The effects of organophosphates on human healthby The Listener
Pesticides credited with boosting food production may also be making us sick.
Organophosphates, or OPs, are a group of pesticides said to have saved millions of people from starvation, and as the chemicals degrade quickly, they are considered an environmentally friendly alternative to the bug-killers that preceded them. Yet as use of these chemicals increases, so are concerns about their effects on human health.
Organophosphates work by deactivating acetylcholinesterase, an enzyme in the human nervous system that breaks down acetylcholine, a chemical that carries signals between the nerves and muscles. As a result the nerves become overactive. This is how the chemicals kill invertebrates and why they can be dangerous to humans. Of course, it’s all a matter of degree.
The dangers of being exposed to high levels of organophosphates are well-established. Acute toxic reactions can include blurred vision, dizziness, headaches, tremors, respiratory and cardiac problems and death. Evidence of harm caused by being exposed to low levels of the pesticides over time is also mounting.
A study by University College London (UCL) published in Critical Reviews in Toxicology at the end of last year found compelling evidence of the link between long-term, low-level exposure to organophosphates and neurobehavioural deficits, such as impairment of memory and information-processing speed. The impact might be too subtle for people to need medical treatment, but appears serious enough to interfere with their ability to do their jobs. Some farmers reported difficulty balancing their books and being unable to follow stock auctions.
The UCL researchers noted there have been dozens of studies looking at the neurotoxic effects of long-term, low-level exposure, but the results have been conflicting. Theirs, however, was a meta-analysis that re-examined the data of 14 high-quality studies conducted since 1960.
The conclusions were no surprise to University of Canterbury professor of toxicology Ian Shaw, who points to similar findings in a review by the Royal College of Physicians and the Royal College of Psychiatrists in 1998. “I’m completely unsurprised. But I’m glad somebody else is saying it because I think we need to control OPs very much more than we do.
“You give animals OPs and they wobble all over the place … You give rats low levels of OPs over time and they can’t find their way through mazes. There’s tons of evidence that
shows that they can have an effect both in the short term and the long term.”
But coming up with strong enough evidence to convince regulators to limit organophosphate use is difficult, partly because the symptoms are often non-specific and could be attributed to other causes.
“That’s always an issue,” says Shaw. “But what you can do is say there’s suspicion around them. We know that theoretically they’ll have an effect on the nervous system. We don’t know if they’ll have a long-term effect, but let’s assume they will. And you need to be doing the following things to be handling them safely.”
Well before the UCL paper was published, the Environmental Protection Agency (EPA) had begun a review of the use of 29 organophosphate (and carbamate-based) pesticides. This was prompted by growing concern about the chemicals, some of which have been restricted or banned by overseas regulators.
The EPA will hold public hearings in March and proposed recommendations include revoking approvals of some, phasing out others and allowing continued use of some but with more controls.
In case all this puts you off your fruit and veges, Shaw says organophosphate levels in our produce are too low to worry about. And as home gardeners would only use such products occasionally, they are also at low risk, although they should still cover up when spraying.
What about farmers? New Zealand legislation requires people working in the agricultural and horticultural industries to use protective clothing and equipment when using the pesticides, but do they? “When you’re a farmer in the Mackenzie Country, and it’s 31°C, you might not feel like wearing a full body suit and mask,” says Shaw.
In which case you might want to read the research.
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