The abnormal reversal of temperature in the troposphere caused by meteorological conditions. This leads to the entrapment of urban air pollution because mixing of surface air into the rest of the troposphere is limited. Under normal circumstances, air in the troposphere is cold at high altitudes and warm at low altitudes, or near the earth's surface. (See lapse rate. ) When air pollutants, such as O₃, SO₂ or NO_x, in urban areas get too concentrated, smog is produced. This can lead to human health disturbances in people with respiratory illnesses such as asthma. Surface temperature inversions are created when the lapse rate goes negative, that is, when the temperature profile of low lying tropospheric air increases with altitude. Air at the earth's surface is trapped there because the air above it is less warmer and less dense. This can be disastrous for the air quality in urban areas when anthropogenic pollutants are not mixed away from the surface and, instead, build up. In London, England in 1952, 4,000 people died during a four-day temperature inversion event in December. In Donora, Pennsylvania in 1948 a five-day temperature inversion trapped smog---made of soot, SO₂, and metal fumes from a zinc works---that killed 20 people.