BEIJING, June 29 -- Chinese police arrested around 8,500 suspects in more than 4,500 environment-related criminal cases in 2014, as the country waged war against pollution, environment minister Chen Jining said on Monday.

Addressing lawmakers during a bi-monthly session of the National People's Congress (NPC) Standing Committee, Chen said stricter enforcement of tough environmental laws was a key factor in curbing air pollution.

Environmental authorities transferred more than 2,000 cases of suspected environmental law violations to the police, more than double the figure for the past 10 years combined.

Close to 3,400 companies and 3,700 construction sites were also found to have violated environment laws and more than 3,100 workshops were closed following air quality inspections by Ministry of Environmental Protection (MEP) officials and drones, Chen said.

The announcement came amid a spell of heavy smog in Beijing and the neighboring Tianjin municipality and Hebei Province for the better part of a week.

One year after the world's second-largest economy "declared war" on pollution after decades of pursuing growth at the expense of the environment, air pollution has become one of the top concerns for Chinese citizens, particularly those living in big, industrial cities in the central and eastern regions of the country.

According to an MEP communique released earlier this month, only 16 of the 161 major cities subject to air quality monitoring met the national standards for clean air in 2014.

But Chen -- who was appointed environment minister just this March amid hopes to breath some new air into the uphill battle against pollution -- said that China's overall air quality improved in 2014 and early this year.

Average PM10 readings in 338 cities monitored by the MEP dropped by 2.1 percent year on year in 2014, and by 5.3 percent in the first four months of 2015.

Readings of PM2.5 -- smaller particulate matter which can penetrate deep into the lungs, thus posing a greater health threat than PM10 -- dropped in 74 cities by 11.1 percent in 2014 and 15.2 percent from January to April this year, the minister said.

Air quality changes were particularly evident in the notoriously smoggy Beijing-Tianjin-Hebei region, where PM2.5 readings recorded faster-than-average drops: 12.3 percent in 2014 and 20 percent this year, according to Chen.