Beijing's air this morning turned from "very unhealthy" to "hazardous" as PM 2.5 - particulate matter with size equal to or less than 2.5 micron and the main gauge to identify pollutant particles in the air - touched 365 compared to yesterday's 256 level.
A grey haze hung over the city at a time when crucial climate talks have entered their final phase in Paris.
Under the red alert, the highest in the four-tier emergency response system created in 2013, all schools have been ordered to be closed, and outdoor construction halted.
The red alert, Beijing's first ever, was issued yesterday will last from 7 am today to noon on Thursday. Officials have also clamped restrictions on factories and traffic.
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The reading of the PM 2.5 the Particulate matter in the air at the US Embassy Beijing Air Quality Monitor showed 365 this morning.
Car use will be limited during the red alert period as cars are allowed on the roads on alternating days depending on the odd or even numbers of their license plates.
In addition, 30 per cent of government cars will be banned from streets on an odd/even basis.
The restriction of car use is expected to push an extra two million passengers onto public transport per day.
Beijing will add 21,000 to 25,000 buses, including 8,182 clean-energy buses, to the roads from today, officials said.
He Jinglong, a physical education teacher of the affiliated school of Beijing Jiaotong University, led students in indoor exercise yesterday.
"We designed a set of exercises specially for smoggy weather to keep students fit," He was quoted as saying by state-run Xinhua news agency.
The Beijing Education Commission issued a notice last night asking all middle, primary schools and kindergartens to suspend classes during the red alert period.
The air quality in Beijing in every winter suffers as the city operates dozens of massive coal fired heaters to keep millions of homes warm, but add to the industrial, vehicular pollution besides burning of agricultural waste in the outskirts.