Parts of China's capital Beijing suffered air pollution more than 20 times recommended levels today, but authorities refrained from issuing the highest smog alert.
Counts of PM2.5 - harmful microscopic particles that penetrate deep into the lungs - reached 529, according to the US embassy, which issues independent readings.
The World Health Organization's recommended maximum exposure is 25 over a 24-hour period. China is often hit by heavy smog, mostly a result of coal burning.
China's state weather observatory issued a yellow alert - the second highest in a four-tier warning system - for smog in Beijing and other parts of north China, the official Xinhua news agency said.
The pollution will linger until tomorrow, it added.
Beijing issued its first red pollution alert - reserved for when authorities forecast an air-quality index of above 300 for at least three consecutive days - earlier this month.
The alert requires construction sites to halt work, while half the city's cars are banned from driving and schools are recommended to halt classes, among other disruptive measures.
Smog is generally worse in winter as coal burning for heating rises, and several northern cities have followed suit with red alerts in recent weeks.
"(Pollution) is off the charts again," said one disgruntled Beijing resident on Chinese Twitter-equivalent Sina Weibo.
Air quality for November and December in Beijing was at a three year low, according to the state-run China Daily, despite measures to tackle the chronic problem.
China's rise to the world's second largest economy was largely powered by cheap, dirty coal.
Even as growth slows, the country has had a difficult time weaning itself off the fuel, even as the pollution it causes wreaks havoc on the environment and public health.
