Severe smog Sunday marked three-straight days of dangerous air pollution in the nation's capital city.

Called the "worst on record", according to the Australian ABC News website, the hazardous airborne smoke and haze led authorities to implement an emergency response plan for the first time, after pollution measurements soared, the Xinhua website said.

People ventured forth, wearing face masks against the unhealthy air.

The emergency measures call for, among other things, requested cutbacks in activities that cause dust and emissions, such as those associated with construction and industry.

Schools were urged to limit or curtail outdoor activities.

Pollution indices Sunday reached 500, the "highest level", Xinhua said.

One measure of airborne particles, called PM2.5, topped 700 (micrograms per cubic meter) at some monitoring stations. One even reached 993 on Saturday evening, Xinhua indicated.

The measurements were "30 to 45 times" higher than recommended safety levels, ABC said.

Pollution from vehicles and burning of soft coal has accumulated in calm, cold air trapped beneath a shallow "temperature inversion".

Breaking of the inversion will require a cooling of the air above it, bolstered by a rise in wind speed.

Forecast tools available to AccuWeather.com meteorologists show that both conditions will not be met until late Tuesday, or even Wednesday, local time. At this time, a shot of cold dry air will waft through greater Beijing from Mongolia.

The month so far has been unusually cold with an average temperature 4.2 degrees below normal as of Saturday.

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