Regulating Naturally Occurring Heavy Metals: How do Genetics, Behavior and the Environment Affect Toxicity?

Substances affecting human health are often analyzed in a vacuum.  Even though exposure is never limited to a single substance, the interactions between chemicals and the effects of small doses over time are largely ignored when setting limits for exposure.  Of the 70,000 organic chemicals currently in use in the United States (U.S), the EPA has designated 654 as “hazardous,” and only 30 as carcinogenic.  According to the National Institute of Environmental Health Sciences (NIEHS), while small amounts of a hazardous substance may not be toxic, the long-term accumulation of those toxins in body tissue can be.  One major source of long-term low does toxicity is the water supply, where trace amounts of chemicals, considered safe, put people at risk for various types of cancer and other life-threatening diseases.

The effects of long-term exposure to chemicals can vary depending on genetic makeup, behavior, and environmental factors.  Of the two major groups of chemicals causing health problems, heavy metals are of particular interest because they appear in the environment both naturally and as the result of the human process of industrialization.  For example, both environmental arsenic, found in bedrock, and arsenic found in herbicides, leach into the groundwater, while mercury enters the atmosphere during volcanic eruptions and the smelting process.  Either way, these substances enter the human environment, which, according to Sandra Steingraber, an ecologist and cancer survivor, encompasses anything humans interact with or consume, dictated either by one’s environment or lifestyle.

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) requires EPA and the Agency for Toxic Substances and Disease Registry (ATSDR) and the National Center for Environmental Health (NCEH) to rank substances commonly found at Superfund sites in order of their potential threat to human health.  The hazard potential for each substance is calculated based on the frequency of the chemical at Superfund sites, its toxicity, and the potential for human exposure (which is based on both concentration and potential for human exposure).  Arsenic receives the highest score of all hazardous substances, while mercury is ranked third.

Environmental arsenic, found in natural deposits in the earth’s crust, account for approximately one-third of the arsenic in the atmosphere; the remaining arsenic is introduced as a result of agricultural and industrial practices.  Arsenic is used as a wood preservative, in paints, dyes, metals, drugs, soaps, semi-conductors, in agricultural applications of pesticides, in mining, and in smelting.  Exposure from both natural and industrial sources is a result of groundwater contamination.  Natural arsenic leaches from bedrock, while industrial and agricultural runoff directly contaminates water sources, both of which can result in mild exposure over many years.  Chronic exposure to arsenic can result in skin damage, impaired circulatory function and increased risk of cancer.

Mercury most commonly enters the atmosphere as a byproduct of industrial practices.  When coal is burned for energy, it releases mercury into the air, accounting for more than 50% of the mercury emitted by human activity.  The remaining emissions are the result of industrial practices, burning hazardous wastes, producing chlorine, and the improper treatment and disposal of products or wastes containing mercury.  Mercury also erodes from natural deposits, is discharged from refiners or factories, or contaminates runoff from landfills, mines and cropland.  Mercury in the air settles into water or onto land, similar to runoff, where it bio-accumulates in animals and fish.  Humans are most commonly exposed after eating fish or shellfish contaminated with mercury, which can affect the human nervous system, heart, kidneys, lungs, and immune system.

Both arsenic and mercury levels in drinking water are regulated under the EPA’s Safe Drinking Water Act (SDWA).  Under the SDWA, EPA sets non-enforceable health goals, based on possible health risks and exposure over an average lifetime, and enforceable limits, which take into account the health goal, cost, benefit, and the ability to detect and remove arsenic from the water supply.  EPA bases health goals on the level below which no adverse health effects are likely to occur.  The health goal for arsenic is zero, while the enforceable limit is 10 ppb.  According to the NIEHS, environmental arsenic is found in the earth’s crust at an average concentration of approximately 5 mg/kg, and arsenic in private wells can range from 1 to 100 ppb.  Both the non-enforceable health goal and the enforceable limit for mercury are 2 ppb.

Regulation of substances like arsenic and mercury are based on a number of factors, and rely on scientific data proving causation and toxicity.  Toxicity is the measure of an element’s ability to impair health, and depends on dose as well as genetic susceptibility. A contaminant’s ability to cause health problems often remains latent for years, making it difficult to assess the exact dose or source of exposure.  Toxicity alone is often insufficient to necessitate regulation.  Data must indicate a correlative effect, or causation, which can be nearly impossible to prove in humans because of an inability to control for various confounding variables.  Genetic variation can alter susceptibility to cancers, and behavior can be dictated by ones environment.  Making regulation even more difficult is that when a toxin occurs both naturally and from industrial practices, the difficulty in teasing out the effects of these influences or the acceptable limits for either can translate into difficulty in regulating various substances.

Toxic substances are regulated under various federal statutes, including the Clean Water Act, the Clean Air Act, the Toxic Substance Control Act (TSCA), and the Resource Conservation and Recovery Act (“RCRA”).  Other statutes, less directly related to the regulation of chemicals, include the Food and Drug Control Act (FDCA) and the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA).  Right-to-know laws provide consumers, industrial workers, and communities with knowledge about the release of toxic chemicals or rates of morbidity and mortality from specific diseases.  Despite this wide array of federal regulation, fewer than 3% of chemicals are tested before entering the market.  This lapse in regulation has resulted in a significant rise in morbidity and mortality beginning in the mid 19th century, a trend that is not likely to decrease unless the current regulatory structure is revised.

In examining the complexity of the human environment, Sandra Steingraber provokes discussion of whether genetics, behavior and the environment can be separated when examining the effects of substances affecting human health.  It can be the subtlest factors that uncover the big picture—seemingly insignificant trace amounts can accumulate over time and space, blurring the lines between environmental and behavioral effects and removing the ability to pinpoint cause and effect.  Perhaps more salient is the silence, or in the absence of silence, the great noise of confounding variables affecting statistical power.  Incomplete data, disparities in the quality of health care, inconsistent registries, the movement of people, and studies with small sample sizes effect the information reported to government agencies responsible for regulating exposure to potentially harmful substances.

The fractured nature of reporting, data collection, and regulation in the U.S. has left populations vulnerable and open to a continual assault by trace amounts of chemicals from a variety of sources.  As research on the health effects related to chemical exposure, the question arises what, and how to regulate natural toxins like arsenic and mercury in light of concurrent industrial use.  To reduce the health impacts, should both industrial and natural sources of contamination be regulated, and should limits for both sources be equal?  Stricter regulation under the Clean Water Act may reduce the emission of mercury from coal-fired power plants, but what about naturally occurring mercury leaching into ground water?  Limits on pesticides containing arsenic may reduce contamination of water supplies, but what about Chinese produce entering the food supply containing trace amounts of arsenic?  Finally, does the patchwork of regulation under various federal agencies undermine efforts to reduce of exposure to safe levels?


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