Vol. 1 No. 2 January 5, 1981


I have been receiving a number of requests for information about malathion with direct reference to its proposed use against the Mediterranean Fruit Fly. Apparently, CDFA and the California Department of Health Services have been getting a lot of questions too, because they just recently (Dec. 16, 1980) released a joint report entitled "Summary Statements Concerning the Possible Human Health Hazards of Applying Fly Bait Containing 2.4 Ounces of Malathion per Acre over Populated Areas". Their Summary and Recommendations are reproduced below for your information.


An assessment has been made of the health risks associated with the proposed aerial application of malathion in Santa Clara County using all available sources of information. Malathion's low level of acute toxicity precludes the possibility of acute toxic effects from the amounts to be applied (2.4 ounces per acre) unless unforeseen accidents occur or high levels of impurities are present in the spray mixture.

Animal studies and short term in vitro tests have shown no results that meet the criteria for evidence of mutagenesis or mammalian teratogenesis. A standard National Cancer Institute bioassay was reported as negative for carcinogenesis. This interpretation has been called into question in an uncorroborated report by a single investigator.

Taking the premise that malathion may be carcinogenic and using conservative, (i.e., health-biased) assumptions and extreme "worst-case" examples the risk of cancer from the malathion applications in the exposed Santa Clara County population has been estimated and found to be insignificant. The greatest health risk may come from the large, low-flying planes and the possibility of a crash in an urban area.

The degree of public concern over possible effects of the aerial spraying program is a major factor in the situation. The people of Santa Clara County should be fully informed and arrangements made to answer their questions.


  1. Maximum effort should be made to inform the public of the details of the operation and to invite and answer questions. This should include the development of a careful risk/benefit evaluation with input from appropriate authorities in both health and environmental protection.

  2. The public should be kept informed by widespread use of the media concerning the times of aerial application and the use of appropriate precautions including the protection of fish ponds, domestic bees and automobiles as well as the avoidance of unnecessary human exposure.

  3. The spray material should be tested before each day of application for the amount of iso-malathion present. An allowable limit for the concentration of this impurity should be established in advance and the justification for such a limit made public.

  4. The amount of maloxon developing in 12 - 24 hours in the material reaching the ground should be monitored.

  5. The droplet size should be monitored. The plan for such monitoring should be developed and made public before the application.

  6. An assessment should be made of the possible risk of a plane accident using data from previous projects of this type. Steps taken to prevent such an accident should be made public. Emergency plans should be developed and made public for dealing with such an accident should it occur.

Before their report was released, I responded to a request from Doug Hamilton from Alameda County by including the following information in a letter to him.

  1. Carcinogenicity - There have been at least 2 definitive studies which have shown that malathion is not carcinogenic in rats or mice. There has been one study done which showed that a metabolite of malathion (maloxon) was not carcinogenic either. In one of the studies, the time weighted average concentration of malathion in the diet was greater than 8000 ppm.

  2. Teratogenicity - Malathion was not teratogenic to rats at oral dose levels up to 300 mg/kg.

  3. Neurotoxicity - I could find no reports that associate malathion exposure or treatment with demyelinization.

  4. General Toxicity - The oral LD50 of malathion in rats is greater than 1200 mg/kg. The LD50 of malathion in young dogs is greater than 1000 mg/kg i.p. The oral LD50 of malathion in chickens is greater than 500 mg/kg. Malathion can be toxic to certain fish at concentrations of .24 mg/liter. The estimated oral lethal dose of malathion to an adult human is 60 grams. Humans have ingested 16 mg of malathion per day for a period of 47 days without suffering any adverse affects. Accidental exposure to concentrated malathion solutions have caused human deaths.

  5. The half-life of malathion in soil varies from one to seven days. Soil temperature and organic matter content directly effect malathion degradation. The half-life of malathion in water is approximately 2 days. As is well known, malathion is a non-persistent pesticide. The non-persistence accounts for its environmental safety.

  6. Other factors - Storage of commercial malathion preparations at environmental temperatures greater than 40C can result in a significant conversion of malathion to iso-malathion. Iso-malathion is about 6 times more toxic to mammals than is malathion. Appropriate handling and storage can prevent this problem.

The proposed treatment of populated areas should pose no health problems to humans, pets or livestock. Because of the sensitivity of bees to the toxic effects of malathion, implementation of protective measures will be necessary to prevent problems. Some species of fish are also very sensitive to the toxic effects of malathion. In most cases water levels of 100 ppb malathion have no detrimental effects. If malathion, at the rate of 2.4 oz/acre (1.56 mg/sq.ft.), were sprayed onto the surface of water 1 foot deep, the resulting concentration would be 56 ppb. There is the possibility that fish in very shallow ponds might be adversely affected by the application of this amount. In ponds deeper than 1', the possibility of adverse effects would be smaller.

Other areas of public concern may involve the removal of residues from windows, cars and backyard fruits and vegetables. Removal of residues from all of them can be accomplished by washing with water. Malathion is soluble in water to about 145 ppm (145 mg/liter). Washing with soap and water will be even more effective in removing the malathion from inedible objects like cars. Studies have shown that washing vegetables in water (without soap) removes from 80-90% of malathion residues, and that cooking reduces residues even more.

Opposition to the spraying for the Mediterranean Fruit Fly control cannot be based validly on concern about major harmful health effects due to malathion because there is no evidence to support this. I would expect that it might be inconvenient to some people. Car washes would probably do brisk business after each application! Entomologists and economists can determine the need for and effectiveness of such spraying. This time I agree with the Department of Health Services Report; the malathion itself should have no detrimental health effects.


2,4-D - Toxicity tests of the dioxin isomers found in 2,4-D are underway. Nothing definitive yet.


Ergot poisoning in Sheep: Monte Bell and I recently investigated some abortions and premature deliveries in sheep pastured on Claviceps paspali infected Dallis Grass. See the next Veterinary Notes for more information.

Arthur L. Craigmill, Ph.D.
Extension Toxicologist
Environmental Toxicology and Veterinary Extension
University of California
Davis, CA 95616
(530) 752-1142