BEIJING, Nov. 20 -- A sizable share of coal samples collected from Beijing, Tianjin and Hebei in north China failed to meet pollution control requirements, the Ministry of Environmental Protection (MEP) said on Friday.

The MEP has tested 203 samples during an inspection on coal quality at 185 coal sales outlets and 10 coal-firing central heating stations across the Beijing-Tianjin-Hebei region.

More than 22 percent of samples from Beijing contain excessive pollutants (volatile phenol not tested). The share rose to 26.7 percent for Tianjin and 37.5 percent for Hebei, the MEP said.

Inspectors also discovered high sulfur content, up to 2.1 percent, in coal at four sales outlets in Binhai New Area in Tianjin, and high content of ashes, up to 30 percent, at two outlets in Hebei Province.

Total sulfur refers to the entire composition of organic and inorganic sulfur in coal. Sulfur, after burning, will become sulfur dioxide which is a major pollutant.

Coal ash, waste material left after coal is burned, contains various heavy metals, many of which are toxic.

Of the 10 inspected heating stations, seven failed to meet coal quality standards. A heating company in Tangshan city of Hebei was found to use coal containing 1.33 percent total sulfur and 29.22 percent ashes. Three in Baoding city were found to burn coal consisting of more than 20 percent ashes.

Coal burning is considered one of major contributors to frequent smog in China, especially when northern China turns on central heating in the winter.