Chemicals have become a part of our life, sustaining many of our activities, preventing and controlling many diseases, increasing agricultural productivity.

However one can not ignore that many of these chemicals may, especially if not properly used, endanger our health and poison our environment.

Most chemical accidents have a limited effect. Occasionally there is a disaster like the one in Bhopal, India, in 1984, with thousands of deaths and many people permanently disabled.

It is not just the worker handling chemicals who is at risk. We may be exposed in our homes through misuse or by accidents, and be contaminated by consumer products including food.

The environment may be affected, chemicals may pollute the air we breathe, the water we drink, and the food we eat. They may have entered into forests and lakes, destroying wildlife and changing the ecosystems.

The harmful effects of chemical substances depend on the toxicity and the exposure to that chemical. Toxicity is a property of the chemical substance, while the exposure depends on the way the chemical is used. The level of exposure depends on the concentration of the hazardous chemical and on the period of contact time. Many substances do not give any warning by odour, even though they may be present at dangerous concentrations in the workplace air.

Dusts, fumes and gases

Dust may be just a nuisance, and the danger depends on the type of material in the dust, and on the amount and the size of the particles.

Asbestos is a natural mineral fibre which is very resistant to fire and to many chemicals. Asbestos fibres are very strong and thin. The dust penetrates the lungs destroying the lung tissue. This condition is called asbestosis. Asbestos can also cause lung cancer. The risk of cancer is many times higher if the asbestos exposure is combined with smoking. Many countries have restricted or banned the use of asbestos.

Exposure to metal fumes can cause damage to the body. `Metal fume fever' is a known health effect when metal fumes, often containing zinc, are inhaled. It usually appears on the day following that of the exposure.

Gases do not necessarily have a warning odour at a dangerous concentration. The odour may be apparent only at very high concentration in the air. Gases may have an irritating effect, or they may enter the blood circulation and cause internal damage.

Sulphur oxides, nitrogen oxides, chlorine and ammonia are toxic gases that are corrosive and irritating to the respiratory system. They are widely used in industry. Phosgene is formed when solvents containing chlorine, such as "TRI" (1,1,2-trichloroethylene), come into contact with hot surfaces or flames. Phosgene can be deadly poisonous even before the odour is detected.

Carbon monoxide is a toxic, odourless, colourless gas which is formed by the incomplete burning of materials of organic origin. It may enter the blood circulation. Some gases can pass through the skin, for example, hydrogen cyanide.

Solvents

Most solvents are liquid organic chemicals. They are used because of their ability to dissolve other substances, particularly fat and grease, which are insoluble in water. Many of them evaporate rapidly at ambient temperatures. They are often flammable and may ignite by heat from smoking, welding or static electricity. Vapours move with air currents and can ignite even by a distant heat source.

Inhalation is the most common way for solvents to enter the body, but some of them penetrate intact healthy skin. Once in the blood stream a solvent can be transported to different organs, such as the brain and liver.

Solvents have different effects on humans, depending on their evaporation rate and their solubility in water. The risks of health effects depend on the period of exposure and the concentration of the solvent in the inhaled air.