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Combining Two or Three Pesticide Ingredients
Increases Neurological Damage

SOURCE: Journal of Toxicology and Environmental Health, 48:35-56, 1996

Although the following research did not specifically pertain to the pesticide malathion, it did research two other pesticides commonly used by the American public. Research such as this raises serious questions regarding the so-called "safety" of pesticides, in that the pesticides included in this research were also considered "safe" when used as stated on the label.

Military personnel operating in the Persian Gulf region between August 1990 and April 1991 have complained of numerous neurological complications after returning home. Several theories have been expressed regarding the occurrence of these problems. One definite fact of exposure to potentially neurotoxic compounds is that soldiers used large amounts of the chemicals DEET (a personal insect repellent) and the pesticide permethrin. Soldiers were also given the drug pyridostigmine bromide (PB) to protect against possible nerve gas attack.

Because of this concern, scientists at the Neurotoxicology Division at the Environmental Protection Agency (EPA) and three other Universities conducted a long term study to determine the effects of combining these chemicals and then observing their effects upon the nervous system of test animals.

The study was conducted by exposing four groups of five hens each to either one or several of these chemicals and at different doses. Researchers continued the exposure daily to the animals for approximately 60 days. During this period investigators looked for various signs of health problems including neurotoxicity.

Results showed animals treated with only PB revealed no difference between controls upon neuropathological examinations (investigations of tissue samples under a microscope). Some animals treated with permethrin or DEET exhibited minor neuropathological changes that consisted of a small increase in the frequency of slightly enlarge axons. (Axons are the main connections between brain cells which send information).

Regarding the effects of combining the chemicals, the researchers stated that the animals treated with the DEET/permethrin combination developed "hyperexcitability" between 1 and 4 weeks of dosing. In one animal, a mild gait disturbance was detectable at 27 days that progressed to a stumbling and unsteady gait accompanied by moderate fine body tremors. Microscopic examination of spinal cord and sciatic nerve found mild neuropathological alterations in two of the animals treated with permethrin/DEET which included a significant increase in both the frequency and degree of enlargement of the axons.

One point of neurotoxic interest - even though the dosages of permethrin were well below the amount needed to kill the animals, it was found that when permethrin was combined with DEET it created brain damage within the test animals that is "similar to those observed following near lethal doses of permethrin."

In conclusion, Dr. Jensen stated,

"This study demonstrates that concurrent administration of any two compounds of PB, DEET, and permethrin results in neurotoxicity that is markedly greater than that resulting from treatment with any individual compound. ....Both DEET and permethrin have been shown to produce tremors and hyperexcitability in experimental animals (Ambrose et al., 1959; Schoening et al., 1993).... In addition, these findings suggest the need for additional studies into potential health risks associated with coexposure of humans to these agents at dosages likely to have been used by the Gulf War veterans."

 Of further concern for the pesticide permethrin, which brings out further questions on the pesticide, is that research has found the chemical undergoes a biological transformation within the human body by what is called esterase and oxidase inhibitors (for the benefit of our scientifically minded), thereby creating a new chemical which researchers say, "may create unanticipated hazards by enhancing pyrethroid toxicity to mammals.(4)" In other words, they are saying that this chemical is most likely more dangerous than tests with animals show since the human body can change the chemical into an entirely new and even more toxic chemical.


DEET is what chemists call an "aromatic amide" and is used as a personal insect repellent against mosquitoes, biting flies, and ticks, among other insects. It has been used since 1946 by the U.S. Army and since 1957 by the general population. Approximately 30% of the U.S. population uses DEET as a lotion, stick, or spray at concentrations between 10 and 100% active ingredient. Extensive and repeated topical applications of DEET resulted in human poisoning including two deaths. Symptoms of poisoning are characterized by tremor, restlessness, slurred speech, seizures, impaired cognitive functions, and coma (McConnell et al., 1986). DEET has been found to be efficiently absorbed through the skin (Windheuser et al., 1982; Spencer et al., 1979). Also, regarding the use of permethrin, this chemical is currently used in schools and homes for general insect or termite treatments. Erroneously, pesticide applicators state that this chemical is the same as pyrethrin (which is a natural pesticide made from the chrysanthemum flower). This is highly inaccurate. Although the molecules are somewhat similar, they are still very different and pyrethroid pesticides such as permethrin are showing increased evidence in the medical research for causing a variety of neurological and immune system damaging effects (3).

Karl F. Jensen (Ph.D.)
Neurotoxicology Division, U.S. Environ. Protection Agency, Research Triangle Park, NC

Journal of Toxicology and Environmental Health, 48:35-56, 1996