Ng Han Guan, Associated Press
A woman wears a mask to protect herself from pollution in Beijing, China, Friday, Dec. 16, 2016. Authorities in Beijing prepares to issue its first red alert for smog, the highest of a four-tier system as heavy smog is forecast to hit the capital city from Friday onwards, triggering vehicle restrictions and forcing schools and construction projects to close.

BEIJING (AP) — China's capital and other northern cities have banned half of all vehicles from city streets and ordered factories, schools and construction sites closed in response to a five-day smog red alert.

The emergency steps significantly reduced traffic in Beijing on Saturday, although it wasn't clear what effect it was having on air pollution. By midday, the capital was enveloped in a smothering layer of smog, with concentrations of microscopic PM2.5 — small, inhalable particles that can penetrate deep into the lungs and are considered a reliable gauge of air quality — rising to more than 10 times the level considered safe by the World Health Organization.

The alert that will run through Wednesday is the first issued this year and comes as coal-fired heating plants are ramping up their output.