People are at a higher risk from PM2.5 exposure indoors than outdoors, with indoor impact four times that of outdoors, according to the latest study released on Wednesday.

This is the first indoor PM2.5 level research in China. It was conducted by the department of electronic engineering and Center for Building Environment Test of Tsinghua University and the Southern Weekly, by setting up PM2.5 sensors in 407 homes in 13 regions across Beijing and monitoring data from November 2014 to January 2015.

The study showed that the concentration of PM2.5 particles indoors is 30 percent lower on average than outdoors. The indoor PM2.5 concentration is 82.6 micrograms per cubic meter on average, which is slightly higher than the pollution upper limit of 75.

"Since we normally spend over 80 percent of a day indoors, the impact of air quality indoors is much greater to our body than when we are outdoors. This means we should pay more attention to indoor air quality and should not only focus on the air quality index," Zhang Lin, associate professor of the electronic engineering department of Tsinghua University who led the research, told the Global Times Wednesday.

The study likewise found that the PM2.5 level decreases the further one is from the ground and main roads, and the air quality is at its best from the 17th storey and up or when the building is at least 500 meters away from the main roads.

Air purifiers and air conditioners also improve PM2.5 levels, the research found, as the ratio of PM2.5 level indoors to that of outdoors would drop to 0.5 from 0.7. The PM2.5 level in offices is also slightly better than that of homes.

"Activities such as cooking, smoking and cleaning are factors that affect the PM2.5 level indoors," Zhang said.

The research team will meet with medical experts and will look at the link between PM2.5 and human health by studying more data.