Depressed by pesticides.

Oct 08, 2008

Beseler, CL, L Stallones, JA Hoppin, MCR Alavanja, A Blair, T Keefe and F Kamel. 2008. Depression and pesticide exposures among private pesticide applicators enrolled in the Agricultural Health Study. Environmental Health Perspectives online September 9.


farmer spraying pesticide.
Cheryl L. Beseler


A study of farmers finds that those with the highest number of lifetime exposure days to agricultural pesticides were 50% more likely to be diagnosed with clinical depression than those with the fewest application days and were 80% more likely if they had applied a class of insecticide called organophosphates. This is the first study to find a link with chronic, low-dose pesticide exposure, although previous studies show an increased risk of depression among people exposed to very high doses or poisoned. This study reinforces concerns that exposure to commonly used pesticides could cause psychiatric disorders.



Agricultural pesticides are a wide class of compounds that include insecticides (used to kill insects), herbicides (used to kill weeds), fungicides (used to kill molds and fungi) and fumigants (gases used to sterilize soil before planting). Pesticides comprise hundreds of compounds with differing chemistry, toxicity and mechanisms of action.

One class, the organophosphate insecticides, garners considerable attention for its known effects on the nervous system. Organophosphates effectively shut down nerve responses in insects, humans and other animals by depressing the enzyme acetylcholinesterase. Acetylcholinesterase stops nerve impulses at the appropriate time after a neuron has fired by degrading acetylcholine. Lower amounts of the enzyme means nerves do not stop firing, which can lead to muscle fatique, spasms, vomiting and eventual death through respiratory shutdown.

The organophosphate pesticides were developed from closely related chemicals that were used as nerve gases during World War II. They were marketed for consumer use after the war. Some of the most well known organophosphates are malathion, chlorpyrifos and diazinon.

Most organophosphate pesticides are banned for use as home pesticides, but they continue to be widely used in agriculture, including on food crops such as fruits and vegetables. They are the most common form of insecticide in agriculture, making up about half of all insecticides used in the United States.  Although they are acutely toxic, they break down quickly when exposed to sunlight and air.

Pesticide applicators, farmers and others making or working with the pesticides have the highest exposures. The chemicals are absorbed through the skin, eaten or breathed in.

What did they do?

Researchers analyzed data from the Agricultural Health Study, a large study of individuals with pesticide applicator licenses in North Carolina and Iowa. Participants in the study are divided into commercial pesticide applicators and private applicators, who tend to be farmers. The researchers limited the current analysis to male private applicators.

More than 17,000 men filled out detailed questionnaires about their pesticide use, their health and their behaviors for the study. Men were asked if they had ever been diagnosed with depression requiring medication or shock therapy. They were also asked about their lifetime use of 50 different pesticides, including the number of days per year and the total number of years each pesticide was applied. Pesticides were grouped into different classes, including herbicides, insecticides, fungicides and fumigants. Insecticides were further broken down into three types: organophosphates, carbamates and organochlorines. Individuals were categorized into low (<225 days), medium (226-752 days) and high (>752 days) lifetime pesticide exposure. Men were also asked if they had ever been diagnosed with pesticide poisoning or experienced an incident of unusually high pesticide exposure.

The researchers then compared the likelihood of being diagnosed with depression among men with low, medium and high cumulative days of pesticide use. Using statistical methods, they took into account factors such as age, education, race and marital status that could impact the results.

What did they find?

About 3% of men in the study reported that they had been diagnosed with depression in the past. Similar to other studies, men who reported a past incident of pesticide poisoning were more than twice as likely to have been diagnosed with depression.

The authors then excluded men who had experienced poisoning or high exposure events. Looking only at regular pesticide use, men who reported the highest number of days of lifetime pesticide application were still 50% more likely to have clinical depression than men with the fewest days of application.

The authors then looked at specific types of pesticides. Herbicides, which are used to kill weeds, were not significantly associated with depression, but insecticides (used to kill insects), fungicides (used to kill fungi) and fumigants (gases used in enclosed spaces or to treat soil) were. The greatest increase in risk was seen for insecticides, particularly for the class of insecticides known as organophosphates. Past use of organophosphates was associated with an 80% higher likelihood of depression compared to men who had never used these compounds.

What does it mean?

This study suggests that long-term, chronic pesticide use may have neurological effects, particularly relating to depression. Previous medical reports have shown anxiety and depression symptoms in pesticide-poisoned individuals. This is the first study to extend those findings to regular pesticide use.

The men in this study were all licensed pesticide applicators who experienced pesticide exposures at levels much higher than the general public. However, pesticide exposure is widespread in the general population because of use in homes, workplaces and food.

The study is strong because it looked at a large population of men with wide variability in their pesticide exposures, allowing the authors to compare individuals with high exposure to those with low exposure in the same population. The researchers had extensive information on participants' backgrounds and pesticide exposure history. The major weakness of the study is that men reported about diagnosis of depression and pesticide use that occurred in the past, without differentiating whether the pesticide exposure came before or after the depression. Self-reporting, large time categories and lack of information about some stress-related events (financial) were additional limitations.

The results indicate that pesticides may have neurological effects at exposure levels well below those that cause clinically identifiable poisoning symptoms. Those who apply pesticides as part of their jobs, such as farmers and pest control applicators, should remain vigilant and use safety precautions to protect their long-term mental health. The authors advise that "physicians should be alert to mood changes in those with a history of applying pesticides."



National Institutes of Health and the US Environmental Protection Agency. The Agricultural Health Study.

Pesticide Action Network. Pesticide Database.

University of Nebraska Cooperative Extension. Neb Guide EC2505. Managing the Risk of Pesticide Poisoning and Understanding the Signs and Symptoms.

Cognitive effects of pesticides

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