Dust pollution is still a big threat to Beijing's air quality as 16 concrete mixing plants and construction sites were still working on Monday despite the ban by the capital's government during the APEC meetings, the city's environmental protection bureau said on Monday.

Mounds of materials were found exposed without the covers that prevent the dust from getting into the air, and seven mixing plants were still working during an inspection by the bureau.

In a recent document from the bureau, dust was listed as one of the four major sources of particulate pollution, known as PM2.5, in Beijing, along with vehicle exhaust, burning coal and industrial production.