CHEMICALS in weedkillers widely used by farmers and gardeners have been found to disrupt animal reproduction at levels well below those supposed to be safe. In fact, their effect is worse at low doses, casting doubt on standards for assessing the safety of any chemical.
Several epidemiological studies looking at farmers or people living in areas where pesticides are used have linked such products to problems such as miscarriages, stillbirths and birth defects. But the studies don鈥檛 directly prove that the chemicals are to blame.
So a team including Warren Porter of the University of Wisconsin-Madison looked at the effects of a popular off-the-shelf weedkiller containing phenoxyacid derivatives, which are found in hundreds of products worldwide. This particular herbicide contained a mixture of mecoprop, dicambra and 2,4-D. When the researchers contaminated the drinking water of pregnant mice with varying doses of it, the mice lost more fetuses and the size of the litters fell by about 20 per cent (Environmental Health Perspectives, vol 110, p 1081).
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But what was striking was that the effect was greatest in the mice exposed to the lowest doses 鈥� only 0.039 parts per million of 2,4-D. That鈥檚 about one-seventh the US Environmental Protection Agency鈥檚 recommended limit for drinking water.
Porter thinks there鈥檚 a specific, very low level at which the chemicals have the most effect. At higher doses, an unknown protective response might kick in, overriding the effect of the chemicals. But no one yet knows where this limit is. 鈥淲e don鈥檛 yet know how low we can go and still get reproductive effects,鈥� he says.
Whether the same applies to people isn鈥檛 clear, although one study has shown that women exposed to phenoxyacid derivatives take longer to get pregnant.
But if such mechanisms are widespread in nature, it poses a serious dilemma for the EPA and other regulatory agencies, who rely on toxicology tests. They assume that any doses lower than the lowest dose initially found to be safe will be safe too. But if the team鈥檚 findings are confirmed, this assumption may not always be true.
Porter admits, however, that asking regulators to test ever-diminishing doses of a chemical before approving a product may be impractical. An EPA spokesman said the organisation isn鈥檛 aware of the study, but takes any published evidence of unexpected low dose effects seriously. They added that the onus is on researchers outside the EPA to investigate these effects. But they do issue warnings or advice on products when such studies are confirmed.
鈥淭he fix is in the public being aware that these materials are potentially hazardous to health,鈥� Porter adds. One effective alternative for gardeners is simply to spray weeds with vinegar, he says, or household cleaners such as bleach.