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.
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.
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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.
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.