Difficulties in Determining Environmental Health Effects at Low Exposure Levels

Pollution levels in most of the industrialized world are now relatively low. Lowering these levels further will be more expensive and, in the absence of convincing public health need, it will be more difficult to create the public will for additional reductions. At the same time, the health consequences of low levels of pollutants are undeniably more difficult to determine. This difficulty arises from two facts. The first is that people differ, often significantly, in their response to environmental toxicants. As mentioned earlier, organisms have evolved a complex environmental response machinery to protect against foreign compounds or xenobiotics. Such machinery includes DNA repair enzymes and metabolizing enzymes in the liver and other organs. Except in a few rare cases, everyone has the genes coding for these enzymes. But there is tremendous variability in these genes and some variants are more effective than others. Thus, some people are more sensitive to specific environmental toxicants and some people less so. Even under the same exposure conditions, there can be wide variation in how people's bodies react to environmental agents. This variability in the general population

can often mask very real effects that occur in a sensitive segment of the population.

The second difficulty is that age and the timing of exposures can 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 can sustain lifelong damage at exposures that have no impact in adults. As an example, the metal lead is a known neurotoxicant. At high exposures of lead (80 pg/dL or higher in the bloodstream), encephalopathy, epilepsy, mental retardation, and blindness are the probable outcomes. Thus fifty years ago a level of 60 pg/dL of lead would be acceptable because immediate neurological symptoms did not occur in adults at these levels.

The "concern" threshold has been steadily dropping, however, because of new information about subtle health effects of lead, as well as the greater vulnerability of children to lead exposures. Now banned from use in products such as household paint and automotive gasoline, the "concern" threshold for lead is currently 10 pg/dL of lead in the bloodstream of children. This level was determined based on research findings that even low lead exposures can cause unexpected problems for children. There is now evidence that every 10 pg/dL of lead in the bloodstream of children is associated with a two- to three-point IQ deficit. Although these decrements are low, they translate into later problems in school, particularly decreased attention spans, increased aggression as juveniles, and failure to graduate from high school. Thus,

Protestors outside the Cleveland, Ohio, city hall on January 20, 1970, protesting the city's air pollution. (©Bettmann/Corbis. Reproduced by permission.)

neurotoxicant chemical that is toxic to neurons, or brain cells

A female student uses an inhaler for her asthma. (© Angela Hampton; Ecoscene/ Corbis. Reproduced by permission.)

regulatory standards based on studies in adults can potentially fail to protect children and other sensitive groups.

Childhood is not the only vulnerable life stage. Puberty could represent another sensitive time point. Exposure to hormonally active agents (HAA) such as DDT, polycholorinated biphenyls (PCBs), dioxins, and certain classes of plasticizers, could adversely affect hormonally sensitive tissue, such as breast tissue. For example, women exposed to these compounds might be at greater risk of breast cancer, particularly if their exposures occur around puberty. The critical exposures could occur much earlier than clinical manifestations of disease, though, making it difficult to establish an association between the exposure and breast cancer. Additionally, the aging body might be less able to hold up against a lifetime of low-level, but persistent, environmental assaults and could begin to experience neurodegeneration, cancers, or heart disease as a consequence.

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