Theron Randolph, the great pioneer of clinical ecology (the study of allergy or sensitivity to substances in the environment), developed the theory that illness is caused when the body's ability to detoxify environmental stressors is overloaded (Ann Allergy, 1978; 40: 333-45). Single, intermittent exposure to chemicals may cause no obvious harm, but if the exposure is repeated, the immune system can become overwhelmed, even at low levels.
For example, painters occupationally exposed to volatile organic compounds (VOCs) were found to have significantly more adverse reactions to VOCs than non painters, even though the individual exposures were below the accepted threshold for harmful effects (Scand J Work Environ Health, 1976; 4: 240-55).Randolph's concept is known as the total load theory, or the more popular 'rain-barrel effect'. When more toxins enter the barrel than the body can detoxify and excrete, the barrel overflows and symptoms develop. This explains why some people may react to environmental toxins while others do not.
Randolph had a wide concept of what environmental stressors might be: as well as chemicals, he included infections and psychosocial factors, such as depression and stress. Thus, a psychological upset in someone already exposed to environmental chemicals might overload the immune system and lead to genuine physical illness. This fact has sometimes led sceptical doctors to dismiss chemical sensitivity as 'psychological'.