In a similar incident in Pennsylvania almost two decades later, six thousand became ill, and eighteen died. In London, an incident of pollution trapped by an air inversion in 1952 caused 4,000 deaths in five days. Sulfur dioxide is released into the air in copious amounts during production of electricity, heat, and other energy with fossil fuels. Asthmatics are shown to be 10 times as sensitive to sulfur dioxide than healthier subjects when exercising. Ozone is a pollutant that acts to make the lungs more susceptible to all other air pollutants by increasing permeability of the bronchial tissues.
Ozone is produced, in part, as a result of the sun's action on automobile exhaust.
Nitrogen dioxide is a potent bronchoconstrictor produced in explosives manufacturing, fertilizer, and cigarette smoking. Any outdoor exercise should be limited, and done in early morning or late evening. When overall pollution levels become high, it may become necessary to increase the level of medication, on advice from the physician. Also avoiding wood burning fireplaces may be helpful. Thermal inversions, high pressure fronts, and static weather conditons can cause pollutants to stagnate and accumulate in the atmosphere.