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Monday 23 March 2009

agricultural pesticides

Agricultural pesticides are divided into five categories, depending on the target pest: insecticides, herbicides, fungicides, rodenticides, and fumigants.

All pesticides are toxic to some plant or rodent species; at higher doses, they can also be toxic to farm animals, pets, and humans. In general, herbicides used to control weeds have low acute toxicity for mammals; fungicides are characterized as moderately toxic. Acute toxicity of insecticides for mammals ranges from low to high.

For example, DDT was widely used as an insecticide in the 1940s and 1950s because it has low acute toxicity for humans. However, DDT persists in the environment and accumulates in the food chain. Birds that ingested DDT-contaminated insects and fish suffered reproductive defects. DDT and its major metabolite, DDE (1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene), accumulate in fat tissue and have been detected in human milk.

Organochlorines, as well as industrial chemicals such as polychlorinated biphenyls (PCBs), are weakly estrogenic. Some of these chemicals are carcinogenic in rodents and cause reproductive dysfunction in amphibians, birds, and fish.

Although several epidemiologic studies have not found increased levels of DDE or PCBs in women with breast cancer compared with matched control subjects, there is still concern that these persistent organochlorines, other potentially estrogenic pesticides, and natural phytoestrogens in plants such as soybeans may have adverse reproductive effects in humans. The mechanisms of action of these xenoestrogens, alone or in combination, in the development of cancer and in reproductive dysfunction are unknown.


- Organochlorines, such as DDT, have low acute toxicity for humans; however, they bioaccumulate and persist in the environment and in fat tissue. These chemicals are absorbed through the skin, gastrointestinal tract, and lungs. As alluded to earlier, the role of DDT and its metabolites as an endocrine-disrupting agent is controversial. Chlordane is representative of cyclodienes that are used to control termites and other soil insects. Acute toxicity causes hypothermia, tremor, and convulsions. Chlordane also causes immune dysfunction and may act as a nongenotoxic carcinogen. These effects may contribute to the increased incidence of lymphoma observed in some farm workers. Lindane is an isomer of benzene hexachloride that is used to control lice and scabies, as a wood preservative, and as a household fumigant. It has been reported to cause immune dysfunction and reproductive problems in women.

- Organophosphates are irreversible inhibitors of cholinesterases resulting in abnormal transmission at peripheral and central nerve endings. These chemicals are absorbed through the skin, gastrointestinal tract, and lungs. Up to 40% of farm workers in the United States show measurable inhibition of red blood cell or plasma cholinesterase activity; fatalities have been reported from organophosphate exposure. Carbamates are reversible inhibitors of cholinesterase that produce acute neurotoxic effects similar to those of organophosphate insecticides. Carbaryl (Sevin) is potentially mutagenic and teratogenic because it poisons the mitotic spindle.

- Herbicides like the dioxin TCDD has received much attention. During the Vietnam War, the defoliant Agent Orange was contaminated with TCDD. A chemical factory explosion in Seveso, Italy, in 1976 caused local environmental contamination and human exposure to TCDD, resulting in chloracne and an increased incidence of leukemia, lymphoma, and sarcomas. TCDD and structurally similar dioxins are also produced in the paper pulp industry using chlorine bleach and by waste incinerators. Low doses of dioxin are present in our food, soil, and water. In some laboratory animals, TCDD is highly toxic, immunosuppressive, teratogenic, and carcinogenic. The sensitivity of some strains of laboratory mice to dioxin is linked to the aryl hydrocarbon hydroxylase receptor. TCDD can induce liver cytochrome P-450 enzyme activity, increase estrogen metabolism, and interfere with development of the male reproductive tract. TCDD also decreases thyroxine levels in adult rats. Extrapolation of these multiple adverse effects observed in laboratory animals to low-dose exposure of humans is difficult.

- Rodenticides are highly toxic chemicals with restricted use. The major health threat is death from suicidal or accidental ingestion.

Prostate cancer

Occupational exposure to pesticides has been associated with increased prostate cancer risk in many epidemiologic studies.

Specifically, several groups or chemicals classes have been linked to prostate cancer, including triazine herbicides, organochlorine insecticides (OC), and organophosphate insecticides (OP), but none of the associations are conclusive and it is unclear which specific pesticides might be driving the group findings.

In the Agricultural Health Study (AHS), a prospective cohort of licensed private and commercial pesticide applicators from Iowa and North Carolina, it has been consistently observed an excess of prostate cancer among AHS men compared to men in the general populations of Iowa and North Carolina, and with continued follow-up of this cohort we recently reported an excess risk of prostate cancer associated with four insecticides, fonofos (OP), terbufos (OP), malathion (OP), and aldrin (OC).


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