The risk to an individual worker from exposure to an organic solvent is more directly related to the uptake of the solvent than its level in the atmosphere. Uptake varies from worker to worker, depending on exercise, respiratory fitness and the amount of stored adipose tissue. Clearance and metabolism of a solvent also vary between individuals, being affected by genetic and environmental factors. Uptake may be estimated by measuring the solvent or its metabolites in breath, blood or urine, but any sampling strategy must be designed to exclude unnecessary variation introduced by different metabolic handling rates occurring in individual workers.