and risks to public health on the other hand. Numerous materials, from food additives to pesticides to manufacturing inputs, have valuable uses but may also threaten the health of the general public or smaller high-risk groups. In many cases such threats can be managed by setting usage guidelines or limiting exposure. In extreme cases they may require taking materials off of the market. Tetraethyl lead, asbestos, DDT, and PCBs are some examples of widely used substances that have been proven harmful (Fig. 3).
Figure 3. Warning sign, Palos Verdes Peninsula, California
See larger image
Source: Courtesy United States Environmental Protection Agency.
Health experts approach these tradeoffs by using risk assessment to systematically evaluate scientific, engineering, toxicological, and epidemiological information on specific environmental hazards. Next they use this factual analysis to develop strategies, such as standards, regulations, and restrictions, that reduce or eliminate harm to people and the environment, a process referred to as risk management. Risk management takes into consideration both the benefits and the costs of controlling or eliminating hazards. It weighs the strength of the scientific evidence along with the social and economic implications of controlling or not controlling environmental risks.
This process has limitations. Epidemiological studies cannot establish causal relationships between exposure and harm. Most toxicological studies carried out in laboratories use artificially high doses to evoke responses within reasonable time periods, whereas real exposures to environmental contaminants often involve low-level exposures over very long time frames. And real exposures almost always involve mixtures of contaminants, such as heavy metals in mine drainage. The time course of exposures and doses is complex, both for individuals and for the population at large: levels, frequency, and intensity of exposure all can affect toxicity.
"We have very good ideas of what individual toxicants can do to people. However, you cannot predict what the ultimate human health impacts might be from simply knowing what the individual toxicants can do. Mixtures can interact in ways that are unforeseen and give you toxic ramifications that are much greater than what can be predicted from the single exposures. On the other hand, in some mixtures toxicants can cancel each other out. So this has to be studied well and properly to understand what the real risks are."
Howard Hu, University of Michigan/Harvard University
Genetic variability in the population adds to the uncertainty of risk assessment. Interactions between humans' genetic makeup and their environment take many forms, including characteristics that either protect individuals from specific risks or make them more susceptible. Both inherited genetic traits and environmental exposures can create genetic susceptibilities, which can then be transferred from one generation to another.
To be effective, risk management must take these uncertainties and sources of variability into account in developing strategies. Managing risks also involves political and philosophical issues. Governments have often acted regardless of the actual magnitude of a risk because of risk perceptions on the part of special interest groups or the general public.
This unit describes the risk assessment process and the central role of epidemiology—studying associations between exposure, risk factors, and outcomes. It then shows how public health experts use evidence to assess cancer and noncancer risks associated with environmental exposures. Next we look at the challenge of balancing risks and benefits and of assigning economic value to proposed environmental actions. The unit concludes with a discussion of the Precautionary Principle, a sometimes-controversial approach to managing health and environmental risks with incomplete knowledge, and with brief summaries of relevant laws and regulations.