An inspection of companies based around Beijing found more than 70% were violating air pollution regulations, Chinese state media says, BBC informs.
Firms pumped out more emissions than allowed, operated without licences or had insufficient pollution control equipment, Xinhua news agency reported.
Checks were carried out at thousands of companies at 28 cities in and around the Beijing-Tianjin-Hebei region.
Air pollution in Chinese cities is notoriously bad.
The findings appear to confirm suspicions that companies ignore strict environmental protection policies and that officials do not enforce them, correspondents say.