专业英语作业

Environmental Pollution

With the coming of the Industrial Revolution the environment pollution increased alarmingly. Pollution can be defined as an undesirable change in the physical, chemical, or biological characteristics of the air, water, or land that can harmfully affect health, survival, or activities of humans or other living organisms. There are four major forms of pollution--waster on land, water pollution (both the sea and inland waters),and pollution by noise.

Land can be pollution by many materials. There are two major types of pollutants: degradable and nondegradable. Example of degradable pollutions are DDT and radioactive material. DDT can decompose slowly but eventually are either broken down completely or reduced to harmless levels. For example, it typically takes about 4 years for DDT in soil to be decompose to 25 percent of the original level applied. Some radioactive materials that give off harmful radiation, such as iodine131, decay to harmless pollution. Other, such as plutonium239 produced by nuclear power plants, remains at harmful levels for thousands to hundreds of thousands of year.

Nondegradable pollutions are not broken down by natural processes. Examples of nondegradable pollutants are mercury, lead and some of their compounds and some plastics. Nondegradable pollutants must be either prevented from enter the air, water, and soil or kept below harmful level by removal from the environment. Water pollution is found in many forms. It is contamination of water with city sewage and factory wastes; the runoff of fertiliser and manure from farms and feed lots; sudsy streams; sediment washed from the land as a result of storms, farming, construction and mining; radioactive discharge from nuclear power plants; heated water from power and industrial plants; plastic globules floating in the world’s oceans; and female sex hormones entering water supplies through the urine of women taking birth control pills.

Even though scientists have developed highly sensitive measuring instruments, determining water quality is very difficult. There are a large number of interacting chemicals in water, many of them only in trace amounts. About 30000 chemicals are now in commercial production, and each year about 1000 new chemicals are added. Sooner or later most chemicals end up in rivers, lakes, and oceans. In addition, different organisms have different ranges of tolerance and threshold levels for various pollutants. To complicate matters even further, while some pollutants are either diluted to harmless levels in water or broken down to harmless forms by decomposers and natural processes, others such as DDT, some radioactive materials, and some mercury compounds) are biologically concentrated in various organisms.

Air pollution is normally defined as air that contains one or more chemicals in high enough concentrations to harm humans, other animals, vegetation, or materials. There are two major types of air pollutants. A primary air pollutant is a chemical added directly to the air that occurs in a harmful concentration. It can be a natural air component, such as carbon dioxide, that rises above its normal concentration, or something not usually found in the air, such as a lead compound. A secondary air pollutant is a harmful chemical formed in the atmosphere through a chemical reaction among air components. We normally associate air pollution with smokestacks and cars, but volcanoes, forest fires, dust storms, marshes, oceans, and plants also add to the air chemicals we consider pollutants. Since these natural inputs are usually widely dispersed