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, and measuring them is considered a more accurate reflection of air quality than other methods."Because these dust particles are relatively fine, they can be directly absorbed by the lung's tiny air sacs," Huang said. "The airway's ability to block the fine dust is relatively weak, and so bacteria and viruses carried by the dust can directly enter the airway." Prolonged exposure could result in tumors, he added. Demand spiked for face masks, with a half dozen drugstores in Beijing reached by phone reporting they had sold out. A woman surnamed Pang working at a Golden Elephant pharmacy said buyers were mainly the elderly and students, and that the store had sold 60 masks daily over the past few days. The bulk of the smog choking Chinese cities is belched out by commercial trucks, but authorities have put off enforcing tougher emissions standards to spare small businesses the burden of paying for cleaner engines. "It is not a problem of technology. It's more about consumer affordability. Increasing the emissions standard greatly increases the cost," said John Zeng, Asia-Pacific director for LMC Automotive Ltd., a research firm. "Most buyers are small business owners, and they are very price-sensitive." Upgrading to cleaner engines would cost about 20,000 yuan ($3,200), adding about 8 percent to a typical sticker price of a vehicle, according to Zeng. The haze even inspired a song parody, widely circulated online. "Thick haze permeates every street in Beijing, the pollutant index is worse than the charts can read. I'm surrounded by buildings in a fairyland and I see people wearing masks all over the city," go the lyrics. "Who is traveling in fog and who is crying in fog? Who is struggling in fog and who is suffocating in fog?"