Thick smog is covering Beijing and much of north China on Monday as the worst period of air pollution this year stretches into a fourth day.

Smartphone air quality apps, popular among residents of Beijing, 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.

There have been similar readings around Beijing, including Tianjin Municipality and cities in Hebei Province.

The center found coal burning to be the major contributor to the high concentration of PM 2.5 after analyzing the composition of the pollutants, according to its head Zhang Dawei.

Complex climate conditions where pollutants are pressed in the lower atmosphere by cold air also contributed to the thickness of the smog, and the situation deteriorated as smog built up around Beijing was blown into the capital by wind, he added.

The air quality in Beijing is expected to improve from 11 p.m. on Tuesday. The capital will have good air quality for at least four days from Wednesday, according to the center's forecast. Endi