BEIJING, April 20 -- China's environmental authority found 285 companies in violation of standards during the latest round of air pollution inspection.

Inspection teams in 28 cities in the Beijing-Tianjin-Hebei region and nearby areas found 285 of 450 companies breaking the rules, the Ministry of Environmental Protection (MEP) said Thursday in a statement.

A limestone factory in Beijing's Shunyi district did not have pollution control facilities and seemed to have used some machines that had already been sealed by local authorities, inspectors found.

In Tianjin, a wooden door maker was operating with neither environmental permission nor pollution controls.

The inspectors have reported the problems to local authorities for further investigation, the statement said.

MEP ordered local authorities to take serious measures against companies with severe violations the statement said.

Early this month, the ministry announced the year's campaign on air pollution in Beijing-Tianjin-Hebei and neighboring areas.

The share of days with good air quality in Beijing-Tianjin-Hebei rose by 14.6 percentage points from a year earlier to 66.3 percent in March, even though the share declined in the first quarter, MEP data showed.