(XINHUA)

A taxi travels on a Beijing street shrouded by fog on January 28.

Several consecutive days of thick smog affected a 1.3-million-square-km area in north and central China in late January, according to the Ministry of Environmental Protection.

The National Meteorological Center has introduced a three-tier color-coded weather warning system to alert the public to the severity of smog. The system uses yellow to indicate moderately smoggy weather, orange for severe conditions and red for extremely severe levels of smog.

The system not only measures the density of fine particulate matter less than 2.5 microns in diameter (PM2.5), but also takes into account visibility and relative humidity to better reflect air-quality conditions, according to the center. PM2.5 is a major component of smog and a serious threat to health as it can penetrate lung tissue.