BEIJING - People refused to venture outdoors and buildings disappeared into Beijing's murky skyline on Sunday as the air quality in China's notoriously polluted capital went off the index.
The Beijing Municipal Environmental Monitoring Center said on its website that the density of PM2.5 particulates had surpassed 700 micrograms per cubic meter in many parts of the city. The World Health Organization considers a safe daily level to be 25 micrograms per cubic meter.
PM2.5 are tiny particulate matter less than 2.5 micrometers in size, or about 1/30th the average width of a human hair. They can penetrate deep into the lungs, so measuring them is considered a more accurate reflection of air quality than other methods.
The Beijing center recommended that children and the elderly stay indoors, and that others avoid outdoor activities.
The U.S. Embassy also publishes data for PM2.5 on Twitter, and interprets the data according to more stringent standards.
In the 24-hour period up to 10 a.m. Sunday, it said 18 of the hourly readings were "beyond index." The highest number was 755, which corresponded to a PM2.5 density of 886 micrograms per cubic meter. The U.S. Environmental Protection Agency's air quality index goes up to only 500, and the agency advises that anything greater than 300 would trigger a health warning of "emergency conditions," with the entire population likely affected.
PM2.5 can result from the burning of fuels in vehicles and power plants.
Weather conditions are a factor in the recent poor air quality, as a lack of wind means pollutants can accumulate and fail to dissipate, said Pan Xiao Chuan, a professor at Peking University's public health department.
"Recent pollution doesn't mean there is an increase in the discharge of pollutants," he said.
Experts say they thought the PM2.5 readings were the highest since Beijing started publishing those data early last year.