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Paraquat and marijuana: epidemiologic risk assessment.

P J Landrigan, K E Powell, L M James and P R Taylor

In March 1978, 13 (21 per cent) of 61 marijuana samples from the southwestern United States were found to be contaminated with the herbicide paraquat, a pulmonary toxin, in concentrations from 3 to 2,264 parts per million. The source of the contamination was an aerial spraying program in Mexico, supported indirectly by United States funds. To evaluate US exposure, a nationwide survey of the paraquat content of confiscated marijuana was conducted. The survey found 33 (3.6 per cent) of 910 marijuana specimens to contain detectable paraquat. In states adjacent to Mexico (Census Region VI), 23 (12.8 per cent) of 180 specimens were contaminated. Combustion testing indicated that approximately 0.2 per cent of paraquat on marijuana passes into smoke. From these data, we projected that 100-200 marijuana smokers in Census Region VI would be exposed by inhalation to 500 micrograms or more of paraquat per year, a dose judged to represent a health hazard; nationally, between 150 and 300 smokers were projected to have such exposure. Another 6,000 persons in Region VI and 9,000 nationally were projected to be at risk of exposure to between 100 and 499 micrograms of paraquat annually. The risk of paraquat exposure was greatest among those smokers who make one large purchase of marijuana per year. No clinical cases of paraquat poisoning were recognized among marijuana smokers during these studies, but no systematic national search for such cases was undertaken.