People wearing masks walk out of the China national tennis center, the venue of the China Open, in Beijing, Oct 6, 2015.[Photo/Asianewsphoto]

China has a four-tier color-coded weather warning system, with red representing the most severe weather, followed by orange, yellow and blue.

The NMC warns people in the affected areas to reduce outdoor activities.

The smog coincides with the week-long National Day holiday, and many tourists are vacationing in Beijing.

The Chinese public have become increasingly sensitive to the health hazard. To address public concerns, the government aims to cut the density of inhalable particulate matter by at least 10 percent in major cities by 2017.