 although the toxicity of lead was recognized centuries ago , concern was restricted to overt symptoms : colic , encephalopathy , anemia , or renal disease . 
 two major reasons for lack of progress in restricting toxicity were that interest was limited to occupational exposures and there was lack of awareness of specific biochemical or metabolic effects . 
 identification of subclinical effects has been possible the last 15 or 20 years because of the development of sensitive measures to detect cognitive and behavioral changes that are not apparent clinically and because of methods to measure the reduced activity of heme enzymes . 
 this progress was driven by basic and clinical research that resulted in a better understanding of cellular toxicology . 
 the new awareness prompted the lowering of acceptable occupational exposures , as measured by blood lead from 80 to 40 to 60 micrograms / dl range , and the establishment of maximum recommended exposures in children to a blood lead level of 25 micrograms / dl . lowering 
 the lead content in gasoline has been accomplished by a nearly 50% decrease in average blood levels of persons in the united states ( nhanes ii data ) . 
 current research implicates lead as a contributing etiologic factor in a number of common diseases affecting large portions of the population such as subtle cognitive and neurological deficits , hypertension , congenital malformations , immunotoxicity , and deficits in growth and development . for each of these disorders there may be multiple etiologic factors 
 ; the scientific challenge is to develop sensitive methodology to detect the specific role of lead . 
 other potential subtle health effects include the influence of small amounts of lead on cell proliferation and lead as a cofactor in carcinogenesis.(abstract truncated at 250 words)imagesfigure 1 . 
Images