Major pollution episodes in brown-air cities usually occur during the summer months, when the sun is most intensive.
Air pollution levels are affected by numerous factors. Wind sweeps dirty air out of cities, rain washes pollutants from the sky. But these pollutants do not disappear. They are transferred from one medium to another. Airborne pollutants can travel hundreds, perhaps thousands, of kilometers to other cities or unpolluted wilderness.
Mountains and hills block the flow of winds and trap pollutants for days. Mountains also block the sun, which helps disperse pollutants.
During normal conditions, air temperature decreases with altitude, thus, pollutants ascend and mix with atmospheric gases. Because of this, ground-level pollution is reduced. Atmospheric mixing is brought about by sunlight. Striking the earth, sunlight heats the rock and soil. This heat is transferred to the air immediately above the ground. The warm air then rises, mixing with cooler air.
Under certain atmospheric conditions, mainly on winter days, the air temperature drops to a certain point. After that the temperature would begin to increase. This inverted temperature profile is called a temperature inversion. Temperature inversion creates a warm-air lid over cooler air. Because the cool dense ground air cannot mix vertically, pollutants become trapped near the ground, often reaching dangerous levels.
Breathing a polluted air may result in a number of diseases, including bronchitis, emphysema, and lung cancer. Symptoms of bronchitis include a persistent cough, mucus buildup, and difficult breathing. Cigarette smoking is a major cause of this disease, but urban air pollution is also a contributing factor. Sulfur dioxide, nitrogen dioxide, and ozone are believed to be the major causative agents.
Emphysema kills more people than lung cancer and tuberculosis combined. It causes the breakdown of small air sacs, or alveoli, in the lungs. This reduces the surface area for the exchange of oxygen with the blood. Breathing becomes more and more labored. Victims suffer shortness of breath when exercising even lightly. Ozone, nitrogen dioxide, and sulfur oxides are the chemical agents believed responsible for this disease.
Not all individuals are affected equally by air pollution. Particularly susceptible are the old and infirm, especially people with heart and lung disorders.