Smog in China's capital Beijing climbed back into the unhealthy range on Saturday as the second pollution red alert of the month went into effect.

A wave of smog settled over the city of 22.5 million overnight and is forecast to last into Tuesday because of a lack of strong winds.

Levels of PM2.5, the smallest and deadliest airborne particles, rose as high as 303 in some parts of the city and are predicted to top 500 in coming days. That is more than 20 times the level that is considered safe by the World Health Organization.

Schools have been ordered closed and half the city's cars forced off the road on any given day. Barbecue grills and other outdoor smoke sources are banned and factory production restricted.

The smog is largely blamed on coal burning power plants, industrial pollution and the booming number of vehicles. Beijing's geography worsens the problem because mountains on three sides trap smog and cold winter air presses down on it to keep it from clearing.

While waiting for promised plans to cut coal pollution to take effect, residents of Beijing and other polluted cities in the country's north adapt by wearing face masks and equipping homes with air filters.

Still, scientific studies attribute 1.4 million premature deaths per year to China's smog, or almost 4,000 per day.

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