BEIJING - Emergency measures cut pollutant emissions in Beijing by 30 percent between the start and late afternoon of Tuesday, day one of the city's first red alert for smog, according to the Ministry of Environmental Protection.

The reduction was made as of 5 pm after the alert, which involved limiting car use and construction and industrial activity, was issued at 7 am, ministry officials said during a press conference on Wednesday.

Without the measures, the density of PM2.5, tiny and particularly hazardous airborne particles, would have risen by 10 percent in that period, estimated environmentalists with Beijing University of Technology.

The density of PM2.5 sulfates, commonly caused by coal-burning, was much higher on Tuesday than on Monday, while the air held far less PM2.5 nitrates, mostly emitted by vehicles, said Chai Fahe, deputy head of the Chinese Research Academy of Environmental Sciences.

That indicated that the traffic restrictions were effective but more work needs to be done to reduce emissions from coal burning, said Chai.

Coal-powered winter heating systems are seen as a big cause of the smog that blights north China around this time of year.

Under the red alert, Beijing banned outdoor operations on construction sites and required some industrial plants to limit or stop production.

Car use has been limited during the period based on odd or even license plate numbers.