The whole field was brown-coloured; should you even go near it let alone eat anything growing there?
Pesticides are a unique group of chemicals in the sense that they are deliberately used to destroy life. In this sense, they are like antibiotics: penicillin and the other antibiotics are used to kill bacteria causing diseases, but not to damage the cells of the person taking the medicine. So when chemists have been developing these substances, the prime aim has been to find selective effects so as to kill only weeds or harmful insects, but have no effects on humans or the environment. In many cases, this challenge has been quite successfully met, if the differences between the pest species and the protected species have been large enough.
Some of the pesticides are very safe for humans, for example, compounds that prevent photosynthesis in plants. These substances do not harm animals, because there are no photosynthetic processes in animals. Nonetheless, there may be other, completely unrelated adverse effects as with any chemical. Since it has not been possible to achieve total selective toxicity, pesticides have to be used with care and professionalism, and their use is regulated in many ways.
Herbicides are the most widely used group of pesticides if measured in tonnes used per year. Herbicides are used to kill weeds in fields and gardens, and to prevent grass and bushes from overwhelming young tree plants in recently planted forests. Many of the more toxic compounds have been withdrawn from the market, but there are differences between countries in this respect. Paraquat was banned in Finland and some other countries a long time ago because of its high immediate toxicity. The European Union permitted its use in 2004, but four EU states, Austria, Denmark, Finland and Sweden brought a case against the EU Commission which led to the directive authorising its use being annulled in 2007. However, this toxic compound is still in use in many other countries. Herbicides may be toxic, primarily to the people handling them, and their use requires a good awareness of the compound in question and strict compliance with the directions of use.
There are different approaches to weed control. Selective substances may be applied during the growing period of the crop plants. Non-selective substances may be used after harvesting to clear the field of unwanted vegetation before the next crop is sowed. For example, phenoxyacetic acid herbicides can be used to kill broad-leaved weeds in corn fields, because grasses are resistant to this agent. Glyphosate is used after the harvest to kill all the vegetation. Since it is only used after harvesting, this means there should not be any residues in the crop, because glyphosate binds to the soil particles, and is broken down rather than being absorbed by the plants growing the following year. Those herbicides that are used after the germination of the crop are applied very early, and therefore the time span until harvest is relatively long. If the directions are followed, herbicide residues are not a problem for the consumer.
Glyphosate is very non-toxic non-selective herbicide, and therefore its use has been increasing. There has been some concern about its filtering into the ground water. In Central Europe where there is intensive agriculture, some glyphosate has been detected in ground water, whereas in the Nordic countries it seems as if this is unlikely to be a problem.
In recent years, the agricultural topic to have aroused the greatest passion and debate has been genetically modified plants, e.g. sugar beets which are resistant to glyphosate. These kinds of resistant plants would allow the use of herbicide during the growth period, and this would further increase the use of the herbicide. Glyphosate toxicity is low, because it inhibits an enzyme that is crucial for plants but does not exist in animals. Its lethal dose in animal experiments is several grams per kilogram. Therefore any risk seems to be very remote. As long as use would be limited to sugar beets, during processing to extract the sugar, any remaining pesticide would be removed. If this technique were used for edible plants such as carrots, then stricter precautions would need to be invoked.
The most controversial group of herbicides has been the phenoxyacetic acid derivatives. Some of the early preparations contained high concentrations of dioxins as impurities, and there was concern of the possibility of dioxin-like toxicity i.e. cancers and developmental effects. However, after 30 years of studies there is still no unequivocal proof on the matter, and this does seem to indicate that the risk cannot be high even in individuals who are exposed to high concentrations through their work.
There does not seem to be any major risk to the consumer from herbicides. Some exposure is possible from picking berries or mushrooms from a recently sprayed forest area. Another source could be exposure of children playing on lawns where herbicides have been sprayed to kill dandelions. Phenoxy herbicides are being replaced by non-toxic sulfonylurea compounds, the doses of which are only a few grams per hectare.
Some herbicides applied to the soil before seeding are known to disturb the functions of the thyroid gland, and also thyroid cancer risk has been a cause for concern. Since these are used early in the summer, there should be no residues in the products, and the potential risks would be occupational and not a concern for the general public.
Fungicides are used to prevent fungal diseases of plants. Previously this group of compounds included very toxic and persistent substances such as organic mercury compounds. These are no longer marketed in industrialised countries, and the toxicity of most modern fungicides is very low. The remaining risks are related to their tumourigenic potency and endocrine disruption (mostly thyroid hormone synthesis). The risk products are fruits transported from distant countries and sometimes even domestic products that are prone to fungal diseases, e.g. apples and strawberries. The problem is application of the fungicides close to the time of harvesting.
Dithiocarbamates are a group of compounds suspected of being carcinogenic because of their chemical relationship with thiourea, a known carcinogen. These compounds (e.g. maneb, zineb, mancozeb) are very non-toxic, but some of them can evoke developmental defects at high doses in experimental animals. Their possible breakdown to carcinogenic compounds has raised the concerns of possible carcinogenicity. However, this has never been demonstrated either experimentally or epidemiologically. Nonetheless it is reasonable to aim at as low residue levels as possible.
Some of the pesticides are very toxic compounds because they are intended to be lethal to some living organisms. Therefore strict control by society is needed. On the other hand, just because this strict control is in place, this group of chemicals rarely is responsible for any major problems. Intake is usually a fraction of the estimated acceptable levels. However, there can be major variation between countries and therefore also trade should be carefully monitored. For instance, in Finland, 91% of pesticide exposure originates from imported foods, mainly fruit. In the developing countries, there may be serious risks of acute toxicity, even deaths due to inadequate control and poor understanding of the way to use these compounds safely.
If the regulations and controls are such as they are in the best countries at the moment, there are no major health concerns from pesticides in food. Unfortunately the situation is not so good in some places, in particular in the developing countries.
Notes and references
- Photosynthesis is a chemical process in plants using the energy of light to synthesize hydrocarbons from carbon dioxide and water. The key molecule capturing the energy of light is chlorophyll, the green coloured pigment of plants.
One level up: Is man defiled by what goes to his mouth?
Previous chapter: If this kills insects, what effect is it having on me?
Next chapter: Should we only eat “organic” products?