Malathion is an organophosphate (OP) insecticide that has been registered for use in the United States since 1956. It is used in agriculture, residential gardens, public recreation areas, and in public health pest control programs. When applied in accordance with the rate of application and safety precautions specified on the label, malathion can be used to kill mosquitoes without posing unreasonable risks to human health or the environment.

How is Malathion Used in Mosquito Control?

The mosquito goes through four distinct stages during its life cycle: egg, larva, pupa, and adult. Malathion is an adulticide, used to kill adult mosquitoes. In mosquito control programs conducted by state or local authorities, malathion is applied by truck-mounted or aircraft-mounted sprayers. Malathion is applied as an ultra-low volume spray. ULV sprayers dispense very fine aerosol droplets that stay aloft and kill mosquitoes on contact. ULV applications involve small quantities of pesticide active ingredient in relation to the size of the area treated. For mosquito control, malathion is applied at a maximum rate of 0.23 pounds (less than 4 ounces) of active ingredient per acre per day, which minimizes exposure and risks to people and the environment.

Does Malathion Pose Risks to Human Health?

Malathion can be used for public health mosquito control programs without posing unreasonable risks to the general population when applied according to the label. We have estimated the exposure and risks to both adults and children posed by ULV aerial and ground applications of malathion. Because of the very small amount of active ingredient released per acre of ground, the estimates found that for all scenarios considered, exposures were several times below an amount that might pose a health concern. These estimates assumed that a toddler would ingest some soil and grass in addition to skin and inhalation exposure. However, at high doses, well above those for normal labeled uses, malathion, like other organophosphates, can overstimulate the nervous system causing nausea, dizziness, or confusion. Severe high-dose poisoning with any organophosphate can cause convulsions, respiratory paralysis, and death.

Does Malathion Pose Risks to Wildlife or the Environment?

Malathion used in mosquito control programs does not pose unreasonable risks to wildlife or the environment. Malathion degrades rapidly in the environment, especially in moist soil. Malathion is highly toxic to insects, including beneficial insects such as honeybees, it is toxic to aquatic organisms, including fish and invertebrates. For that reason, we have established specific precautions on the label to reduce such risks. Additionally, previously completed screening-level risk assessments and exceedances of agency levels of concern indicate a need to further examine and potentially refine estimates of acute and chronic risk to terrestrial and aquatic animals during registration review.

On November 18, 2008, the National Marine Fisheries Service issued its biological opinion for endangered Pacific salmonid species. The opinion found that use of malathion will jeopardize 27 evolutionarily significant units (ESU) and is likely to adversely affect, but not jeopardize, one ESU. An ESU is a term used by NMFS to identify distinct population segments of Pacific salmon. The biological opinion further finds that use of malathion will adversely modify designated critical habitat of all but one ESU(PDF)(484 pp, 11.04 MB) We are currently evaluating how we will respond to the NMFS biological opinion and how to implement it.

What is the Current Regulatory Status of Malathion?

As part of our responsibility to reassess all pesticides, we reviewed malathion in 2006 under the reregistration process and initiated the registration review process in June 2009. All pesticides distributed and sold in the United States must be registered by the EPA, based on scientific data showing that they will not cause unreasonable risks to human health, workers or the environment when used as directed on product labeling. The new registration review program makes sure that, as the ability to assess risk evolves and as policies and practices change, all registered pesticides continue to meet the statutory standard of no unreasonable adverse effects. The Registration Review process for malathion began in June 2009. Read more about the malathion registration review.

Malathion product labels have been amended to include the application rates and risk mitigation measures specified in the 2006 Reregistration Eligibility Decision and 2009 revised RED.

What is Malaoxon and Does it Pose Risks to Human Health?

Since the completion of the Malathion: Revised Human Health Risk Assessment for the Reregistration Eligibility Decision, dated July 31, 2006, the agency received confirmatory data, which has allowed us to refine several toxicological assumptions for malaoxon, a more toxic compound that is formed from malathion under certain conditions. For example, malathion runoff and spray drift may reach drinking water sources downstream from where the malathion was used.

Malathion present in untreated water will form malaoxon during the chlorination process in water treatment facilities. Malaoxon can also form more slowly when malathion is deposited on hard, dry surfaces and exposed to air over time. Our assessment shows that, even when considering the presence of malaoxon on surfaces following applications of malathion for mosquito control, the relatively low application rates and small droplet sizes used in these types of applications result in minimal exposure to people in the treated area.

Furthermore, the refinement of the toxicity data confirms that our conclusions in the acute, short-term and intermediate-term exposure scenarios and risk assessments were adequately conservative to protect human health. Therefore, the overall human health risk from exposure to malathion and malaoxon would be lower than was calculated in the human health risk assessments developed for the malathion RED.