BACKGROUND: Organophosphorus pesticides such as chlorpyrifos and malathion are 
widely used insecticides. They do not bioaccumulate appreciably in humans and 
are rapidly metabolized and excreted in the urine. In nonoccupational settings, 
exposures to these pesticides are typically sporadic and short-lived because the 
pesticides tend to degrade in the environment over time; however, dietary 
exposures may be more chronic. Biologic monitoring has been widely used to 
assess exposures, susceptibility, and effects of chlorpyrifos and malathion; 
thus, the information base on these compounds is data rich. For biomonitoring of 
exposure, chlorpyrifos and malathion have been measured in blood, but most 
typically their urinary metabolites have been measured. For assessing early 
effects and susceptibility, cholinesterase and microsomal esterase activities, 
respectively, have been measured.
OBJECTIVES: Although many biologic monitoring data have been generated and 
published on these chemicals, their interpretation is not straightforward. For 
example, exposure to environmental degradates of chlorpyrifos and malathion may 
potentially increase f urinary metabolite levels, thus leading to overestimation 
of exposure. Also, the temporal nature of the exposures makes the evaluation of 
both exposure and effects difficult. We present an overview of the current 
biomonitoring and other relevant data available on exposure to chlorpyrifos and 
malathion and the use of these data in various environmental public health 
applications.