As much as 80 percent of the PM2.5 inhaled by Beijing residents are in their home and office rather than the open air, a recent survey has revealed.

The capital city's residents' indoor exposure to the PM2.5 - particulate matter with a diameter smaller than 2.5 microns that can penetrate the lungs and harm health – is four times higher than that tested outside a building, and that is because most people spend up to 90 percent of their time within doors, researchers at Tsinghua University in Beijing said at a news conference on Wednesday that released the survey's findings.

The indoor air quality in Beijing would be classified as being polluted for almost eight hours every day as the concentration of PM2.5 exceeds 75 micrograms per cubic meter, said Zhang Lin, chief scientist of the research.

A total of 407 volunteers took part in the survey that started in mid-November and lasted more than 70 days. They used a portable particulate matter sensor developed by the university to test 7,703 rooms in 13 districts of Beijing, generating a huge database of about 110,000 hours of PM2.5 readings, he said.

The survey's findings will facilitate the research on air pollution's impact on human health and help people protect themselves from indoor PM2.5, said Tian Geng, a health expert with the university's School of Medicines.