What are pyrethroids?

Potent neurotoxins

Pyrethroids are a family of insecticides, all originally derived from six chemicals called pyrethrins, found in the chrysanthemum. Pyrethrum, the extract of chrysanthemum, is a very potent insecticide. However the pyrethrins are sensitive to sunlight and quickly become inactive; pyrethroids are synthetic derivatives designed to be stable in ultra-violet light, whilst retaining the potent insecticidal properties.

Thanks to their great effectiveness and relative safety pyrethroids are popular and are produced industrially for use around the globe. Like many other insecticides pyrethroids are neurotoxic and work by incapacitating the creature, preventing it from feeding on the crop, or buzzing around the front room. Thankfully mammals are much less sensitive to the effects of pyrethroids, requiring doses thousands of times larger before they are toxic.

To serve global demand, a large family of pyrethroid compounds has been developed, each with different characteristics. The pyrethroids have been divided into two classes on the basis of their biochemical actions and behavioral indices of toxicity. Generally they can be classified as Type I or Type II, each producing distinct symptoms of poisoning. Type I pyrethroids are best suited to small-scale, personal applications whilst type II are preferred for large-scale uses due to their higher potency.

Humans and other animals are regularly exposed to pyrethroids; sometimes direct exposure through agriculture or fly sprays, and one can receive tiny doses from drinking water or food. Thanks to judicious regulations governing pyrethroid use in most areas poisonings are rarely lethal, yet there is still a substantial risk associated with such potent neurotoxic chemicals. Conservative estimates of safe exposure levels are made for each individual insecticide before its general release. However the interactions between different compounds are largely uninvestigated, partly due to the sheer number of combinations.

Assessing the risk

Public perception of pesticides is often marred by the rarest case scenarios of poisoning, which are set against a background of hysteria mediated by advertising schemes to sell 'pure' products. It seems unlikely that the general population is at any great risk, although groups of people may be more susceptible because of greater exposures or idiosyncratic phenotypes The true risk of small cumulative doses is unclear as unfortunately little research has been able to gather enough information to draw firm conclusions [Ray, 2000].

Exposure to pyrethroids through foodstuffs is closely monitored in the EU, making ingestion a minor risk. The risk of poisoning occurs mainly where large quantities of pyrethroids are available, such as in agriculture. Misuse is a primary cause of overexposure, and occasionally pyrethroids are a choice for suicide.

Acute poisoning with pyrethroids incurs life-threatening symptoms, and is of very real and obvious danger. It is with this more imminent kind of poisoning that this study is concerned. It has become apparent that the two classes of pyrethroid interact with each other, with a key findings by Song et al. and Motomura et al. showing competitive effects between fenvalerate, tetramethrin in cultured neurons.

Understanding the nature of interactions between pyrethroids is important for realistic risk assessment of exposure in humans. At this time knowledge of the combined effects of multiple pyrethroids in vivo is lacking, a hole in toxicological research. To allow informed guidance by pesticide regulatory bodies, this gap needs to be filled.