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 medicalscience. 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 make 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 hasalways been pathogenic organisms. Improved sanitation, nutrition, and modernmedicine in the industrialized countries have reduced or eliminated many of the communicable diseases that once threatened us. Some authorities argue thatthe greatest "medical" advance in industrialized countries actually has beensewers and water purification, which all but eliminated many major infectious diseases from our society. 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 affluencewill 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 ourchances of exposure and potential harm might be from those released into theenvironment remains highly controversial. Toxins are poisonous, which means that they react specifically with cellular components or interfere with uniquephysiological processes. A particular chemical may be toxic to one organismbut not another, or dangerous in one type of exposure but not others. Becauseof 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 mosttoxic 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 everyonein 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), andcarcinogens (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.
The German physician Paracelsus 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.
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 reactvery 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 weassume that we are as sensitive as the most susceptible animal, as resistantas 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 saccharinand bladder cancer in male rats. Critics pointed out that humans would haveto 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 benefitsfrom this sugar substitute. The upper estimate, however, suggests that 3,640people 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 biologistsbegan 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 andjuvenile 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, seagullsin 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.
Ever since the 1970s, air pollution has been a major concern of both citizensand government policy makers in the United States. This concern is partly because air pollution can be widely visible as smog or haze. The EnvironmentalProtection Agency has implemented stronger and stronger limits on emissions from smokestacks of power plants and other industries, as well as emissions from cars and trucks. Even stronger regulations are in effect in some parts ofthe United States, such as Southern California.
These regulations have worked: Generally speaking, air is visually cleaner and contains fewer pollutants than it once did. However, scientists express continued concern about high levels of ground-level ozone, particularly in and around large cities. This pollutant, which is colorless and odorless and therefore not obvious to the general public, seems to cause or exacerbate respiratory diseases such as asthma and emphysema. It is thought to be part of the explanation for the greatly increased occurrence of asthma in inner cities of the United States.
It is important to distinguish this ground level problem--too much ozone--from the problem caused by a reduced level of ozone some miles above the Earth'ssurface. Ozone at that level reduces the amount of ultraviolet radiation reaching the surface. Increasing levels of ultraviolet radiation are a factor incausing cataracts and skin cancer, among other disorders. Thus, even thoughthe Ozone Layer is not part of our immediate environment, even cataracts andskin cancer are environmental diseases in a sense.
Before the advent of sewer systems (to carry away waste) and water treatmentplants (to purify water) in the developed word, water-borne illnesses causedterrible epidemics. Diseases such as cholera and yellow fever decimated almost every city from time to time, and even in the ancient world the wealthy would flee hot, low-lying cities for cooler hill regions every summer. Problemsof this type continue to afflict poorer regions of the world.
Even in the developed world, however, water-borne illnesses occur from time to time. During the 1990s a single-cell organism called cryptosporidia survived water treatments in some large American cities and caused severe illnesses.These illnesses were particularly serious in infants and frail elderly people, but they caused widespread problems. Water purification plants have adapted their processes to minimize such outbreaks in the future, but authorities believe that some other organism could emerge to cause a new set of problems.
Another potential source of water-borne illnesses is landfills, runoff from abandoned industrial sites, and the like. Awareness of this hazard became acute in the early 1970s, when people living near the Love Canal near Buffalo began to suffer from clusters of unusal illnesses. Since then, the EnvironmentalProtection Agency has devoted enormous sums of money to cleaning up "Superfund" sites, which carry residues from environmentally thoughtless industies orpoorly designed landfills. Because of aggressive legislation throughout thedeveloped world, waste disposal in landfills and from industrial sites is greatly reduced. These hazards still create real misery in many developing countries, and even in industrial areas of the former Soviet Union and Eastern Europe.
Radioactivity in the environment is another cause for long-term concern. Thenuclear disaster at Chernobyl in 1986 caused a handful of immediate deaths and a large number of cases of radiation illness. News reports since that disaster indicate that people living near the Chernobyl plant at the time of the disaster suffer from a wide range of medical problems, and children born sincethen suffer from an increased level of obvious and less-obvious genetic disorders and birth defects. Thousands of square miles around Chernobyl are now closed off to any residents, although workers continue to tend both the functioning reactors and the ruined unit at the plant itself.
Throughout the world, many nuclear power plants are reaching the end of theiruseful lives, at least as projected at the time they were built. In poorer industrial countries, such as Russia, there are no current plans to replace the power generated by these plants, so it seems that they will remain in use.Although it is logical to think that these very old plants will become increasingly dangerous, only time and experience will tell.
Even safe and well-run nuclear plants present environmental problems. In theUnited States, attempts to provide for long-term storage of "high-level" nuclear waste have been tied up in political disputes for decades. The chief concern is for the future environment: Wastes from nuclear power plants will be extremely dangerous for thousands of years, and how can we devise a disposal site that can remain secure for a period that is roughly the same as the period from ancient Egypt to the present? Currently waste is stored on-site at nuclear power plants, and this arrangement, which is supposed to be temporary, will continue for some time into the future.
In addition, "low-level" nuclear waste is generated every day from certain medical tests and scientific research. These wastes are not as acutely dangerous as high-level wastes, and the danger from these wastes tends to persist fordecades rather than millennia. These wastes are handled by individual states, or groups of states, in the United States.
The Indoor Environment
Environmental health concerns can also involve the environment indoors, in our homes, schools, and offices. In many older buildings, asbestos was used asinsultation, and was even a component of ceiling and floor tiles. This material can cause deadly respriatory problems if inhaled. Similarly, paint manufactured before the 1970s uses lead-based pigments. If eaten or inhaled, lead can cause neurologic problems. These problems are most harmful to the developing brains of infants and young children. Neither material causes great dangersunless it develops cracks or flakes, or if it is disturbed by building renovations, repainting, or similar activities. This means that older buildings must be examined very carefully before any repairs begin. If the inspection reveals asbestos or lead-based paint, it will probably require specialized preparation to ensure the safety of occupants and workers.
Other indoor environmental hazards can include chemical fumes from new carpeting, or even a buildup of stale air in very tightly insulated buildings. In other tightly sealed buildings, molds and spores can cause a range of allergicsymptoms. Authorities report hundreds of incidents each year of carbon monoxide poisoning, generally caused by defective heating furnaces.
Radon, a colorless, odorless gas that is radioactive, is an environmental hazard in some homes. Because radon molecules are much heavier that most gas molecules in the atmosphere, radon gas tends to collect in basements in some regions of the country. It can contribute to cases of lung cancer, particularlyin smokers, and authorities believe that the increase in radioactivity can have other long-term health effects, as well.
In very cold weather, well heated buildings will tend to have very low relative humidity. This can lead to irritated skin, among other problems. Finally,warm, heated, well insulated buildings provide a breeding ground for virusesto multiply and spread. Respiratory illnesses such as colds and flu are the most likely to cause problems in such buildings.
Legionnaire's Disease is a special case of an environmental respiratory disease. The bacterium that causes this disease can multiply in slow-flowing watersupplies. Often this involves water used for cooling in large air conditioning units, but it may involve drinking water supplies as well. This disease usually occurs in people with pre-existing risk factors such as smoking, asthma, and other lung diseases. Outbreaks continue to occur sporadically; typically about 15% of those who get the disease will die, and tracing the source ofoutbreaks is often difficult.
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 riskof 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 ourlives, avoid dangerous jobs, lower indoor pollutants, practice safe sex, avoid sun exposure, and prevent household accidents. Many of these factors overwhich we have control are much more risky than the unknown, uncontrollable, environmental hazards we fear so much.
Hazards of the Natrual Environment
Beyond pollutants, the natural environment itself can cause major health problems under certain conditions. For instance, heat can cause serious problemsfor many people. Heat exhaustion or heat stroke can occur among people who over-exert themselves or fail to consume enough water in hot weather. More tragically, frail elderly people sometimes become sick or even die in overheateddwellings. Especially in northern big cities, older apartments and houses were designed to keep heat in during cold winters. Unfortunately, they also trapheat during summer heat waves. Elderly people with heart or respiratory conditions are particularly vulnerable to this stress.
Similarly, cold weather can affect almost anyone through disorders ranging from frostbite to cold-induced asthma. Again, the frail elderly are particularly vulnerable. To save money, they may use indoor heating only sparingly, making them subject to hypothermia, or low body temperature. This can be fatal, and every winter it causes the deaths of some older people.
As mentioned earlier, excessive exposure to sunlight can cause skin cancers,particularly in light-skinned people. As well, cataract development seems tobe related to exposure to sunlight. Lenses for both sunglasses and even regular spectacles sometimes offer UV-protective lenses. So far, there is little direct evidence about the effectiveness of these lenses, but the added cost isrelatively low and may appeal to prudent people.
High altitudes are a fascinating special case of the natural environment. Physicians have performed a large body of research on climbs up Mt. Everest andother major peaks, and they also investigate problems of high-altitude flight. Greatly reduced oxygen available from the thin air, combined with cold temperatures and sometimes increased exposure to ultraviolet light, create uniquemedical problems.