Exposure risk assessment involves evaluation of damages which a contaminant present in air, water, soil or food supplies is likely to inflict on a living system, a population or a community.

The presence of toxic material in any of the component of the environment could directly or indirectly affect the entire biotic spectrum resulting ultimately into adverse changes in the entire ecosystem. It is the adverse effect caused to the living beings, the evaluation of which is being discussed here.

A biological system is an exceedingly complex machinary on which action of toxic material is usually highly diverse and complicated (Ariens et al., 1976). Most of the toxic responses stem from the following action of the toxic agent:

1. Direct chemical irritation of tissue systems.

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2. Interference in the activity of the enzyme systems.

3. Blockade of Oxygen transport by haemoglobin molecules.

4. Disturbances in general cellular functions.

5. Hypersensitivity and allergy.

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6. Interference in synthesis and/or function of nucleic acids and proteins.

The toxic effects produced by a toxic agent may be reversible or irreversible. Reversible toxic effects are those which disappear soon after the exposure ceases and the toxic agent is detoxified and eliminated. Irreversible toxic effects on the other hand, persist even after the exposure is discontinued. Effects like carcinomas, mutations, damages to neurons and liver cirrhosis are obviously irreversible as they are usually produced by permanent damage or changes in the tissue systems.

There are some toxic materials which cause reversible effects when administered in low concentrations and irreversible toxic effects when higher concentrations are introduced in the system. The chances of recovery from exposures which cause irreversible effects are very low and they are considered very dangerous.

Certain toxic effects are considered irreversible though they disappear after the exposure has ceased. Insecticides which inhibit the activity of enzyme cholinesterase for short duration which is approximately the time required for the synthesis of the replacement enzyme are examples of this type. The toxic effect is considered irreversible because the enzyme once affected is made useless. The recovery is actually due to the formation of fresh enzyme.

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The toxic affects produced by a toxic agent may appear immediately after the exposure or there may be some time gap between the exposure and the appearance of toxic response. For example, Cyanide poisoning the toxic effect occurs immediately after the exposure, while some effects, particularly those caused by the interference of toxic agent in the synthesis and/or function of nucleic acids and proteins may appear months or years after the actual exposure, or at times they may appear in the following generation. To determine the delayed effects of toxic agents on a living system low term studies are essential. Such affects are considered to be the most dangerous ones.

(1) The Basis of Exposure Risk Assessment:

The magnitude of exposure risks and the problems associated with the presence of toxic material in any of the component of the environment depend on a number of factors. Evaluation of exposure risk, therefore, involves study and observations to obtain the following set of information:

1. The concentration of toxic material in the medium carrying it.

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2. The route of entry of the toxic material in a living system.

3. The quantity of toxic material absorbed by the organism.

4. The nature and properties of the toxic material.

5. Behaviour of the toxic material within the living system.

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6. Threshold concentration of toxic action and LD50 or LC50.

7. The nature of adverse effects caused by the toxic agent.

(2) Acceptable and Unacceptable Risks:

With a variety of chemical in daily use all around us, living organisms are almost invariably exposed to smaller or larger concentrations of a number of toxic agents which are absorbed through various routes into the living system. Sooner or later the toxic material absorbed in smaller Quantities is detoxified and/or excreted from the biological system. In many cases, the toxic response does not appear. The toxic response, however, appears only when the concentration of the toxic material exceeds the threshold concentration for toxic action. In practice exposure risks may be considered manageable when?

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1. Absorption of the toxic material by the living system is well below the threshold concentration for toxic action.

2. When the exposures may cause some toxic response which is reversible and can be tolerated with a little inconvenience only.

A large number of unwanted and toxic chemicals enter a living system in small quantities, are retained, modified or detoxified and excreted out in due course of time. The entry presence or toxic action of these materials is not even felt by the organism concerned.

The living system works incessantly to modify, detoxify and eliminate the toxic material so that the toxic concentrations are never attained with the system. However, life in such an environment is more taxing for the organism. Due to socio-economic reasons, however, such conditions are usually tolerated as there is no other alternative for the organism to go to.

Exposures which cause irreversible toxic responses are considered more dangerous and are usually avoided. There are no chances of recovery from such exposures – the damages caused are of permanent nature. In this high risk category are also placed chemicals which are responsible for causing late and latent toxic effects.

The toxic symptoms do not appear immediately and there may be a considerable gap of time between actual exposure and the appearance of toxic responses. In most of the cases delayed toxic responses are due to exposures which involve interference in the synthesis and/or function of nucleic acids and proteins resulting in carcinogenic, teratogenic and mutagenic action. Often exceedingly small quantities of chemicals trigger these responses which are magnified by the biological system itself into irreversible changes in structure and function of tissues (Williums 1976).

(3) Criteria for regulating the use of Chemicals:

A multitude of standards are used to regulate the use of chemicals in the society and determination of acceptable risks. Different substances are often regulated according to different standards. The same substances may be subject to one regulatory standard when used in agriculture and another when used in industries and still other when used for domestic purposes. The statutory risk standards are commonly vague and often confusing (Annonymous 1987). The criteria usually adopted for regulating the use of chemicals in various spheres of our daily life are:

1. No-risk criteria:

For the chemicals which cause delayed toxic effects such as cancers or mutations, are to be avoided completely. No chances have to taken with such chemical whatever may be the cost of imposing such restrictions (Cohrrsen and Covello, 1989). The criteria are oblivious of the cost of regulation as the effects produced by such exposures are of a very serious nature. The chemical could, if allowed to circulate in the environment, change the very course of evolution.

2. Margin of Safety criteria:

If a substance causes reversible toxic effects which are manageable within a comfortable margin of safety, it’s use may be permitted. Smaller concentrations of such chemicals may be allowed as they cause little obvious toxicity and even if the toxic response is stronger remedial measures could be adopted as there is no danger of late and latent effect. Regulatory agencies permit the use of these chemicals with little additional margin of safety (Williams 1976).

3. Criteria using Risk-benefit analyses:

At times it becomes necessary to use a chemical even if it causes some toxic effects. Regulatory agencies are required to weigh the damages caused by the exposures to such chemicals and the benefits which acrue from its use. If the benefits derived are substantial as compared to the costs involved in restricting the use of the chemical or the cost involved in repairing the damages caused by the exposures the use of the chemical is allowed (Cohrrsen and Covello, 1989).