A report released by Tsinghua University on April 22 shows that on average the indoor PM2.5 concentration in Beijing stands at 82.6 ug m-3 (microgram per cubic meter) and air quality is lightly polluted. During one third of the period monitored, the PM2.5 concentration was higher than 75 ug m-3, the level at which air quality is judged to be polluted. 

The indoor PM 2.5 concentration was monitored over two and half moths in 7,703 locations of 13 districts or counties in Beijing. A total of 407 volunteers participated in the study.

Zhang Lin, the research group leader and a researcher from the Department of Electronic Engineering with Tsinghua University, said the volunteers occupied an environment with a daily average PM 2.5 concentration amounting to 2018.6 ug m-3 per hour, and each person was inhaling 787.3 ug of potential pollutants into his lungs per day, of which more than 80 percent came from indoor pollutants.

This is mainly because people spend about 20 hours of their day indoors and only four hours outdoors, Zhang Lin said. Therefore people inhale four times as much PM2.5 indoors as outdoors.

For indoor PM2.5 concentrations, neither the world nor China currently has any standards and norms.

The report shows that under the same external conditions, PM 2.5 level in an office environment is lower than in a living environment, because there are lots of sources of pollution in the family home such as cooking, smoking and others. Additionally, there are usually central air conditioners in an office, which helps a lot in reducing the concentration of PM2.5, said Zhang Lin.

In the same building, on the 17th floor and above, indoor PM2.5 level is lower. Buildings which are 500 meters from a main road have relatively higher air quality. This is mainly because the air flows better at higher altitudes and main roads are affected by emissions from motor vehicle exhausts.

In addition, when outdoor PM2.5 concentration is below 75 ug m-3, ventilation will help to improve indoor air quality, but when the outdoor PM2.5 concentration is higher than 150 ug m-3, ventilation will worsen the indoor air quality.

The study included indoor environments such as offices, homes, and public spaces, but did not include mobile environments such as the subway.