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As citizens of the 21st century, we are guinea pigs in an unprecedented worldwide experiment. No one is in charge of the research, nor have we been asked to participate in the experiment. But we all have a life or death stake in the outcome.
Here's how it works. Almost every day, we may be exposed to any of an estimated 100,000 chemicals, many of them brand-new, man-made substances that the human body has never encountered before. We meet up with them everywhere—in the foods we eat, the clothes we wear, the air we breathe. No one yet knows what their health effects will be. Some environmental chemicals have already been shown to cause cancer—asbestos, for example, and the contaminants of cigarette smoke. Others, including lead and pesticides like DDT, are linked to birth defects and mental retardation.
Now researchers fear another danger. Some chemicals in the environment are immunotoxic—poison to our immune system. And their effect may be insidious. Immunotoxins building up in the environment may be depleting our immune reserves and weakening the very defenses that should protect us. That could leave us open to the spread of cancer and more susceptible to infections as we grow older, a time when immune function normally declines. Young children, whose immune systems are still developing, and people with chronic illnesses such as asthma, may be especially vulnerable. Environmental pollutants, some experts fear, may already be posing a threat to longevity.
Perhaps most worrisome of all is what researchers don't know. Most studies focus on the effects of exposure to single contaminants. In real life, we're being exposed to many of these chemicals at the same time. Experiments usually look at the effects of high doses over short periods. But researchers know very little about the effects of low doses over a lifetime—the way most of us are exposed. And the number of new, man-made chemicals in the environment is constantly growing. "I'm not convinced that the current levels of exposure are dangerous at present," said one professor of pharmacology and toxicology. “But if we don't keep the levels of some of these environmental toxins down, we could have very real problems."
What are the risks to longevity? Which chemicals are potentially most damaging? And what are scientists doing to make sure the growing number of chemicals in the environment doesn't end up leaving us defenseless?
Sounding the Alarm
Doctors already know how vulnerable we are to devastating infections and diseases when our immune systems are deliberately suppressed for medical reasons. Physicians routinely use drugs to do this. For example, powerful immune suppressing drugs like cyclosporine are used to keep the immune system from rejecting a transplanted heart or kidney. Such drugs are administered under careful medical surveillance. Even then, patients can fall prey to serious, sometimes fatal infections.
That's why there's growing alarm about what toxins in the environment may be doing to our immune systems. Consider the evidence: In 1971, oil accidentally laced with dioxin was used to hold down dust on roads near the Quail Run Trailer Park outside St. Louis. Afterward, researchers detected reduced levels of protective antibodies, indicating abnormal immune responses in exposed residents.
To date, experts in the new field of immunotoxicology have identified some dozens immune-damaging chemicals, ranging from the sulfur dioxide and nitrogen dioxide spewed out of power plants and autos to metals like lead and mercury. Only a small number of environmental chemicals have been thoroughly tested for their effect on the immune system.
More and more pollutants were released into our air, water, and soil weaken our immune systems just during the past decade. Chemicals called organotins, used in paint to keep barnacles from forming on the bottom of boats, have been shown to damage lab animals' immune systems. And like several immunotoxic pesticides, they appear to be building up in our water supply. Heavy metals like lead and mercury can weaken our ability to fend off all illnesses. So can the volatile organic compounds used in many paints and solvents. There are the invisible threats of radiation and other potentially harmful electromagnetic waves. Even keeping your laptop on your lap is being considered dangerous by some researchers. Repeated exposure to the sun's rays can damage immune cells in the skin. There's continued concern that electromagnetic fields created by high-voltage power lines also may weaken the immune system. Immune cells rely upon a slight electrical field to communicate with each other. Researchers speculate that exposure to the power lines' higher electrical fields may impair cell-to-cell communication.
Every Breath You Take
But perhaps most worrisome of all are the immunotoxic chemicals that we breathe. Inhaled gases readily find their way into the bloodstream. And although the lungs swarm with scavenger cells whose job it is to engulf and devour particles from the atmosphere, some pollutants—asbestos and silica, for example—can't be digested and may seriously undermine this early line of defense.
One of the most prevalent and widely studied is nitrogen dioxide (NO2). It is responsible for the blight of brownish smog that hangs over Los Angeles, New York City, Chicago, and other American cities most of the year, and it provides a case study of the damaging effects of pollution on the immune system. Let alone the damage it causes to the environment. Hairlike cells, called ciliated cells, which line the airways and help expel bacteria and foreign debris, are highly susceptible to NO2 damage; this may explain why ailments like asthma and bronchitis are more common in smoggy areas.
Nitrogen dioxide's injurious effects reach far beyond the lungs. When NO2 hits the mucous membranes of the nose, mouth, and airways, it reacts with water to form nitric acid and other chemical by-products that, within minutes, spread throughout the body. Research has shown that the NO2 levels you might breathe on a typical LA morning can impair the function of macrophages, which alert other immune cells to the presence of potentially dangerous bacteria or viruses. Lab animals exposed to NO2 levels found in many smoggy cities also show decreased antibody production, as well as abnormal disease-fighting T cells.
No one knows for certain how airborne pollutants like NO2 do their damage. Both NO and Ozone (another component of urban Smog) release free radicals—highly reactive molecules that have been implicated in the aging process. One effect may simply be to hasten the loss of lung-tissue elasticity that accompanies aging. Another may be to break down the immune system's lines of communication. Immune cells communicate with one another via proteins and protein receptors on their surfaces. Communication is crucial to defense strategy. If those receptors are damaged, the cell's ability to communicate may be knocked out.
Of equal concern, researchers at UCLA found that urban smog destroys connective tissue called elastin, which forms the scaffolding of the lungs—and skin—supporting all the other cells, including immune cells. Once elastin is destroyed, it may be impossible for the body to restore the structure of the lung.
Pollutants may also make us more vulnerable to cancer. A healthy immune system constantly spots and tries to destroy cancer cells as they arise—long before they can develop into tumors. But some pollutants appear to be linked to both immune damage and cancer. While many forms of polycyclic aromatic hydrocarbons exist (the PCBs and PBBs, for instance), only some appear to cause cancer. What we're finding is that some of the cancer causing forms of these hydrocarbons also happen to be the ones that suppress the immune system.
Pollutants may contribute to cancer in a variety of ways. Cancer is a complex, multistep process. The first step occurs when a single gene—an oncogene—is transformed within a healthy cell. A variety of exposures, from ultraviolet radiation to substances like asbestos, seem to be able to trigger these genes.
A single oncogene isn't enough to create cancer. We now suspect that at least two mutations are necessary. And the chances of that are about one in 100 trillion. Cancer experts now think that the real cancer danger may be substances that favor the growth of mutated cells, thus promoting the likelihood that a second, cancer-causing mutation will occur.
It's possible that air pollution may be a factor in promoting skin cancer and perhaps other cancers that strike lining tissues, as in the lungs. And certain pollutants are what researchers call complete carcinogens. They both trigger a cancerous gene change and promote the growth of these mutated cells. Some researchers believe that benzo(a)pyrene, one of the most common car-exhaust contaminants, may pack just such a double whammy.
Laboratory findings are one thing, real world proof is another. It has been much harder for researchers to prove that the levels of pollution most of us are exposed to are making us sick or shortening our life through their effects on the immune system. If pollutants began killing us at a sudden rate, we'd know about it. But it's much harder to document subtle effects. How can we measure exactly how long a cold lasts or how bad a case of influenza becomes?
The Health Effects Institute's Thomas J. Kulle, Ph.D. exposed human subjects to NO2 levels commonly measured in US cities and then tested their resistance to influenza viruses. People breathing NO2 tended to become infected more often than those breathing clean, filtered air. But Over the three-year study, the difference in numbers was too small to be deemed significant.
The stormy debate over the health effects of dioxin-containing Agent Orange provides another case in point. In animal studies, dioxin targets the thymus—a small gland behind the breastbone where T cells mature. Dioxin can substantially impair the developing immune systems of young mice. But When the US Air Force and, more recently, the US Centers for Disease Control (CDC) studied soldiers exposed to Agent Orange, they found no evidence of immune-system abnormalities. Veterans groups have protested that government agencies can't be trusted to investigate such issues objectively. And in truth, the CDC study leaves disturbing questions.
Animal studies provide some insights. But what harms a mouse immune system may have no human effects at all. Medical ethics prevent researchers from exposing human subjects to potentially dangerous chemicals, so the only really instructive human experiments have taken the grim form of accidental exposures like the contamination of meat and dairy products in Michigan. But accidents usually expose people to high doses over a relatively short time. We know very little, at least in terms of direct evidence, about the effect of low-level exposures over long periods of time. That's really the kind of exposure most of us have to environmental pollutants.
Nor do researchers know very much about the effects of exposure to a multitude of pollutants at the same time. One tough question is, what combinations should we look at? Combined exposures in one part of the country may be very different than in another. Even in the same city, the combination of exposures varies widely from location to location.
Protecting Immune Reserves
Perhaps the biggest unanswered question remains the immune system itself. Researchers can measure, with precision, the number and function of the immune system's many cell types. But it's still not clear what their numbers mean in terms of health or susceptibility to illness. Immune parameters among healthy people vary widely. And within an individual, immune-cell counts are constantly changing, affected by everything from stress to depression.
Do low numbers of immune cells necessarily mean declining health? In extreme cases, yes. When enough immune cells are killed off by the AIDS virus, for example, serious infections begin to appear. And the use of immune-suppressing drugs, mentioned earlier, after transplant surgery does pose a danger of infections.
One reason may be that the immune system, like many of the body's systems, has a built-in functional reserve, which provides protection even when the system is weakened. Significant immune injury can take place before the health effects show up.
But when it comes to environmental toxins, should we wait that long? As long as you don't need those reserves, it doesn't matter. The loss of reserves could mean that what might have been a simple case of influenza develops into pneumonia. Or a bout of pneumonia that might have been mild proves fatal.
Maintaining a healthy immune reserve may be particularly important for the young and old. Young children may be more susceptible to the effects of toxins like dioxin. And as we grow older, our immune systems decline, making us more susceptible to such infections as influenza and pneumonia, and to more severe bouts. It is therefore increasingly important as we age to maintain healthy diets and—a temporary boost to the immune system—regular exercise.
Along with growing interest in the science of immunotoxicology have come calls for stricter screening of new chemicals. Toxicologists have proposed more stringent immunotoxicity testing for pesticides. And the evidence of hidden damage to the body's health reserves has led Scientists like USC's Sherwin to advocate stricter environmental controls. It's not enough to base standards on the health of the whole person, he argues. Our ultimate well-being—how well we feel, how much energy we have, how long we live—depends on the health of the cells in Our bodies. Anything that threatens them, threatens us.