Which of the following air pollutants is known to be a human carcinogen?

Asbestos fibres are a recognised human carcinogen that inflame lung tissue and cause cancer.

Carbon monoxide poisons by blocking oxygen transport but is not a carcinogen.

Nitrogen dioxide irritates the airways yet is not classed as a human carcinogen.

Sulphur dioxide harms breathing and forms acid rain but is not carcinogenic.

Inhaled asbestos can cause lung cancer and the rare cancer mesothelioma.

Its dangers led many countries to ban asbestos in construction.