165
compounds that could be seen or smelled - a far cry from the extensive list of harmful substances
known today. As technology has developed and knowledge of the health aspects of various
chemicals has increased, the list of air pollutants has lengthened. In
the future, even water vapor
might be considered an air pollutant under certain conditions.
Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and
nitrogen oxides, are found in nature. As the Earth developed, the concentration of these pollutants
was altered by various chemical reactions; they became components in biogeochemical cycles.
These serve as an air purification scheme by allowing the compounds to move
from the air to the
water or soil. On a global basis, nature's output of these compounds dwarfs that resulting from
human activities.
However, human production usually occurs in a localized area, such as a city. In such a
region, human output may be dominant and may temporarily overload
the natural purification
scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The
concentrations at which the adverse effects appear will be greater than the concentrations that the
pollutants would have in the absence of human activities. The actual concentration need not be large
for a substance to be a pollutant; in fact, the numerical value tells us little until we know how much
of an increase this represents over the concentration that would occur naturally in the area. 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 of 0.1 ppm and is not
usually a pollutant until its level reaches about 15 ppm.
Chia sẻ với bạn bè của bạn: