11.   Cars are a major cause of air pollution.

12.   Cars are still one of the principal contributors to air pollution.

13.   Cleaner fuel has contributed to a reduction in air pollution.

14.   Copies were distributed far beyond West Virginia to scientists and officials concerned with the effect of air pollution on Eastern forests.

15.   Fumes from cars are a major cause of air pollution.

16.   Great strides have been made in reducing air pollution in US cities.

17.   Instead they see low concentrations of magnesium as a natural phenomenon exacerbated by air pollution.

18.   Most air pollution is caused by cars.

19.   She says that transporting goods by rail instead of road would cut air pollution dramatically.