TAIYUAN - The coal-rich north China province of Shanxi shut down more than 7,400 polluting firms in 2017 as part of its fight against pollution.

Local authorities also dismantled 11,000 small coal-fired boilers last year, the provincial environmental protection bureau said Thursday.

Meanwhile, more than 1 million households in the province switched from coal to gas- or electricity-powered heating systems, and 59.8 percent of buses and 21.3 percent of taxis were replaced by new energy vehicles, it said.

Shanxi is a major energy base and its traditional mining, coking, steel, and coal chemical industries have caused severe pollution.

Local authorities took harsh measures to tackle air pollution last year, sending environmental inspectors to all 11 cities across the province.

They filed 5,502 illegal sewage discharge cases and levied 580 million yuan (about $91 million) in record fines, up 123 percent year-on-year.

The efforts have paid off. The levels of PM2.5, smaller airborne particles more damaging to health, and sulfur dioxide, fell 27.6 percent and 49.2 percent year-on-year, respectively, between Oct 1 last year and Jan 18 this year.

The air quality is improving, but challenges remain, said He Zhongwei, an official with the Shanxi provincial environmental protection bureau, citing isolated coal burning in the rural areas.

More polluted days are expected from late January to early February with forecasted stable weather conditions, He said.

China has set a target to reduce the average PM2.5 level by at least 15 percent in the cities around the Beijing-Tianjin-Hebei region between October 2017 and March 2018, compared to last year's level.

Shanxi has vowed to cut the levels of PM2.5 and sulfur dioxide by 40 percent during the period.