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 enacted Friday night 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 to help guard homes and offices against the frigid north China winter.

Beijing has become notorious for heavy pollution in recent years, although experts said air quality actually improved in the first half of 2016.

The number of days in which air quality was rated good grew by 19 to 107, while the number of days with heavy pollution fell by two to 14, the bureau said.

That was attributed to the closure of 174 heavily polluting factories, along with the switching of 463 communities from coal to alternative energy sources, the retirement of tens of thousands of exhaust-spewing cars, trucks and buses, and the addition of 6,803 vehicles running purely on electricity.

Still, retired worker and lifelong Beijing resident Yu Changhai said the situation remained dire.

"I think it is getting more and more serious every year. I'm a native who live in Beijing since I was born. The air of Beijing was much better in old time. In the past 10 years, it has been deteriorating fast," Yu said.

Researchers at Germany's Max Planck institute have estimated that smog has led to 1.4 million premature deaths per year in China, while the nonprofit group Berkeley Earth in California has had a higher figure, 1.6 million.