With booming industries, Beijing finds it difficult to control SO₂ and particulate matter. NO₂ and toxic hydrocarbons add to its problems. After seven years of controls, SO₂ and CO are down by nearly 25 per cent, but NO_x is down by 17 per cent and PM₁₀ levels marginally. Inhalable particles are very high on days that do not qualify as blue sky days.

Scientists from Peking University released data in 2006 to show that levels of the tiniest and most harmful particles