Environmental Health

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Environmental health

Environmental health is concerned with the medical effects of chemicals , pathogenic (disease-causing) organisms, or physical factors in our environment . Because our environment affects nearly every aspect of our lives in some way or other, environmental health is related to virtually every branch of medical science. The special focus of this discipline, however, tends to be health effects of polluted air and water, contaminated food, and toxic or hazardous materials in our environment. Concerns about these issues makes environmental health one of the most compelling reasons to be interested in environmental science .

For a majority of humans, the most immediate environmental health threat has always been pathogenic organisms. Improved sanitation , nutrition, and modern medicine in the industrialized countries have reduced or eliminated many of the communicable diseases that once threatened us. But for people in the less developed countries where nearly 80% of the world's population lives, bacteria, viruses, fungi , parasites , worms, flukes, and other infectious agents remain major causes of illness and death. Hundreds of millions of people suffer from major diseases such as malaria , gastrointestinal infections (diarrhea, dysentery, cholera ), tuberculosis, influenza, and pneumonia spread through the air, water, or food. Many of these terrible diseases could be eliminated or greatly reduced by a cleaner environment, inexpensive dietary supplements, and better medical care.

For the billion or so richest people in the world—including most of the population of the United States and Canada—diseases related to lifestyle or longevity tend to be much greater threats than more conventional environmental concerns such as dirty water or polluted air. Heart attacks, strokes, cancer , depression and hypertension, traffic accidents, trauma, and AIDS lead as causes of sickness and death in wealthy countries. These diseases are becoming increasingly common in the developing world as people live longer, exercise less, eat a richer diet, and use more drugs, tobacco , and alcohol. Epidemiologists predict that by the middle of the next century, these diseases of affluence will be leading causes of sickness and death everywhere.

Although a relatively minor cause of illness compared to the factors above, toxic or hazardous synthetic chemicals in the environment are becoming an increasing source of concern as industry uses more and more exotic materials to manufacture the goods we all purchase. There are many of these compounds to worry about. Somewhere around five million different chemical substances are known, about 100,000 are used in commercial quantities, and about 10,000 new ones are discovered or invented each year. Few of these materials have been thoroughly tested for toxicity. Furthermore, the process of predicting what our chances of exposure and potential harm might be from those re-leased into the environment remains highly controversial. Toxins are poisonous, which means that they react specifically with cellular components or interfere with unique physiological processes. A particular chemical may be toxic to one organism but not another, or dangerous in one type of exposure but not others. Because of this specificity, they may be harmful even in very dilute concentrations. Ricin, for instance, is a protein found in castor beans and one of the most toxic materials known. Three hundred picograms (trillionths of a gram) injected intravenously is enough to kill an average mouse. A single molecule can kill an individual cell. If humans were as sensitive as mice, a few teaspoons of this compound, divided evenly and distributed uniformly could kill everyone in the world. By the way, this points out that not all toxins are produced by industry. Many natural products are highly toxic.

Toxins that have chronic (long-lasting) or irreversible effects are of special concern. Among some important examples are neurotoxins (attack nerve cells), mutagens (cause genetic damage), teratogens (result in birth defects ), and carcinogens (cause cancer). Many pesticides and metals such as mercury , lead , and chromium are neurotoxins. Loss of even a few critical neurons can be highly noticeable or may even be lethal making this category of great importance. Chemicals or physical factors such as radiation that damage genetic material can harm not only cells produced in the exposed individual, but also the offspring of those individuals as well.

Among the most dread characteristics of all these chronic environmental health threats are that the initial exposure may be so small or have results so unnoticeable that the victim doesn't even know that anything has happened until years later. Furthermore the results may be catastrophic and irreversible once they do appear. These are among our worst fears and are powerful reasons that we are so apprehensive about environmental contaminants. There may be no exposure—no matter how small—of some chemicals that is absolutely safe. Because of these fears, we often demand absolute protection from some of the most dread contaminants. Unfortunately, this may not be possible. There may be no way to insure that we are never exposed to any amount of some hazards. It may be that our only recourse is to ask how we can reduce our exposure or mitigate the consequences of that exposure.

In spite of the foregoing discussion of the dangers of chronic effects from minute exposures to certain materials or factors, not all pollutants are equally dangerous nor is every exposure an unacceptable risk. Our fear of unknown and unfamiliar industrial chemicals can lead to hysterical demands for zero exposure to risks. The fact is that life is risky. Furthermore, some materials are extremely toxic while others are only moderately or even slightly so.

This is expressed in the adage of the German physician, Paracelsus, who said in 1540 that "The dose makes the poison." It has become a basic principle of toxicology that nearly everything is toxic at some concentration but most materials have some lower level at which they present an insignificant risk. Sodium chloride (table salt), for instance, is essential for human life in small doses. If you were forced to eat a kilogram all at once, however, it would make you very sick. A similar amount injected all at once into your blood stream would be lethal.

How a material is delivered—at what rate, through which route of entry, in what form—is often as important as what the material is. The movement, distribution, and fate of materials in the environment are important aspects of environmental health. Solubility is one of the most important characteristics in determining how, when, and where a material will travel through the environment and into our bodies. Chemicals that are water soluble move more rapidly and extensively but also are easier to wash off, excrete, or eliminate. Oil or fat soluble chemicals may not move through the environment as easily as water-soluble materials but may penetrate very efficiently through the skin and into tissues and organs. They also may be more likely to be concentrated and stored permanently in fat deposits in the body.

The most common route of entry into the body for many materials is through ingestion and absorption in the gastrointestinal (GI) tract. The GI tract, along with the urinary system are the main routes of excretion of dangerous materials. Not surprisingly, those cells and tissues most intimately and continuously in contact with dangerous materials are among the ones most likely to be damaged. Ulcers, infections, lesions, or tumors of the mouth, esophagus, stomach, intestine, colon, kidney, bladder, and associated glands are among the most common manifestations of environmental toxins. Other common routes of entry for toxins are through the respiratory system and the skin. These also are important routes for excreting or discharging unwanted materials.

Some of our most convincing evidence about the toxicity of particular chemicals on humans has come from experiments in which volunteers (students, convicts, or others) were deliberately given measured levels under controlled conditions. Because it is now considered unethical to experiment on living humans, we are forced to depend on proxy experiments using computer models, tissue cultures, or laboratory animals. These proxy tests are difficult to interpret. We can't be sure that experimental methods can be extrapolated to how real living humans would react. The most commonly used laboratory animals in toxicity tests are rodents like rats and mice. However, different species can react very differently to the same compound. Of some 200 chemicals shown to be carcinogenic in either rats or mice, for instance, about half caused cancer in one species but not the other. How should we interpret these results? Should we assume that we are as sensitive as the most susceptible animal, as resistant as the least sensitive, or somewhere in between?

It is especially difficult to determine responses to very low levels of particular chemicals, especially when they are not highly toxic. The effects of random events, chance, and unknown complicating factors become troublesome, often resulting in a high level of uncertainty in predicting risk. The case of the sweetener saccharin is a good example of the complexities and uncertainties in risk assessment . Studies in the 1970s suggested a link between saccharin and bladder cancer in male rats. Critics pointed out that humans would have to drink 800 cans of soft drink per day to get a dose equivalent to that given to the rats. Furthermore, they argued, most people are not merely large rats.

The Food and Drug Administration uses a range of estimates of the probable toxicity of saccharine in humans. At current rates of consumption, the lower estimate predicts that only one person in the United states will get cancer every 1,000 years from saccharine. That is clearly inconsequential considering the advantages of reduced weight, fewer cases of diabetes, and other benefits from this sugar substitute. The upper estimate, however, suggests that 3,640 people will die each year from this same exposure. That is most certainly a risk worth worrying about.

An emerging environmental health concern with a similarly high level of uncertainty but potentially dire consequences is the disruption of endocrine hormone functions by synthetic chemicals. About ten years ago, wildlife biologists began to report puzzling evidence of reproductive failures and abnormal development in certain wild animal populations. Alligators in a lake in central Florida, for instance, were reported to have a 90% decline in egg hatching and juvenile survival along with feminization of adult males including abnormally small penises and lack of sperm production. Similar reproductive problems and developmental defects were reported for trout in the Great Lakes , seagulls in California, panthers in Florida, and a number of other species. Even humans may be effected if reports of global reduction of sperm counts and increases of hormone-dependent cancers prove to be true.

Both laboratory and field studies point to a possible role of synthetic chemicals in these problems. More than 50 chemicals, if present in high enough concentrations, are now known to mimic or disrupt the signals conveyed by naturally occurring endocrine hormones that control almost every aspect of development, behavior, immune functions, and metabolism . Among these chemicals are dioxin , polychlorinated biphenyl, and several persistent pesticides. This new field of research promises to be of great concern in the next few years because it combines dread factors of great emotional power such as undetectable exposure, threat to future generations , unknown or delayed consequences, and involuntary or inequitable distribution of risk.

In spite of the seriousness of the concerns expressed above, the Environmental Protection Agency warns that we need to take a balanced view of environmental health. The risks associated with allowable levels of certain organic solvents in drinking water or some pesticides in food are thought to carry a risk of less than one cancer in a million people in a lifetime. Many people are outraged about being exposed to this risk, yet they cheerfully accept risks thousands of times as higher from activities they enjoy such as smoking, driving a car, or eating an unhealthy diet. According to the EPA, the most important things we as individuals can do to improve our health are to reduce smoking, drive safely, eat a balanced diet, exercise reasonably, lower stress in our lives, avoid dangerous jobs, lower indoor pollutants, practice safe sex, avoid sun exposure, and prevent household accidents. Many of these factors over which we have control are much more risky than the unknown, uncontrollable, environmental hazards we fear so much.

[William P. Cunningham Ph.D. ]