BEIJING - A study conducted by an environmental group found that second-hand smoke is the single greatest source of indoor air pollution in Beijing.

The Daerwen Nature Quest Agency, a civil environmental group, surveyed 43 restaurants, bars and Internet cafes in Beijing over the past six months for the study. The results were released this week.

The concentration of small particles in the air -- measured using PM2.5 readings -- in Beijing's restaurants, bars and Internet cafes was significantly greater than the standards recommended by the World Health Organization, the survey results showed.

Small particles from tobacco smoke made up 90 percent of the pollutants measured by the survey, said Wang Qiuxia, the group's project manager.

Li Qiang, a researcher with the National Office of Tobacco Control, said the fine particles are very harmful to the respiratory and cardiovascular systems.

China has the world's largest number of smokers, totaling more than 300 million, statistics show. Smoking kills roughly 1 million people in China annually, the Ministry of Health has warned, saying the toll will rise to 3 million by 2050 if the current trend continues.

China does not have a comprehensive national tobacco control law. Although smoking is outlawed in most hospitals, schools and public transportation facilities, very few cities plan to ban smoking in public places like restaurants, bars and Internet cafes.