[url=http://www.bbc.co.uk/news/world-asia-china-20998147]BBC link.[/url] With visibilty just a few hundred metres, Beijing's air is extremely unhealthy. WHO says that PM2.5, tiny pollution particles, should remain under 25 micrograms per cubic metre. At 100 micrograms, air is considered unhealthy. At 300 micrograms, children and elderly should stay inside. Beijing is currently measuring at 400+ micrograms. Unofficial readings from a US embassy say that the air is currently measuring at roughly 800 micrograms. Once inhaled, PM2.5 can cause respiratory infections and increase mortality rates from lung cancer and heart disease. Some citizens have decided to wear masks to protect themselves.