Exposure to pesticides 鈥� even at relatively low levels 鈥� may increase an individual鈥檚 risk of developing Parkinson鈥檚 disease by 70%, according to a study of more than 140,000 people. Researchers say that the findings strengthen the hypothesis that such chemicals somehow promote the development of the disease.
In recent years, experts have identified genetic mutations that apparently predispose people to develop Parkinson鈥檚. But some mystery remains because not everyone with the mutations will get the devastating neurological disorder, which is characterised by rigid and slow movement or other problems with body coordination. Likewise, Parkinson鈥檚 can develop in individuals with no known genetic predisposition.
As a result, many scientists maintain that environmental factors ultimately trigger the development of Parkinson鈥檚 disease.
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The idea that pesticides may be to blame has now received a boost from the first large-scale, prospective study to examine this possible link. Alberto Ascherio of the Harvard School of Public Health in Boston, Massachusetts, US, and colleagues looked at data from roughly 143,000 people involved in a cancer and diet study, of whom 413 were diagnosed with Parkinson鈥檚 disease in the 1990s.
Toxin exposure
In 1982 these volunteers each completed an initial questionnaire, providing information about their occupation and levels of exposure to various toxins.
Ascherio and colleagues found that those who reported exposure to pesticides had a 70% greater risk of developing Parkinson鈥檚 disease than those who said they had no such exposure. But exposure to other toxic compounds 鈥� such as asbestos and formaldehyde 鈥� did not increase their chances of acquiring the illness.
Ascherio stresses that the absolute risk of developing Parkinson鈥檚 is low. So while about 2% of the population as a whole may be at risk of developing the disease, exposure to pesticides might increase this risk to little more than 3%.
Of the 413 patients with Parkinson鈥檚 disease, 43 reported exposure to pesticides. But surprisingly the study found that farmers 鈥� many of whom presumably had high levels of exposure 鈥� and non-farmers shared a similarly increased risk. This contradicts a previous, smaller study reporting that risk rises with exposure levels (see Exposure to pesticides can cause Parkinson鈥檚).
Garden pesticides
Ascherio suggests that non-farmers may have encountered pesticides while gardening. 鈥淢aybe the pesticides used in agriculture are not the most harmful,鈥� he speculates. He regrets that the initial questionnaire did not include more details about the type, duration and intensity of pesticide exposure.
Experts stress that many people unknowingly consume pesticides on a daily basis. 鈥淚f you analyse the fruit and vegetables we eat, they鈥檙e full of chemicals,鈥� says Serge Przedborski of Columbia University in New York, US. He adds that traces found in such foods can accumulate over a lifetime to potentially harmful levels.
Przedborski describes the new study as 鈥渆xcellent鈥� because researchers collected data about pesticide exposure years before participants developed Parkinson鈥檚 disease, ruling out potential bias. But he notes that it does not prove that pesticides are the main cause of Parkinson鈥檚 disease.
Moreover, Przedborski explains that because the initial questionnaire did not ask about specific pesticides, we are no closer to knowing which particular chemicals are the culprits. 鈥淚n reality, we have no idea,鈥� he says.
Journal reference: Annals of Neurology (DOI:10.1002/ana.20904)
