Certain pesticides may lead to asthma in farmers

Certain pesticides may lead to asthma in farmers

American discovery revealed at 17th ERS congress in Stockholm

17 Sep 2007

A study of over 20,000 American farmers shows for the first time that pesticides are an independent risk factor for adult asthma.

Farmers' airways were already known to be at risk from many substances, including pollen, animal fur, plant mites, dust of various kinds and mould spores. Some pesticides may now need to be added to that list in the light of the study presented to the ERS Congress in Stockholm by Jane A. Hoppin, of the Epidemiology Branch at the US National Institute of Environmental Health Sciences.

This is a breakthrough: never before has a large-scale study demonstrated that farmers' exposure to insecticides, fungicides and herbicides can contribute to the prevalence of asthma, independently of other risk factors.

Over fifteen suspect pesticides

The research presented to the ERS Congress was carried out on 20,183 male farmers in the US states of Iowa and North Carolina. It formed part of the Agricultural Health Study, a vast American prospective study conducted jointly by the National Cancer Institute, the National Institute of Environmental Health Sciences, the US Environmental Protection Agency and the National Institute for Occupational Safety and Health.

Hoppin's project involved analysing the prevalence of adult-onset asthma, both allergic and non-allergic, in relation to the individual's exposure to a range of pesticides. The results presented to the Congress are quite alarming: asthma was found to be linked to use of sixteen different pesticides!

"Asthma was linked to specific chemicals", Hoppin explains, "since we did not identify a link either with particular pesticide classes or with a particular method of use. In addition, we show that a history of a high pesticide exposure event was associated with a doubling of asthma risk", she adds.

What about women?

This link remains statistically significant after adjustment for age, smoking, body mass index and state of residence.

Another analysis is under way, this time on farm women, to determine whether the results are also independent of gender. But there is no reason to suppose that women would react differently to occupational exposure, according to Hoppin.

Pesticides give rise to a different level of risk depending on the type of asthma (allergic, i.e. atopic, or non-allergic). Of the 452 farmers who had developed asthma after the age of twenty, 129 had allergic asthma while the remaining 323 had the non-allergic version. "We were expecting this higher rate of non-allergic asthma", Hoppin explained, since farmers were usually raised in a rural environment, their allergy risk should be relatively low."

Captan: a common and dangerous fungicide

Twelve pesticides show a statistically significant link to a rise in the risk of allergic asthma, and four are linked to a rise in the rate of non-allergic asthma.

Of the first group, almost half are no longer sold in the USA, as the team told the Stockholm Congress. These include 2,4,5 - TP, chlordane, heptachlor, carbon tetrachloride/carbon disulphide 80:20 mix, and ethylene dibromide.

Which leaves seven chemicals still on the market: EPTC, paraquat, lindane, parathion (although rarely used), coumaphos, diazinon and captan (all three widely used).

Coumaphos had the strongest association, Hoppin told the Congress, since its relative risk of asthma is the highest at 2.3, suggesting a two-fold increased prevalence of allergic asthma.

The pesticides that increase the prevalence of non-allergic asthma include the now prohibited DDT, as well as phorate, which is rarely used, and two commonly applied preparations: malathion and petroleum oil herbicide. All these were associated with a 30-40% increased prevalence of asthma.

What about pesticides in urban areas?

"But these are only the active ingredients", Hoppin emphasises. "It would obviously be interesting to find out what exactly goes into the preparations on sale, in order to identify the possible role of solvents or additives.

"And the data we are presenting to the ERS Congress are exploratory", she adds. "But they make it possible to reassess the way we should be treating certain pesticides." It is true, however, that the use of these data to protect agricultural workers requires decisions at the political level, and that's out of the researchers' hands.

Nonetheless, the possible scope of the link between pesticides and adult-onset asthma raises a problem of broader interest, given the considerable quantities of pesticides used in the domestic and urban environments.

Their impact on a population which, while less exposed, has a greater risk of allergies and a higher prevalence of asthma, remains to be determined.

More information on asthma