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Demand spiked for face masks, with a half dozen drugstores in Beijing reached by phone reporting they had sold out.

A woman working at a Golden Elephant pharmacy said that buyers were mainly the elderly and students, and that the store had sold 60 masks daily in 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 $3,200, adding about 8 percent to a typical sticker price of a vehicle, Mr. Zeng said.

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?”