1.   There is one-third less oxygen for efficient fuel combustion, and cold layers of air in winter trap toxic emissions with thermal inversions.

2.   The crisis was caused by the climatic phenomenon of thermal inversion, which trapped emissions at near-ground level.

3.   The pollution is worst during winter, when thermal inversions trap the warmer polluted air above the city.

4.   Pollution is worst during winter months, when cold air over the city produces a thermal inversion effect, trapping the warmer, polluted air close to the surface.

5.   Proponents of the scheme believe the fans would form artificial tornadoes of polluted air, which would be propelled up through the thermal inversion cap.

6.   A succession of unyielding thermal inversions made January especially asphyxiating.

7.   But if winds die down or turn, Paris, like other European cities, has thermal inversions on winter days and a soupy smog in mid-summer.

8.   But if the western breezes die down or turn, car fumes are trapped in thermal inversions in winter or make for a soupy summer smog.

9.   Thermal inversion is the name of the wintertime weather event that turns the air over Mexico City into a grayish-yellow pudding of pollutants.