Human health effects of benzene, arsenic, cadmium, nickel, lead and mercury: report of an expert consultation

Benzene, arsenic, cadmium, lead, mercury and nickel are ubiquitous pollutants in ambient air. The main sources are industrial processes, electricity generation and fuel combustion. The main routes of exposure are inhalation for benzene, and diet for arsenic, cadmium, lead, nickel and mercury. Inhalation of benzene, arsenic and cadmium is relevant for exposure in active tobacco smokers and people exposed to second-hand tobacco smoke. Epidemiological studies show that exposure to these pollutants is associated with adverse effects on the cardiovascular (‎cadmium, lead, mercury)‎; haematological (‎benzene, lead)‎; immunological, neurological and reproductive (‎benzene, lead, mercury)‎; respiratory (‎cadmium, nickel)‎; renal (‎cadmium, lead)‎; and skeletal (‎cadmium)‎ systems. Limited epidemiological evidence on ambient air pollution suggests adverse effects on the cardiovascular system (‎arsenic and nickel)‎. Since benzene, arsenic, cadmium and nickel are classified as carcinogenic, the lowest possible exposure level is suggested to minimize the risk for cancer development in view of the no-effect threshold paradigm. Lead and methylmercury compounds are classified as possibly carcinogenic to humans. However, the available evidence is insufficient to warrant updating the air quality guidelines for these air pollutants. Evidence gaps are identified and these should guide future research efforts.