Concern over the impact on human health of chemicals in the environment has increased in recent years. It has become clear, for example, that a number of chemicals act specifically on the heart muscle and thus contribute to the development of heart diseases. Living and working conditions and activities, in conjunction with other stressors, can also cause an increase in morbidity rates of (for example) ischaemic heart disease, metabolic disorders and nervous conditions. Relationships with environmental factors have also come to light, such as that between soft water and the incidence of heart diseases, fluorine deficiency and excessive dental decay, increased arsenic in soil and water and the enhanced occurrence of cancer, and chlorine compounds and male infertility. In evaluating human exposure to potentially hazardous chemicals it is therefore necessary to consider all pathways - in air, water, soil and food, and both the home and working environment. The rate of circulation of many elements through the environment has been greatly increased by man's activities. In the mid-1970s over 1,500 substances were produced in quantities exceeding 500 tonnes per year (worldwide), and over 50 substances were produced in quantities over 1 million tonnes. In 1950, world production or organic chemicals was 7 million tonnes; in 1970, 63 million tonnes; and the 1985 figure is expected to be 250 million tonnes. Despite increasingly stringent industrial controls, some substances that now occur in negligible concentrations (say less than one part per million) may present serious problems to the next and succeeding generations.