PM2.5 the culprit

The US Embassy in Beijing has reported its own monitoring of air quality hourly since 2008. The readings are for ozone and for the concentration of PM2.5, the tiny airborne pollutants that cause haze and can travel deep into the lungs and damage people's respiratory systems.

Particulate matter currently measured in China is smaller than 10 microns and is known as PM10, while matter smaller than 2.5 microns is called PM2.5.

Tang Xiaoyan, a professor at Peking University's College of Environmental Sciences and Engineering, said research shows that Beijing's haze is caused mostly by PM2.5. The contribution of larger matter "is rather limited", she said.

As a result, readings from the US Embassy usually paint a gloomier picture than the official ratings from Beijing Environmental Protection Bureau. For Monday, the embassy report described the PM2.5 reading as "hazardous", the worst ranking on the pollution scale according to US standards.

The embassy's reports are posted automatically to twitter.com/beijingair. Accountant Wang Changyu, 32, receives the updates through an iPhone app called Beijing Air Quality, developed by SolarJoke, a Chinese programmer.

"I know the readings from the US Embassy may not reflect the overall air quality of Beijing accurately, but I am feeling very uncomfortable about the air," he said. "I try not to check the updates often because it really makes me feel depressed."

Du dismissed the accuracy of the pollution information on Monday, saying the US Embassy has made it clear that the monitoring is only for internal reference.

Seeking protection

Even so, some Beijing residents are seeking ways to protect themselves from the pollution, at least for psychological relief. Wang said his company started to distribute face masks Tuesday morning and encouraged employees to wear them when they go outside.

Yang Jiachuan, 34, a consultant at an accounting firm, recently spent 18,000 yuan ($2,800) to install a ventilation system to improve indoor air quality at home.

"I have never seen so many hazy days before. Beijing's fall used to be my favorite time. But now, it is totally different," Yang said.

Continual poor air quality has boosted business for Guo Kai, who sells indoor air purifiers in Changsha, Hunan province. His company's sales volume for Beijing rose 30 percent during the past month, Guo said.

"I've seen lots of discussion about air quality on micro blogs, and wanted to share my opinion as an industry insider," he said. Now he answers questions online - not only about his own products, but also types of masks for purchase, a protection many people turn to in outdoor activities.

Luo Yin, mother of a 4-year-old boy, started shopping online for masks after she read about the US Embassy's Beijing air quality report.

"I forbid my kid to play outdoors these days," she said. And she told him to wear two masks to school. "I hate to wear the mask as it ruins my makeup, but I definitely want to make sure that my kid is safe from the pollutants in the air."