Much of our scientific knowledge about the health effects of chemicals comes from laboratory studies of animals. While humans are not necessarily susceptible to all of the diseases that chemicals cause in test animals, there are many similarities between the way humans and other species respond to toxic exposures. Former U.S. Assistant Surgeon General David Rall noted that every chemical known to cause cancer in humans also causes cancer in experimental animals. While some species are more sensitive to toxic effects than others, Rall wrote, laboratory studies have proved to be good predictors of health effects in humans. Fundamental biological systems are remarkably similar in all mammals, and - at the cellular and biochemical level - most species respond to toxic substances in similar ways. Based on years of comparative studies, biologists believe that many of the health problems observed in animals could also occur in people at some exposure level. However, conclusive proof is often elusive. Since researchers cannot ethically dose humans with suspected toxicants, they must rely on studies of people accidentally poisoned or exposed in the workplace. The normal variables of human life make such studies less than scientifically ideal. For instance, workers may be exposed to multiple chemicals, or poisoning victims may have smoked cigarettes, confounding efforts to assess the damage done by a single chemical. But combined with animal studies, this epidemiological research has provided enough evidence to raise concerns about many chemicals in common use. Sorting out the health effects of low level exposures may be the most difficult task of all. Because most toxicity studies look at people exposed in the workplace, where exposure levels are much higher than what most of the world's population would see, the health effects of the average levels of chemicals found in people are largely unknown. However, according to Dr. Michael McCally of Mt. Sinai, the study's lead doctor, "Current 'normal' body burdens of dioxin and several other organochlorines in humans are at or near the range at which toxic effects occur in laboratory animals." Research has made one thing clear – there is tremendous variability between individuals in their responses to toxic substances. Even the highest level exposures will not make everyone sick. Among workers with similar exposures to vinyl chloride, for example, one might be stricken with a brain tumor, another with liver cancer, and a third with bone disease - but no cancer at all. Still others may remain completely healthy. On the other hand, people with far lower exposures may also suffer health problems. The Environmental Protection Agency acknowledges that some groups of people are far more susceptible to toxic effects than others. Children are among the most vulnerable. It is known that women pass some of their body burdens to their children during pregnancy. Most scientists concur that exposures in utero and in early childhood pose far greater risks than exposures later in life. (See Children at Risk) "There's another notion that tiny doses, if given at the right time, can be terribly effective and terribly injurious." - Michael McCally, M.D., Ph.D.