For most of us, genetic inheritance plays a limited role in determining our health. More important is where and how we live, work, and play — the quality of what we drink, eat, and breathe. From the time of conception, throughout development, and into early and late adulthood, environmental factors either directly impact biological tissues or influence gene expression and shape subsequent disease risks.
Although links between exposures to environmental contaminants and health impacts have been known for centuries, recent research documents an expanding list of previously unrecognized effects occur after fetal or infant exposures. The developing fetus and child are particularly vulnerable to toxic insults. During this time cells are rapidly dividing, and growth is dramatic. Various events, including development of the brain and endocrine, reproductive, and immune systems, are uniquely susceptible to disruption that is often permanent. To compound the problem, pound for pound, children are often disproportionately exposed to toxic environmental agents because of the way they breathe, eat, drink, and play. Moreover, immature detoxification pathways in children frequently result in increased impacts of toxic exposures when compared to adults.
To the limited degree that health care providers address environmental factors at all, most focus nearly all of their attention on personal behaviors, like smoking, substance abuse, or use of sunscreens. These are more easily addressed by individuals than more complex problems like air and water pollution, hazardous waste sites, agricultural systems that inevitably result in farmworker pesticide exposures, and mercury contamination of dietary fish. Global environmental conditions, however, are changing, along with the changing pattern of disease and disability, and our increasing understanding of the importance of environmental factors in determining the health of individuals and populations places a new and special responsibility on the medical profession.
** The release of ozone-depleting chemicals used for industrial and agricultural purposes has depleted the stratospheric ozone layer and is likely a major contributor to the increased incidence of malignant melanoma.
** Carbon dioxide concentration in the atmosphere has increased by nearly 30 percent in the last 150 years. Carbon dioxide is a greenhouse gas that contributes to global warming. Hazardous air pollution, in general, is the norm in most parts of the U.S. and elsewhere in the world.
** Humans are responsible for more atmospheric nitrogen fixation than all other sources combined. Nitrates contaminate groundwater, surface water, and air at toxic concentrations.
** Humans are responsible for most of the mercury deposition on the surface of the earth. Mercury makes its way into the food chain, where it bioconcentrates. In most states, freshwater and marine fish are sufficiently contaminated with mercury to require warnings to women of reproductive age to limit consumption because of risks to fetal brain development.
** In addition to naturally occurring products like lead and mercury that are mined from the earth, novel synthetic industrial chemicals contaminate the world’s ecosystems, its human and non-human inhabitants, their breast milk and egg yolk, ovarian follicles, amniotic fluid, and meconium. The toxicity of most is little known.
** Of the approximately 85,000 chemicals on the federal inventory, nearly 3,000 are produced in excess of 1 million pounds annually. For these high-production volume (HPV) chemicals, toxicity data are surprisingly sparse. Even basic toxicity testing results are not publicly available for 75 percent of them.In the U.S., according to the 2000 Toxics Release Inventory, over 6.2 billion pounds of the listed toxic chemicals, including 2 billion pounds of known or suspected neurotoxicants, were released into the environment by major emitters required by federal law to file reports. Emissions from small industries and neighborhood shops are unquantified. The extent of exposure from these releases and from the use of various consumer products that contain them is also largely unknown, but population-based surveys give an indication of the ubiquity of exposures.
Small exposures to substances like lead, mercury, or polychlorinated biphenyls (PCBs), which have no discernible impact on adults, can permanently damage the developing brain of a child, if the exposure occurs during a window of vulnerability. Early exposures to dioxin or polychlorinated biphenyls (PCBs), chemicals from industrial activities that bioaccumulate in dietary fat, damage the developing immune system, making the child more prone to infections, risks of asthma and high blood pressure are increased by early environmental exposures. Recent research from Sweden concludes not only that environmental factors play a more important role than genetic inheritance in the origin of most cancers, but also that cancer risk is largely established during the first 20 years of life.
Some birth defects, including disorders of the male reproductive system and some forms of congenital heart disease, are increasingly common. Sperm counts and fertility are in decline in some areas of the U.S. and other parts of the world. Asthma is more common and more severe than ever before. Genetic factors explain far less than half of the population variance for most of these conditions. Although smoking and sun exposure are well-recognized risk factors for some conditions, improved understanding of development of the brain and the immune, reproductive, respiratory, and cardiovascular systems leads to the conclusion that other environmental factors play a major role in determining current patterns of disease.