Pollution levels in the city were five times higher than normal.

By the 1930s the pollution levels were too high to allow for swimming.

As a result, pollution levels have dropped in recent decades.

High pollution levels were also detected in water running off from the fire.

The lack of large farms in the area and a small population means a low pollution level.

See if pollution levels where you live could make it hard to breathe today.

This has a strong effect on the pollution levels in the delta.

Pollution levels on bridges do not build up because they are open to the air.

The current law does not spell out safe pollution levels.

The aim is to reduce pollution levels to those of the 1930s.