(C)         A useful definition of an air pollutant is a compound added directly or indirectly byhumans to the atmosphere in such quantities as to affect humans, animals, vegetation, ormaterials adversely. Air pollution requires a very flexible definition that permits continuouschange. When the first air pollution laws were established in England in the fourteenthcentury, air pollutants were limited to compounds that could be seen or smelled - a far cryfrom the extensive list of harmful substances known today. As technology has developed andknowledge of the health aspects of various chemicals has increased, the list of air pollutantshas lengthened. In the future, even water vapor might be considered an air pollutant undercertain conditions.        Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, andnitrogen oxides, are found in nature. As the Earth developed, the concentrations of thesepollutants were altered by various chemical reactions; they became components inbiogeochemical cycle. These serve as an air purification scheme by allowing the compoundsto move from the air to the water or soil on a global basis, nature's output of thesecompounds dwarfs that resulting from human activities. However, human production usuallyoccurs in a localized area, such as a city. In this localized region, human output may bedominant and may temporarily overload the natural purification scheme of the cycle. Theresult is an increased concentration of noxious chemicals in the air. The concentrations atwhich the adverse effects appear will be greater than the concentrations that the pollutantswould have in the absence of human activities. The actual concentration need not be large fora substance to be a pollutant; in fact, the numerical value tells us little until we know howmuch of an increase this represents over the concentration that would occur naturally in thearea. For example, sulfur dioxide has detectable health effects at 0.08 parts per million (ppm),which is about 400 times its natural level. Carbon monoxide, however, has a natural level of0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
73. What does the passage mainly discuss?
(A) The economic impact of air pollution.
(B) What constitutes an air pollutant.
(C) How much harm air pollutants can cause.
(D) The effects of compounds added to the atmosphere.