Large amounts of organic nitrogen compounds were found in Beijing smog in January, the worst month in recent years. According to a recent report by the Chinese Academy of Sciences, the chemicals are believed to be the major components that killed more than 800 people in the Los Angeles photochemical smog over six decades ago.

Beijing's sky were charcoal grey for most of January.

PM 2.5 monitoring stations in the Beijing-Tianjin-Hebei area recorded five bouts of heavy pollution, lasting a good 22 days.

Smog persisted throughout January and returned with fresh bouts of pollution this month. Experts believe the cause for the prolonged pollution is a direct consequence of both man-made emission and disruption of natural ecological balance.

The Chinese Academy of Sciences says the prolonged smog is caused by a combination of intensive coal burning, car emissions, cooking pollutants and a particular weather pattern created an inversion layer over the low-lying city.

Wang Yuesi, Atmospheric Physicist of Chinese Academy of Sciences, said, "Car exhaust is the primary cause in Beijing, contributing about a fourth of the air pollution. Coal fumes are the secondary cause. "

Most alarming is that the Academy says they've found organic nitrogen particles, a key component in the deadly photochemical smog in Los Angeles in the 1950s and the Great Smog in London.

Professor Pan Xiaochuan of School Of Public Health, Peking Univ., said, "Nitrogen compounds, when reaching a higher density, could cause acute respiratory symptoms such as coughing and shortness of breath. It's a stressor for asthma. And our clinical records show an appreciable increase in respiratory patients last month."

But Professor Pan doubts that photochemical smog formed in Beijing. The conversion requires the presence of sunlight to form ground-level ozone. Beijing has hardly any sunlight on foggy days.

Professor Pan Xiaochuan said, "Photochemical smog usually contains large amounts of ozone, aldehyde, and a chemical compound called PANS. The last chemical in particular has an almost tear gas effect on humans: irritated eyes, dry mouth and serious coughing. In Los Angeles, 800 people died in three or four days. In Beijing, the situation is much less severe."

But no one disputes the cause for the smog. Experts have called to reduce car emissions, and improve fuel quality. They have also suggest limiting industrial production and improving the process of coal burning so people in China can breathe easier.