Human toxicity results from ingestion or dermal contact with the agent prior to their application to weeds and soil. In Malaysia, paraquat is used without adequate regulation and protection. Workers have been found stirring vats of the deadly poison without protection for their arms and hands. Bipyridyliums are poorly absorbed by inhalation owing to the large size of the suspended droplets and significant toxicity has bot been reported by this route. The mechanism of action is related to single electron reduction, oxidation with flavoproteins and molecular oxygen and peroxidation of lipid cellular membranes.
Paraquat presents a very high hazard due to irreversible lung damage.
The most specific effect for paraquat is pulmonary fibrosis. In acute intoxication, hepatic failure and renal insufficiency are also very common. Gastrointestinal and central nervous system impairment may also occur. Occupational intoxications most result from local contamination. Nail damage following repeated contamination with diluted paraquat also occur.