by LinDiyang
Last updated 5 years ago

World Languages

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As more people live closer together, and as they use machines to produce leisure, they find that their leisure, and even their working hours, become spoilt by a byproduct of their machines – namely, noise, The technical difficulties to control noise often arise from the subjective-objective nature of the problem. You can define the excessive speed of a motor-car in terms of a pointer reading on a speedometer. But can you define excessive noise in the same way? You find that with any existing simple “noise-meter”, vehicles which are judged to be equally noisy may show considerable difference on the meter. Though the ideal cure for noise is to stop it at its source, this may in many cases be impossible. The next remedy is to absorb it on its way to the ear. It is true that the overwhelming majority of noise problems are best resolved by effecting a reduction in the sound pressure level at the receiver. Soft taped music in restaurants tends to mask the clatter of crockery and the conversation at the next table. Fan noise has been used in telephone booths to mask speech interference from adjacent booths. Usually, the problem is how to reduce the sound pressure level, either at source or on the transmission path.

Land can be polluted by many materials. There are two major types of pollutants: degradable and nondegradable. Examples of degradable pollutants are DDT and radioactive materials. 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 decomposed to 25 percent of the original level applied. Some radioactive materials that give off harmful radiation, such as iodine-131, decay to harmless pollutants. Others, such as plutonium-239 produced by nuclear power plants, remains at harmful levels for thousands to hundreds of thousands of years.


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.


Nondegradable pollutants 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 entering the air, water, and soil or kept below harmful levels by removal from the environment.


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 throughout the world, they normally don’t build up to harmful levels. And when they do, as in the case of volcanic eruptions, they are usually taken care of by natural weather and chemical cycles2.

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.


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 30,000 chemicals are now in commercial production, and each year about 1,000 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 organisms1.





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