Questions and Answers:

EPA’s Risk Assessment on

Malathion

Q. What is malathion and how is it used?

A. Malathion is a commonly used organophosphate

insecticide. In an agricultural setting, malathion is

used to eliminate pests, such as boll weevils and fruit

flies, that threaten the productivity of important crops

like cotton and citrus. Human exposure is minimized

in agricultural programs by targeting host crops or

plants, and, in some programs, by using bait formulations with low malathion concentrations. For example, the cooperative boll weevil eradication program

uses careful precision to apply malathion only to

infested cotton fields. Because of application techniques and the rural location of most cotton fields,

the risk of human exposure is low.

In an urban setting, malathion is used to control

mosquitoes that can carry West Nile virus and other

diseases of public health importance. It is also used

as an insecticide to control pests in homes and personal gardens. Malathion is approved by the U.S.

Food and Drug Administration as a prescription drug

for the treatment of head lice on humans.

Q. What document is the Environmental

Protection Agency releasing?

A. The Environmental Protection Agency (EPA) is

undertaking an extensive reregistration effort involving hundreds of pesticides, including malathion, to

ensure that they meet the requirements of the Food

Quality Protection Act of 1996. As part of its review,

in September 2005, EPA released a revised risk

assessment on malathion for public comment.

EPA currently classifies malathion as having

“suggestive evidence of carcinogenicity, but the evidence is not sufficient to assess human carcinogenic

potential.” 

Q. So, does this mean malathion causes cancer?

A. According to EPA’s document, studies conducted

on rats and mice “showed tumors only at very high

dose levels [of malathion] or that the number of

tumors was so low that they could have occurred by

chance rather than as a result of exposure to

malathion. In addition, studies showed that

malathion did not act as a mutagen to cause cancer.”

EPA states that, “Human exposures to malathion are

generally quite low.” Because potential human exposure is several orders of magnitude lower than dose

levels in animal studies and the studies show no firm

evidence of cancer caused by malathion, EPA concluded that when malathion is used according to

label directions cancer risk is not a concern.

Q. What are the ecological effects of malathion?

A. EPA’s document acknowledges that malathion

degrades rapidly in the environment and does not

pose a risk to birds. However, EPA does note that

malathion is toxic to beneficial insects, and some

concerns exist about malathion’s effects on aquatic

animals.

In addition, malathion, if left on hard, dry surfaces

(e.g., swingsets, backyard toys, and decks) or in

chlorinated water, can result in an increase in

malaoxon levels. Malaoxon is a degradation product

of malathion under certain conditions. Studies are

being done to determine the environmental impact of

malaoxon under such conditions. The EPA has

recently suggested that malaoxon, previously viewed

as a low-level contaminant of malathion, may be up

to 77 times as toxic as malathion. However, the high

purity of the malathion used in boll weevil eradication

(typically 97- to 99-percent pure malathion) means

that the amount of malaoxon, and any human health

risk associated with such treatments, is negligible. 

Comprehensive risk assessments that evaluate

the way malathion is used in the U.S. Department of

Agriculture’s Animal and Plant Health Inspection

Services’ (APHIS) cooperative programs indicate little

to no effects on nontarget organisms. Years of environmental monitoring conducted in conjunction with

APHIS’ programs confirm that if ecological effects

occur, they are minimal, limited in duration, and confined to treatment areas.

Q. Does this assessment place new restrictions

on the use of malathion?

A. This revised risk assessment will be considered

by EPA as it develops any risk mitigation measures

that it considers necessary. Thus far, the assessment has not resulted in new restrictions on the agricultural uses of malathion. However, the assessment

is subject to review, comment, and further revision.

Q. Does EPA’s revised risk assessment change

any of APHIS’ programs that are currently underway?

A. APHIS has announced no changes to programs

that are currently underway.
EPA expressed concern about the potential risk

from malaoxon to toddlers near cotton fields, and in

chlorinated drinking water near citrus production

areas. However, the high purity of malathion used in

boll weevil eradication (typically 97- to 99-percent

pure malathion) means that the amount of malaoxon,

and any human health risk associated with such treatment is negligible. 

APHIS is working closely with EPA to further

refine the risk assessment.

Q. How does this affect APHIS programs in the

future?

A. To comply with existing laws, APHIS prepares

environmental documentation that analyzes the

potential impacts of its programs. The environmental

documentation considers alternative courses of action

and ensures that APHIS is using the most efficacious

means to protect U.S. agriculture, while remaining

sensitive to human health and other ecological concerns. 

APHIS is considering the implications of EPA’s

revised risk assessment. Upon publication of EPA’s

final risk assessment, APHIS will consider the need to

revise the environmental documentation for programs

that use malathion.

Q. Who can I contact for more information?

A. For additional information regarding malathion and

the boll weevil eradication program, please call

APHIS’ Environmental Monitoring staff in Riverdale,

MD, at (301) 734–8247.

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