BEIJING, Sept. 4-- Beijingers breathed perfect air from August 20 to September 3 as half of the city's cars were barred from roads and industries in nearby provinces were temporarily shut down.

The average density of PM 2.5, airborne particles measuring less than 2.5 microns, dropped by 73.2 percent after efforts were made to ensure clean air for IAAF World Championships and WWII V-Day military parade, the Beijing Municipal Environmental Protection Bureau said on Sunday.

On September 3, the level of every major air pollutant dropped to a historical low since the city begun monitoring these pollutants, said Zhang Dawei, head of the Beijing Municipal Environmental Monitoring Center.

Beijing began to adopt temporary measures to ensure air quality on August 20. The density of nitric oxide, a vehicle exhaust, more than halved in the morning rush hours.

Nearly 2,000 industrial firms, including petrochemical and cement plants, suspended or cut production in Beijing.

Similar measures were adopted in neighboring Tianjin, Hebei, Shanxi, Shandong, Inner Mongolia and Henan.

Without the measures, the density of PM 2.5 could have been 70 percent higher, said Pan Tao, deputy director of Beijing Municipal Research Institute of Environmental Protection.

Such measures contributed to blue skies during the 2008 Summer Olympics and the APEC meetings last year.