Waste is hazardous which because of its quantity, concentration or physical, chemical or infectious characteristics may cause or contribute to an increase in mortality or an increase in illness or pose a substantial present or future hazard to human health or the environment. Hazardous wastes result from manufacturing processes and from non-manufacturing sources such as waste oil from filling stations, ash from power plants, dredged materials, and waste from hospitals. The problem is two fold: (a) the ongoing treatment of waste as it is continuously produced and (b) remedying damage to the environment and human health cause by improper disposal of wastes in the past. Undisciplined disposal of these wastes can cause fires, explosions, air, water and land pollution, contamination of food and drinking water, damage to people who get them on their skins or inhale their vapours, and harm to plants and animals.
One of the most worrying features of the problem is that very little is known about the long-term consequences of exposure to hazardous chemicals, although a good deal is known about their short-term effect. It is known, however, that consequences over long periods can include cancer, delayed nervous damage, malformations in unborn children, and mutagenic changes that could produce disability and disease in future generations. The situation is made even more difficult because, once they are in the environment, chemicals spread in a very complex way and may be converted into other substances which have different effects.
The output of hazardous wastes worldwide was about 400 million tonnes a year in the early 1990s, of which some 300 million tonnes were produced by OECD countries, mainly from chemical production, energy production, pulp and paper factories, mining industries, and leather and tanning processes.
Some 90% of hazardous wastes are produced in the industrialized countries. As developing countries industrialize and urbanize they are generating ever greater amounts of dangerous waste. Much of the resultant pollution spreads beyond national borders. In China, India and Thailand, for example, burning low quality coal adds substantially to the global problem of acid rain and greenhouse gases.