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Beijing haze at a glance in November 2015

Beijing is enduring the worst air pollution of the year on Monday, with monitoring sites throughout the city reporting highest levels of pollutants.

The National Meteorological Center issued an orange alert for heavy smog on Sunday, the second highest possible and the highest smog warning so far this year.

A shocking comparison of Beijing National Stadium under blue sky and in smog

Smartphone air quality apps showed an Air Quality Index (AQI) reading of 500, the maximum, for almost all monitoring stations across the city, advising people to avoid going outdoors.

Concentrations of PM2.5, tiny airborne particles that embed deeply in the lungs, peaked at 900 micrograms per cubic meter in southern Beijing, the municipal environment monitoring center said. The World Health Organization's recommended maximum is 25 microgram per cubic meter.

The reading of fine particulate matter (PM2.5) in Beijing on November 30, 2015

There have been similar air pollution readings in much of north China recently, including Tianjin Municipality and cities in Hebei Province.

According to Zhang Dawei, head of municipal environment monitoring center, coal burning is the major contributor after analyzing the composition of the pollutants. And the cold air has also pressed the pollutants, contributing to the thickness of the smog, he added.