As described above, the potential hazardous effects of some chemicals in the environment are receiving considerable attention because of their estrogen-like or antiandrogenic properties. Compounds that affect thyroid function are also of concern. Since 1998, the process of prioritization, screening, and testing of chemicals for such actions has been undergoing worldwide development. These chemicals mimic, enhance, or inhibit a hormonal action. They include a number of plant constituents (phytoestrogens) and some mycoestrogens as well as industrial chemicals, persistent organochlorine agents (eg, DDT), PCBs, and brominated flame retardants. Concerns exist because of their increasing contamination of the environment, the appearance of bioaccumulation, and their potential for toxicity. In vitro assays alone are unreliable for regulatory purposes, and animal studies are considered indispensable. Modified endocrine responses in reptiles and marine invertebrates have been observed. In humans, however, a causal relation between exposure to a specific environmental agent and an adverse health effect due to endocrine modulation has not been fully established. Epidemiologic studies of populations exposed to higher concentrations of endocrine-disrupting environmental chemicals are underway. There are indications that breast and other reproductive cancers are increased in these patients. Prudence dictates that exposure to environmental chemicals that disrupt endocrine function should be reduced.
Asbestos in many of its forms has been widely used in industry for over 100 years. All forms of asbestos have been shown to cause progressive fibrotic lung disease (asbestosis), lung cancer, and mesothelioma. Every form of asbestos, including chrysotile asbestos, causes an increase in lung cancer and mesothelioma. Lung cancer occurs in people exposed at fiber concentrations well below concentrations that produce asbestosis. Very large-scale studies of insulation workers have shown that cigarette smoking and exposure to radon daughters increase the incidence of asbestos-caused lung cancer in a synergistic fashion. Asbestos exposure and smoking is a very hazardous combination.
All forms of asbestos cause mesothelioma of the pleura or peritoneum at very low doses. Other cancers (colon, laryngeal, stomach, and perhaps even lymphoma) are increased in asbestos-exposed patients. The mechanism for asbestos-caused cancer is not yet delineated. Arguments that chrysotile asbestos does not cause mesothelioma are contradicted by many epidemiologic studies of worker populations. Recognition that all forms of asbestos are dangerous and carcinogenic has led many countries to ban all uses of asbestos. Countries such as Canada, Zimbabwe, Russia, Brazil, and others that still produce asbestos argue that asbestos can be used safely with careful workplace environmental controls. However, studies of industrial practice make the “safe use” of asbestos highly improbable. Recent attempts to limit international trade in asbestos have been thwarted by heavy pressure from the asbestos industry and the producing countries. Information on countries that currently ban asbestos and the International Ban Asbestos movement may be found at http://ibasecretariat.org/ alpha_ban_list.php.
Occupational and environmental poisoning with metals, metalloids, and metal compounds is a major health problem. Toxic metal exposure occurs in many industries, in the home, and elsewhere in the nonoccupational environment. The classic metal poisons (arsenic, lead, and mercury) continue to be widely used. (Treatment of their toxicities is discussed in Chapter 57.) Occupational exposure and poisoning due to beryllium, cadmium, manganese, and uranium are relatively new occupational problems. In 2016, cobalt and cobalt-releasing compounds were listed by the National Institute of Environmental Health Sciences as “reasonably anticipated to be” human carcinogens.