Malathion was used in the 1980s in California to combat the Mediterranean Fruit Fly. This was accomplished on a wide scale by the near weekly aerial spraying of suburban communities for a period of several months. Formations of three or four agricultural helicopters would overfly suburban portions of Alameda County, San Bernardino county, and Santa Clara County releasing a mixture of malathion and corn syrup, the corn syrup being a bait for the fruit flies. Malathion has also been used to combat the Mediterranean Fruit Fly in Australia.
Malathion was sprayed in many cities to combat West Nile virus. In the Fall of 1999 and the Spring of 2000, Long Island and the five boroughs of New York City were sprayed with several pesticides, one of which was malathion. While it was claimed by some anti-pesticide groups that use of these pesticides caused a lobster die-off in Long Island Sound, there is as of yet no conclusive evidence to support this. Research, however, is still continuing on this topic.
Manitoba, Province of Canada, ordered the city of Winnipeg, Manitoba to be sprayed in July 2005 as part of the West Nile virus campaign. Prior to this, Malathion was used over the last couple of decades on regular basis during summer months to kill nuisance mosquitoes, but homeowners were allowed to exempt their properties if they chose. Today, Winnipeg is the only major city in Canada with an ongoing Malathion nuisance-adult-mosquito-control program.
Malathion is also used in conjunction with diesel fuel to fog an area where there is an infestation of mosquitoes. By diluting the mixture, it becomes much weaker. It is possible to dilute the mixture to the point where mosquitoes are not killed, but become more resistant to the mixture, making it less effective in subsequent foggings.
Malathion itself is of low toxicity; however, absorption or ingestion into the human body readily results in its metabolism to malaoxon, which is substantially more toxic. Chronic exposure to low levels of malathion have been hypothesized to impair memory, but this is disputed. There is currently no reliable information on adverse health effects of chronic exposure to malathion. Acute exposure to extremely high levels of malathion will cause body-wide symptoms whose intensity will be dependent on the severity of exposure. Possible symptoms include skin and eye irritation, cramps, nausea, diarrhea, excessive sweating, seizures and even death. Most symptoms tend to resolve within several weeks. Malathion present in untreated water is converted to malaoxon during the chlorination phase of water treatment, so malathion should not be used in waters that may be used as a source for drinking water, or any upstream waters.
In 1976, numerous malaria workers in Pakistan were poisoned by isomalathion, a contaminant that may be present in some preparations of malathion. It is capable of inhibiting carboxyesterase enzymes in those exposed to it. It was discovered that poor work practices had resulted in excessive direct skin contact with isomalathion contained in the malathion solutions. Implementation of good work practices, and the cessation of use of malathion contaminated with isomalathion led to the cessation of poisoning cases.
Malathion breaks down into Malaoxon, which is 61 times more toxic than Malathion. For this reason, if Malathion is used or somehow enters an indoor environment, as it breaks down into Malaoxon, it can seriously poison the occupants living or working in this environmnent.
Malathion is classified by US EPA as having “suggestive evidence of carcinogenicity but not sufficient to assess human carcinogenic potential.” This rating implies that insufficient evidence is available to either rule out malathion as a carcinogen, or to state that it is a carcinogen. No studies on carcinogenicity have been performed in humans; however, studies in rats and mice have yielded conflicting results. Liver tumours were found to be induced in rats, but only at excessive doses. On the other hand, malaoxon, a structurally related chemical, was found not induce tumour formation in rats. A review of the classification of malathion as 'suggestive' was carried out in 2000, by the FIFRA Scientific Advisory Panel. The conclusion of this panel was that there was still insufficient evidence to either declare malathion as non-carcinogenic, or to declare it a carcinogen.