The red alert - the most serious warning on a four-tier system adopted a little over two years ago - means authorities have forecast more than three consecutive days of severe smog.
Expecting heavy #smog, Beijing issues first ever red alert for air pollution pic.twitter.com/yN4oxdSM52
— China Xinhua News (@XHNews) December 7, 2015
Beijing hotel staffer Fan Jinglong said the smog forecast was "really worrisome."
"We have no choice but to step up preventative measures like wearing a mask outdoors at all times," he said.
Readings of PM2.5 particles climbed toward 300 micrograms per cubic meter on Monday and are expected to continue rising before the air begins to improve with the arrival of a cold front on Thursday. The World Health Organization designates the safe level for the tiny, poisonous particles at 25.
While pollution in the capital improved slightly in the first 10 months of the year, heavy smog that can be seen from outer space regularly forces Beijing schools to suspend outdoor activities and can even prompt highway closures because of reduced visibility.
Convenience stores in Beijing were doing a brisker than usual business in air filtering face masks Monday night as residents began stocking up for the days ahead.
"You have to do whatever you can to protect yourself," said Li Huiwen, who stopped by a market on her way home from work. "Even when wearing the mask, I feel uncomfortable and don't have any energy."
#Beijing issues first red alert for heavy #smog. Odd-even car ban imposed; schools suspended pic.twitter.com/F3ZNU0Mvyg
— China Xinhua News (@XHNews) December 7, 2015
Most of the city's international schools canceled Tuesday classes, although it wasn't clear how many Chinese schools planned to do so.
There previously have been stretches of severe smog in Beijing that lasted more than three days. However, those had initially been forecast to last three days or less, so they did not trigger a red alert. The alert requires a forecast of more than 72 straight hours with PM2.5 levels of 200 micrograms per cubic meter or more.
Most of the pollution is blamed on coal-fired power plants, along with vehicle emissions and construction and factory work. China, the world's biggest carbon emitter, plans to upgrade coal power plants over the next five years to tackle the problem, and says its emissions will peak by around 2030 before starting to decline.
While emissions standards have been tightened and heavy investments made in solar, wind and other renewable energy, China still depends on coal for more than 60 percent of its power.

