Beijing, Tianjin and cities in Hebei and Henan provinces are covered by heavy air pollution that started on Monday and was forecast to last through Saturday, the Ministry of Environmental Protection said on Monday.

Beijing issued an orange alert on Monday afternoon, the second-highest level in the four-tiered emergency response against air pollution, which requires some plants to suspend production. Beijing did not disclose when the alert will be lifted.

On Wednesday, the first working day after the three-day Tomb-Sweeping Festival, residents in Tianjin as well as Zhengzhou and Anyang, Henan province, are expected to face severe air pollution.

On Thursday and Friday, the air pollution will expand to cover some cities in Hebei province as well, including Baoding, Langfang and Tangshan, which surround Beijing, results from the ministry showed.

Amid concerns by residents that smog has been frequent even after emissions from heavy coal consumption for winter heating had ended, experts blamed vehicles and active industrial production.

According to the ministry's experts, the coming smog is mainly because of increasing pollutants emission from more vehicles and more active industrial production in the region, especially those plants that resumed production after heating-season restrictions ended. The reasons were pinpointed through economic data and monitoring results of satellites and equipment installed at plants.

The monitoring network showed that 55 cement plants that suspended production during the heating season, from Nov 15 to March 15, have all resumed production.

Additionally, industrial production, especially in the iron and steel and glass-making industries, has increased since January, driving the growth of coal-fired power generation, experts said.

In Tianjin, crude steel production has grown by 22.5 percent year-on-year in the first three months of the year, and coal-fired power generation grew by 16.8 percent year-on-year, the ministry's data showed.

In Henan province, glass production has soared, increasing by 107.7 percent year-on-year in the first three months, it added.

The soaring production has discharged massive pollutants, excessive for the environment, the experts said, adding that the vehicle amount in the Beijing-Tianjin-Hebei region increased by 11.3 percent year-on-year.

The excessive emissions were the root reason for the smog, they said.

Environmental Minister Chen Jining required the affected governments to take restriction efforts and strictly monitor to reduce air pollution.

Anyang had experienced severe air pollution lasting for about a week in late March, and it found companies with excessive emissions during an inspection from Feb 15 to March 18.

On Sunday, Zhao Yingmin, deputy environmental minister, led inspectors to check several plants in Anyang to push forward the government's efforts to tackle air pollution.

Seven more inspection teams will monitor cities in the region beginning on April 1, the ministry said.