Pesticides are an example of contaminants deliberately introduced into the environment that have played a significant role in the decline of species and the loss of habitat diversity. They interfere with and cause the breakdown of fundamental biogeochemical processes that support life support systems, including decomposition, mineral oxidation, nitrogen fixation and photosynthesis.
Pesticides are chemical compounds used to control plants and animals that are classified as pests. They are most widely used on crops, but they are also used in and around the home on insects, rodents, weeds, and plant diseases; in wood processing and preserving; in paint; in food storage; and in public health programmes. Insect and weed control are the two most common uses. Some pesticides are applied directly to plants or soil. Soon after application, they are dispersed into the environment, so that applications are often repeated. As the pesticides accumulate in the soil and wash into streams and rivers, they can affect fish and birds. Because of the wide spectrum of life forms which they attack, some call these pesticides biocides.
Because of the toxicity of pesticides in concentrated form and because of the frequency of exposure, the most serious human health effects are found among agricultural and production workers. Long-term and chronic health effects occur as the chemicals are ingested and inhaled. Bioaccumulation (the buildup of toxic materials in tissues) is evident in fish and birds as well as in humans.
Two different sets of problems are posed by the occurrence of persistent pesticides in the environment: (1) localized problems, tending to be acute, leading to recognizable effects with assignable origins, which can be dealt with if successfully sorted out and if the will and powers exist; and (2) more widespread problems, tending to be inferential with postulated effects, due to the universal presence of the materials in question. There is, for example, growing evidence from the amount of pesticide residues found in specimens of affected species as well as in the animals, fish, invertebrates, or plankton they feed upon, that there is a process of pesticide concentration in the food chains. As small amounts of persistent pesticides become more and more widely spread throughout the entire natural environment, they are absorbed by low forms of life. Where large numbers of these species serve as food for higher animals, some of the total pesticide remains in the eater. This can lead to a certain concentration of pesticides in a form of life at the end of the food chain. Eventually lethal doses may be reached for certain populations, or the species may be reduced because of adverse effects on reproduction or behaviour.
The importance of changes in the environment is that they may be irreversible: species may disappear altogether. This will cause ecological imbalance - certain forms of life whose numbers are controlled by the disappearing species will no longer be held in check. Thus an apparently harmless species may become a pest. Another result of reduction of species diversity may be the loss of the genetic possibilities that each organism disturbance of the selective pressures of the environment will reduce the chances of future development of desired plants and animals. Furthermore, as humans are at the top of food chains, they too tend to concentrate residues in their bodies with as yet unknown effects on them. While perhaps sufficient attention has been directed to acute toxicity problems, too little attention has been paid to the effect of long term ingestion by human beings of small amounts of these chemicals.
A serious dilemma is that the cheapest pesticides are the organochlorines, known to accumulate in human fat. The substitution of organophosphorus pesticides brings both higher cost and a need for more carefully trained operators, both of which pose problems.
Recent insight into the toxic effects of pesticide exposure suggests that early-life exposure is of greatest concern, especially prenatal exposure that may harm brain development. Most insecticides are designed to be toxic to the insect nervous system, but living creatures depend on similar neurochemical processes and may therefore all be vulnerable to these substances; potential adverse effects on the nervous system may apply to many herbicides and fungicides as well. However, there is no systematic testing for neurotoxicity – especially developmental neurotoxicity – as part of the chemical registration process. This despite at least 100 different pesticides that are known to cause adverse neurological effects in adults. Such adverse effects are likely to be lasting and one main outcome is cognitive deficits, often expressed in terms of losses of IQ points. growth, brain functions and sexual development.
Worldwide, about a million people are poisoned by pesticides each year; ten thousand of these victims die from such poisonings. The risks are greatest in developing countries. Ninety-nine percent of the deaths caused by agricultural chemicals occur in those countries.
According to the International Food Policy Research Institute, every dollar spent on pesticides causes five to ten dollars worth of damage in poisonings, accidents, contamination and pollution.
Concern about pesticide residues is largely associated with the organochlorine insecticides - notably dieldrin and DDT - for they are exceptionally persistent, a quality which also makes them highly useful in many pest control situations. It has been argued that their residues in foods, though initially minute, could accumulate in the human body and build up levels sufficient to affect health. The organochlorines are fat-soluble, so that any retained in the human body are mainly to be found in fatty tissues. Each year almost 0.5 kg of pesticides is used on crops, parks, forests, and lawns for each person on earth. In the USA, 2.7 kg per person are used. Half a million people are poisoned by pesticides each year, and five thousand die. Although developed nations like the USA have restricted the most dangerous chemicals, they do not forbid their production for export. More than 64 million kg of restricted or unregistered pesticides were exported by the USA in 1976. Though its use is banned in the USA, DDT is still produced for export and traces of it can be found on most coffee beans imported into the country. More than 360 million kg of pesticides are used in developing countries that lack the resources or expertise to evaluate their safety. As pesticide use increases, so does resistance among pests. Pesticide use is then increased or new poisons are found to fight these new pests.
In the Cañete Valley of Peru, the use of organochlorine and organophosphorous pesticides gave rise to highly resistant super-pests whose ravages made cotton production uncommercial until the pesticides were abandoned and equilibrium was restored. A switch to chemical from non-chemical control methods was responsible for similar declines in cotton production in Mexico, Central America and part of the Middle East. In Malaysia and elsewhere, the use of pesticides brought on heavy attacks by pests on oil palm, rubber and other crops. Perhaps more tragically, the incidence of malaria has been on the rise again in a substantial number of developing countries, where it appears that the main vector, the anopheline mosquito, developed a resistance successively to dieldrin, DDT, malathion and propoxur.
In 1972, a shipment to Iraq of 80,000 tons of imported wheat and barley coated with organic mercury fungicide caused the death of at least 400 persons and the hospitalization of another 5,000. The fungicide whose domestic use had already been banned in the country of export, had worked slowly by entering ecological cycles and food chains, eventually reaching people through poultry, meat, river fish and bread. In rural areas of Guatemala, the accumulation of DDT and similar compounds in mother's milk was found to have attained levels over 400 times greater than the maximum acceptable daily intake limit for human infants.
Pesticide residues travel long distances and build up in the food chain, and so can affect people and other creatures far from the places where they were applied. They kill or injure large numbers of non-target organisms, including fish, birds, insects that pollinate crops, and animals that prey on others attacking crops. They have left unsolved many pest problems and created new ones.
Chemical compounds that persist in the environment and which can affect the health and reproduction of organisms at the molecular or the reproductive level are now considered to be a much greater global problem than many of the 'old' poisons, such as lead, where adverse effects and the measures needed to reduce them are now reasonably well understood.