ZHENGZHOU, Jan. 16 (Xinhua) -- Severe smog continued to affect central China's Henan Province on Tuesday, spreading to five more cities in the province, according to the local environmental protection bureau.

The first round of smog to hit the province this year started last Friday and continued to affect the region this week.

As of noon Tuesday, the air quality of nine Henan cities was severely polluted and another eight cities were heavily polluted.

Zhengzhou, the capital of Henan, was among the most polluted cities in the province. Many citizens on the streets of Zhengzhou wore protection masks.

Henan has issued an orange alert for the smog since last Friday. The province is expected to see smog from Jan. 20 to 25 and from Jan. 29 to Feb. 5, according to the provincial environmental protection authorities.

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