Pesticides are poisonous chemical substances which are sprayed over crop plants to protect them from pests (harmful small animals) and diseases. These include insecticides, fungicides, herbicides, rodenticides and weedicides.
The earliest chemical pesticides were nicotine, rotenone and pyrethrum from plant sources. The first synthetic pesticide was Paris Green – copper acetoarsenate – an inorganic compound. Other inorganic pesticides include lead arsenate, lime sulphur and fluorides.D.D.T. and B.H.C. are some of the common pesticides used today. These pesticides are chlorinated hydrocarbons (Chlorine containing hydrocarbons) which are very toxic or poisonous. It has been found that urban people (city people) have damaging amount of pesticides like D.D.T. concentrated in their bodies.
The total amount of pesticides applied world wide is currently put at 2.5 million tones. Of this 50-60 per cent are herbicides, 20-30 per cent insecticides and 10-20 per cent fungicides.
Types of Pesticides
1. Organochloro insecticides: The first synthetic organic pesticide DDT (dichlorodiphenyltrichlorothane) was introduced in 1939. This was followed by other organochloro insecticides like Aldrin, BHC, Dieldrin and Heptachlor. The hazard associated with the use of Organochloro insecticides were realised when scientists discovered residues of these compounds building up in the fatty tissues of animals and even humans. According to one report in 1970, the human fat from residents of Mew Delhi contained as much as 26 ppm, the highest in the world at that time. Their residues persist in the environment for long periods extending upto a few years causing adverse health effects like cancer.
2. Organophosphates: Organophosphates were introduced in 1930. These include Parathion, Malathion, Ronnel, Methoate and several others. Gnlike organochloro compounds these chemicals break down quickly when exposed to weather. But they are more poisonous to mammals than the organochlorines and have to be handled carefully.
3. Carbamates: A third class of insecticides, the carbamates, contain one or more amino groups. They do not leave harmful deposits on food. But some carbamates are harmful to warm-blooded animals. Carbaryl is a general purpose carbamate insecticide. Aldicarb is the most toxic carbamate.
4. Pyrethroids: A new class of insecticides called pyrethrcids was introduced in 1970. These chemicals are similar to pyrethrum, a natural insecticide, but do not break down as quickly as the natural product. They attack the nervous system. They are not toxic to mammals but are lethal to fish. For this reason their use in steams, rivers and lakes has to be strictly controlled.
Ways of entry of pesticides in our bodies
The poisonous pesticides can enter our body in the following ways:
1. The pesticides enter our body directly when we don’t wash the fruits and vegetables properly before eating them on which pesticides had been sprayed in the fields.
2. When excessive pesticides are sprayed over the crops during their growth, then some of the pesticides goes inside the fruit (or grains etc.) of the crop and becomes a part of it. When men and animals eat such contaminated fruits or grains etc., then the pesticides enter their body and do the damage.
3. Pesticides which are sprayed over crop plants to protect them mix up with the soil and water. From soil and water, these pesticides are absorbed by the growing plants along with water and other minerals. When they are eaten, these chemical pesticides go into the bodies of animals and human beings.
4. Some of the chemical substances used on crop plants get washed down into the ponds, lakes, rivers and underground water bodies. Drinking of such contaminated water or eating the fish grown in such water also leads to the entry of these chemicals inside the human body.
Gradual build up of pesticides in food chain
Pesticides enter the food chain at the plants level. When the man and other animals eat these plants or their products like food grains, these poisonous chemical substances get transferred to their bodies. These harmful chemicals get concentrated at each successive higher level in food chain. Let us consider the following food chain for better understanding:
Grass -> Sheep or Goat -> Man
In this food chain, grass absorbs harmful chemicals from the soil. The sheep or goat receives these chemicals when they eat grass. When man drinks the milk of sheep or goat or when man eats the meat of sheep or goat, then these harmful chemicals present in the milk or meat of sheep or goat get transferred to his body and start accumulating there.
It has been found by experiments that the harmful chemical substances present in soil and water (due to spraying of pesticides) get concentrated at each successive trophic level (Plants -> animals -> man) with the result that the creature occurring at the highest trophic level in the food chain (e.g. man) has the maximum quantity of harmful chemicals in it. The algae and the protozoa which utilize this water contain a higher proportion of 5 ppm of the harmful chemicals. The fish which feeds on these organisms has still higher amount of 240 ppm of these chemicals, and the birds and human beings which eat these fish have been found to contain as much as 1600 ppm of the harmful chemical substances in their bodies. This can be represented as
Water —> Algae & Protozoa –> Fish –> Birds & Man
.02 ppm 5 ppm 240 ppm 1600 ppm
of chemicals of chemicals of chemicals of chemicals
It has been found that the amount of poisonous chemical substances accumulated in human bodies is higher than that of other organisms which are lower in the food chain. For example, the concentration of harmful chemical called DDT is maximum in human bodies.
Harmful effects of pesticides on living beings
According to one report, only 10 per cent of the 35,000 pesticides introduced since 1945 have been tested for potential health effects. Exposure to these pesticides as well as their unregulated use is causing environmental health problems of serious dimensions. Annually about 0.5 million people are subjected to pesticide poisoning while deaths number 10,000 worldwide. According to one observer, in China there were people working in clouds of pesticides all the time and they were using the dirtiest possible pesticides. Such scenarios are very common in all developing countries. In fact, quite a large number of pesticides like the organochloro pesticides which are banned in developed countries are still widely used in developing countries because they are relatively cheap and easily available. The following are some of the detrimental effects from the use of various pesticides
- Many domestic animals are poisoned due to ingestion of pesticides and therefore significant quantities of meat and milk obtained through them are also contaminated by the pesticides.
- When pesticides are applied to crops, natural enemies that are important for controlling some pests (e.g. birds) are frequently destroyed. This causes outbursts of pests that have to be controlled with additional applications of pesticides.
- The development of pesticide-resistant pest population is proving to be a great problem. Where this occurs, additional treatment and sometimes stronger pesticides become necessary to kill these pests.
- Large number of honey bees and wild bees are poisoned by pesticides resulting in honey losses and reduced pollination.
- Some pesticides, especially when applied through aerial spray (e.g., form aircraft) and during periods of strong wind, drift into adjacent agricultural land and destroy the crops.
- Significant losses of fisheries and wildlife occur from pesticide use.
- Pesticides cause destruction of soil, invertebrates (eg. earthworm), microflora and microfauna.
- Pesticides cause chronic health problems such as allergies or diseases like cancer.
- Pesticides cause ground water contamination. It is reported that 10.4 percent of US community water supply wells and 4.2 per cent of US domestic water supply wells contain detectable levels of pesticides.
Some recommendations to lower the effects of pesticides
Although there is no way one can escape fully from the effect of chemicals and pesticides used in food but following measures will help to some extent in reducing their harmful effects.
- Wash all fruits and vegetables in plenty of water before eating, specially where skin is also to be eaten.
- Keep the food grains, fruits and vegetables for sometime in sunshine. This will dilute the effect of chemicals/ pesticides to some extent.
- For eating root vegetables in raw form (e.g. Carrot, Radish, Lettuce etc.) skin should be preferably peeled off to ward off the effect of Pesticides. Leafy vegetables should preferably be eaten after boiling and not eaten raw for reduced ingestion of pesticides sprayed over them in the field.