Several studies have reported the occurrence of sensory neuropathy with exposure 
to chlorpyrifos and other organophosphorus insecticides, at levels not 
associated with overt toxicity. We evaluated 113 chemical workers, including 53 
of 66 (80%) eligible chlorpyrifos workers and 60 of 74 (81%) randomly selected 
referent workers, to identify evidence of sensory neuropathy or subclinical 
neuropathy. Compared to referents, chlorpyrifos subjects had significantly 
longer duration of work in chlorpyrifos-exposed areas (9.72 vs. 0.01 years; P < 
0.0001), greater cumulative chlorpyrifos exposure (64.16 vs. 0.69 mg/m(3). day; 
P < 0.0001), higher urine 3,5,6-trichloro-2-pyridinol (TCP) excretion (108.6 vs. 
4.3 microg/g creatinine; P < 0.0001), and lower plasma butyrylcholinesterase 
(BuChE) activity (7281 vs. 8176 mU/ml; P = 0.003). Despite exposures among 
chlorpyrifos subjects to levels at which well-described physiological effects on 
B-esterases exist, the frequency of symptoms or signs of neuropathy did not 
differ significantly between groups, and the only 2 subjects fulfilling criteria 
for confirmed neuropathy were both in the referent group. Mean nerve conduction 
study results were comparable to established control values and did not differ 
significantly between groups. We found no evidence of sensory neuropathy or 
isolated peripheral abnormalities among subjects with long-term chlorpyrifos 
exposure at levels known to be associated with the manufacturing process.