Popular poisons

Toxic pesticides that contaminate the soil and enter the food chain need to be replaced with non-toxic ones. But research in this area is still in its infancy

By N P Agnihotri
Published: Thursday 15 January 1998

-- (Credit: Amit Shanker /CSE) the ever-increasing pressure to increase food production to meet current demand requires protection of crops from pests. The use of pesticides is, therefore, inevitable, and constitutes an integral part of modern crop-management practices. It has been estimated that crop damage is as high as 20 per cent where pesticides are not used.

Pesticides include chemicals to protect crops from insects (insecticides), weeds (herbicides), disease-causing micro-organisms (fungicides and bactericides), mites (acaricides), nematodes (nematicides), and rodents (rodenticides). Pesticides also include chemicals that control pests directly hazardous to animals and humans, particularly vectors -- such as mosquitoes, flies, fleas, ticks and lice -- that transmit diseases.

At present, food production in India is around 190 million tonnes, which is just enough to meet domestic requirements. By the turn of the century, the population is likely to increase to one billion. It will then require 210 million tonnes of food grain. The increase in production will depend mainly on introduction of high-yielding crop varieties and increasing the productivity of agricultural land. Reduction of crop losses caused by pests and diseases will thus be a key factor in achieving this goal.

In India, 143 pesticides are registered, with an annual consumption of about 85,000 tonnes. Organochlorine insecticides form the bulk of pesticides used in India, of which hch , dichlorodiphenyl trichloroethane (ddt) , malathion, methyl parathion, monocrotophos and endosulfan are the most extensively used. Pesticides are currently used on 25 per cent of the total cultivated area.

Most pesticides, particularly insecticides (constituting over 70 per cent of the pesticides used in India), are inherently poisonous. Plants sprayed with pesticides become toxic. However, the toxicity of pesticides is not permanent. Soon after they are applied on a crop, pesticides start breaking down due to the action of plant enzymes or environmental factors such as light, temperature and wind. After some time, the concentration of pesticide falls below maximum residue limit (mrl), a level of pesticide residues considered safe for daily consumption.

During application, some of the pesticide falls on the ground, contaminating the soil. The pesticides are detoxified in the soil by adsorption and degradation. However, their presence in the soil for a long time could adversely affect its fertility, besides contaminating nearby water bodies.

Pesticides can also contaminate groundwater as the soil soaks in rain and irrigation water. The presence of pesticides in groundwater is a matter of grave concern since pesticides cannot be filtered out using most filtration techniques. In addition, if contaminated groundwater is used for irrigation, it can be a potential source of pollution for crops.

The harmful effects of a pesticide depend on its toxicological properties and the degree of exposure of humans to the residues. Thus, the hazards associated with pesticide residues depend mainly on two factors:

l Concentration or level of residues

l Toxicity to humans or other life forms

Of the two, the level of residues is of greater significance. The mere presence of pesticide residues in food does not mean that it is hazardous. To be toxic, residues have to be present in quantities large enough to be considered unsafe or toxic (greater than mrl), which cause some disorder or disease in the body.

To determine levels of residues 'safe' for consumption, several studies have been carried out on animals which were fed on food containing known small doses of pesticides.

In addition, several other studies on the toxicity of pesticides were carried out -- their role in causing mutations in genes (mutagenecity) or cancer (carcinogenecity), and their effect on the brain and the nervous system (neurotoxicity) -- to establish tolerance levels. Surprisingly, it was found that a pesticide which is highly toxic in cases where a large dose has been consumed can be relatively less hazardous than one with which is not as harmful in the same dose. That is, a seemingly less harmful pesticide may harm the body more in the long term.

In the past, organochlorine pesticides like ddt , benzene hexachloride ( bhc), and aldrin have been popular among Indian farmers because of their simplicity in application, efficiency and good returns. Their continuous use has resulted in widespread contamination of food commodities, which is a serious matter from the viewpoint of both domestic use and food export.

Pesticides in water are toxic at very small concentrations - of the order of a few thousand nanogrammes (one billionth of a gramme)per litre (ng/l). It has been reported that all major rivers of India are contaminated with ddt (21,900 ng/l), bhc (272,000 ng/l), endosulfan (2,890 ng/l) and aldrin (1,500 ng/l). Pesticides in cultivated soils in India are found to range from 0.02-15 parts per million (ppm) for ddt , 0.0124-0.02 ppm of bhc . These pesticides find their way into crops from the contaminated soil. Pesticides have also been reported in urban drinking water and rural groundwater. ddt and bhc were detected most frequently, besides endosulfan, aldrin, and heptachlor.

Almost all food commodities in India are re-contaminated with residues of pesticides, particularly ddt and bhc . Milk samples have been found to contain as much as 127 ppm ddt and 13 ppm bhc , while vegetable oils contained upto 26 ppm ddt.

A two-year study on pesticide residues in food commodities, sponsored by the Food and Agriculture Organisation, was carried out at Punjab Agricultural University, Ludhiana. Samples were collected from Punjab, Haryana, Delhi, Maharashtra, Andhra Pradesh and Uttar Pradesh. In the 1,651 samples of cereals analysed, 53 per cent contained residues of ddt (ranging from traces to 12 ppm) and 31 per cent bhc (traces to 20 ppm).

The majority of the samples were contaminated with ddt and 60 per cent of the samples exceeded the mrl value (1.25 ppm). Besides ddt , residues of bhc in the form of alpha and beta isomers (a known carcinogen), were also detected. About 66 per cent samples of a popular brand of butter contained ddt residues above the mrl . Similarly, out of 2,154 samples of vegetables analysed, 27 per cent contained ddt (traces to 35 ppm) and 28 per cent bhc (traces to 60 ppm).

According to a recent report published by the Indian Council of Medical Research, in which about 2,000 samples were analysed, 97.5 per cent of the samples of food commodities analysed were contaminated with pesticide residues and 25 per cent contained residues above the mrl . Even human milk was found to be contaminated with residues of ddt and bhc.

Samples of milk taken from 120 mothers in Punjab contained residues of ddt (0.24-0.50 ppm) and bhc (0.07-0.18 ppm).

Organochlorine pesticides are fat soluble. Intake of these chemcals results in their accumulation in body tissue. As a result, these pesticides can be transferred from mother to babies in the foetus or through mother's milk. According to studies, residents of India contain the highest level of ddt in the world - of the order of 28 ppm.

Thus, extensive and excessive contamination of soil, water and food commodities with pesticide residues poses a serious threat to humans and the environment. Interestingly, residues of organophorus, carbamate, or synthetic pyrethroid insecticides were either not found or much below the mrl values among Indian populations. This suggests that if use of persistent organic compounds like organochlorine insecticides is banned or restricted, the problem of pesticide residues can be minimised to a great extent.

The consumption of pesticides in India is very low - about 400 g/ha, as compared to 1,470 g/ha in the us , 1,870 g/ha in Europe, and 10,790 g/ha in Japan. Yet, the level of pesticide residues in human tissue is much lower in these countries and regions. The main reason for high residues in India is the indiscriminate use of persistent organochlorine pesticides. Farmers are mostly illiterate and ill-equipped to use these pesticides as per recommendations.

Beside leaving residues, consistent use of these pesticides has also led to development of resistance among pests and vectors, and adverse effect on non-target organisms. The negative aspects have thus overshadowed the benefits of the use pesticides in alleviation of poverty and in dealing with hunger. There is an urgent need to think of alternative ways.

Twelve pesticides - endrin, chlordane, heptachlor, dibromochloropropane, toxaphere, pentachloro-nitrobenzene, pentaclorophenol, ethyl parathion, nitrofen, tetradifon, paraquat dimethyl sulphate and aldrin - were banned in India in June 1993. bhc was banned on April 1, 1997. Most of these pesticides are toxic and persist in the environment for long periods. Alternatives are now available. For example, ddt can easily be replaced by synthetic pyrethroids which are biodegradable, effective at low dosages with a low toxicity for mammals. However, these alternatives are more expensive and require proper handling and application. So, while ddt is being phased out, it is still being used in public health applications, primarily for economic reasons.

Considerable research has been conducted to develop non-chemical pest control methods, but it has not reached a stage where pesticides could be dispensed with altogether. Botanic preparations and biopesticides are emerging as viable alternatives to synthetic pesticides. These bio-control methods can supplement the use of chemical methods to some extent. However, there is a general consensus that use of synthetic pesticides will continue in the near future.

Recently, some pesticides have been developed which are biodegradable, are safe for mammals, have a low residual life and do not harm non-target organisms. There has also been an emphasis on integrated pest management (ipm) where chemical and biological pesticides are used together. These developments are most encouraging and will help check use of toxic chemicals, while ensuring food security through higher crop yields

N P Agnihotri is the all India project coordinator, Pesticide Residues, at the Indian Agricultural Research Institute, New Delhi

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