Malathion is a man-made organophosphate insecticide that has been registered for use primarily as a mosquito control with the United States Environmental Protection Agency (EPA) since the mid-1950s. Specifically, it is an organophosphate parasympathomimetic. This means that it belongs to a class of chemicals that impact the parasympathetic nervous system. Other members of this chemical family include tear gas and sarin.
The mechanism of an organophosphate parasympathomimetic is to trigger acetylcholine release, the chief neurotransmitter utilized by the parasympathetic nervous system. This is achieved by either stimulating nicotinic or muscarinic receptors, or by suppressing cholinesterase release. The insecticidal properties of malathion are due to the latter.
To treat mosquito infestations, malathion is often applied as a fog or fine mist containing diesel fuel. It has also been mixed with corn syrup and sprayed from the air to combat the Mediterranean Fruit Fly in the Western U.S. In the late 1990s, malathion was one of the pesticides sprayed in the New York City vicinity to help deter West Nile virus. In addition to being used in public mosquito control programs, the insecticide is also approved for use in parks and other recreational areas and in agriculture.
The U.S. Food and Drug Administration approves of the inclusion of malathion in a prescription lotion to treat head lice. The agency also permits its use in products formulated to kill ants and other pests in residential yards and gardens. It may also be found in indoor pest control products, as well as in pet care products intended to treat fleas and ticks.
While the EPA maintains that there is not enough evidence to declare malathion a carcinogen, it is still suspected as such. Experiments using rodents have resulted in tumor formations of the liver, although exposure amounts exceeded levels that most humans are likely to encounter. However, the results of other animal-based studies suggest that the insecticide may be an endocrine disruptor.
Malathion itself is considered relatively safe for mammals and birds, but as an insecticide, does pose a risk to honeybees. This substance also degrades quickly in the environment if sufficient moisture and light are present. However, under certain conditions, it may degrade into malaoxon, which is more than 60 times more toxic than the original compound. For instance, drinking water exposed to malathion spray may later form malaoxon at the treatment plant during chlorination. For this reason, care should be taken to limit contact with reservoirs and streams.
This insecticide will also convert to malaoxon in the human body if absorbed or ingested. Symptoms of malathion toxicity include skin rash, blurred vision, vomiting, cramping, seizures, heart palpitations, headaches, and profuse sweating. In cases of high toxicity, depression of the central nervous system may occur.