Inspections of 18 cities in north China's Beijing-Tianjin-Hebei (BTH) region and nearby areas used unannounced checks at night and undercover methods, the Ministry of Environmental Protection (MEP) said.
Handan city of Hebei Province continues to illegally operate coal-fired boilers even though officials had ordered them closed.
After the inspections, the boilers were dismantled and a police investigation opened.
In Cangzhou, also in Hebei, an oil pipe maker, a major source of emissions, was not included in the list of companies to halt production on heavily polluted days, state-run Xinhua news agency reported.
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China is intensifying efforts to fight pollution and environmental degradation after decades of growth left the country saddled with problems such as smog and contaminated soil.
Chinese cities suffered from more days of air pollution in January, with northern areas being the worst affected.
The share of days with good air quality in BTH were a mere 36.2 per cent in January, a year-on-year drop of 19.6 percentage points.
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