The limit, set by the European Union, is 40 micrograms of NO2 per cubic metre of air.

BBC: Dundee Biomass Centre

One study by the American Cancer Society found that death rates from lung cancer increased 6% for every additional 10 microgrammes of fine particles in a cubic metre of air.

BBC: Traffic

Visitors will be able to see scientists' laboratories and a 300-metre tunnel used for air strikes.

BBC: China to open atomic bomb site to tourists

It is a U-shaped underground villa, built of metre-thick concrete slabs to withstand air and artillery attack.

BBC: De-mining in the Horn

As he travelled around the fruit farms of California, it became clear that most growers were relying on the 11 metre-tall galvanised towers and rotor blades for the air movement needed to prevent frost from forming.

BBC: Graham Hewitt and his wind machine