If this kills insects, what effect is it having on me?
When Rachel Carson published in 1962 her famous book Silent spring, she predicted that persistent insecticides such as DDT would destroy the whole ecosystem. Her book was an important landmark in the way it focussed attention on this topic. Subsequently many problematic compounds were removed from the market. On the other hand, it created the impression that pesticides were evil in general. It is uncertain how she as a scientist would have responded to such beliefs. Pesticides are like medicines: used correctly and according to strict guidelines they are beneficial, but their use requires always skill and discretion. If there is no good and specific reason to use these compounds, then they should not be used.
The insecticides used in agriculture are nerve poisons with the exception of some mineral oils. Their mechanisms of action are very different, however. The most important groups used in the developed countries at the moment are organophosphates (and similarly acting carbamates) and pyrethroids.
Not so long ago also organochloride insecticides were in common use. Some of these are well known to the general public, DDT, metoxychlor, lindane, endosulfan, chlordane, heptachlor, aldrin, dieldrin and endrin. Many were banned because of their general toxicity (e.g. endrin, dieldrin), some because of their environmental effects (like DDT which has very low toxicity to mammals). Traces of chlorinated pesticides can still be found all over the world. It is not surprising that they are found in bottom sediments and biota of the Baltic Sea and North American Great Lakes, but they have even been transported by air currents to Arctic regions where they were never used, such as Greenland and Alaska. Fortunately, concentrations in fish have decreased since the 1970s, and they no longer cause major problems for human consumers.
There has been much discussion on the hormone disrupting potency of DDT for many years. The antiandrogen effect or antagonism of male sex hormone of DDE, a metabolite of DDT, is the most certainly proven effect. In the natural environment, the effects of DDT on wildlife are quite clear: birds, seals, alligators and fish have experienced problems in reproduction. There is no direct evidence for any effects on humans, and now the decreasing concentrations mean that this is even less likely. Today, the concentrations are a mere fraction of those in the 1970s. DDT causes tumours in experimental animals at very high doses, but there is no evidence that this occurs in humans.
Nicotine was used as an insecticide for many years especially in greenhouses. Its use has been largely abandoned because of its high toxicity which is also difficult to prevent even by wearing protective clothing and other measures. Since it has such specific and limited use, it is not a risk to the general public.
Organophosphates and carbamates have a common mechanism of action. They increase the concentrations of a very important neurotransmitter, acetylcholine. They differ from organochlorine compounds in that they are broken down rapidly in the environment. On the other hand, many organophosphates are acutely toxic, some of them extremely lethal. Therefore they pose a real threat to the people handling them, especially in a closed space such as a greenhouse. Many toxic nerve gases such as sarin, soman and tabun belong to this group of compounds. These compounds were created as warfare agents, and the chemists planned compounds which would be both extremely toxic and easily absorbable through intact skin.
In the U.S., when DDT was replaced by parathion, one of the organophosphates, there were a number of serious poisonings, because people handling these agents assumed them to be equally innocuous. This caused several hundred deaths. Subsequent development has produced compounds that are much less toxic to humans than parathion. One such example is malathion which is a sulphur-containing organophosphate. It is not poisonous as such to either insects or humans, but it is metabolized in insects to malaoxon, which is relatively slowly broken down. In humans, on the contrary, malaoxon is formed very slowly and broken down very rapidly. Therefore its concentrations never reach toxic levels, and thus this compound is really only toxic to insects.
Some of the carbamates can be quite toxic compounds, but most of them are not truly lethal and they break down easily in human body. Some toxic carbamates, such as aldicarb, have been banned in Europe.
Since organophosphates and carbamates break down rapidly in the environment due to moisture and they are also eliminated rather rapidly should they be absorbed by animals, their residues do not pose a great risk. Their main risk is occupational, and the sprayers should follow rules of protection depending on the substance used. Preparations intended for agricultural use should never be used indoors e.g. to spray flowers in the home.
Pyrethrins are natural toxins made by plants of the Chrysanthemum family. These agents are also used in organic farming. There are technical problems with the pyrethrins – they break down rapidly in sunlight, and they also cause allergies easily. Therefore a number of synthetic pyrethroids have been developed which are more resistant to light and rarely cause allergies. Mammals eliminate pyrethroids easily, and therefore these compounds are only toxic to insects.
On the other hand, these compounds are probably the kinds of insecticides used by the general public rather in their homes. Therefore very strict control is needed both in registration and also in educating the consumers on how to use them properly. Since pyrethroids are expensive to produce, they are not commonly used in agriculture. Due to their rapid elimination, any small residues in products are not problematic to consumers.
Consumers may worry that insecticides, like other pesticides, may still be present in their in food. In most countries, the residue levels are very low, but control is needed in international trade. These agents are toxic by definition, and therefore one should be careful when using them in the home. Spraying flowers at home is best done when there are no children in the vicinity. In some countries, professional exterminators may use these compounds to spray against cockroaches or termites. Insecticides used by these experts may be much more toxic than those intended for the general public.
Notes and references
- See the chapter "The dirty dozen – not just an old movie?"
- Neurotransmitter substances are active natural chemicals that convey messages from one nerve cell to the next or from a nerve to muscle. Organophosphates prevent the breakup of one of these neurotransmitters, acetylcholine, increasing its concentrations to abnormal levels.
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