Shanghai warned children and the elderly to avoid prolonged or heavy outdoor activities as PM2.5 readings exceeded four times the World Health Organization’s recommended level of daily exposure.

The air quality index was 185 as of 9 a.m., signaling “moderate pollution,” the third worst in a six-tier warning system, according to the website of the city’s environmental monitoring center.

The reading for particles smaller than 2.5 microns in diameter that are most dangerous to health was 109.6 micrograms per cubic meter. That compares with the WHO guidelines of no more than 25 for 24-hour exposure.

Shanghai saw record levels of smog last month, forcing flight cancellations and prompting the government to issue warnings to keep children indoors and order vehicles off the road. PM2.5 levels reached 602.2 on Dec. 6.

via Shanghai Warns on ‘Moderate’ Air Pollution With PM2.5 Above 100 – Bloomberg.