Without an indoor source of pollution, levels of particles in your home would be less than in the air outside. But this is seldom the case.
Roy Harrison, professor of environmental health, and colleagues at the University of Birmingham have studied the indoor/outdoor relationship of particulate matter. His team monitored pollution inside and outside nine homes over 12 months, while the occupants kept a diary of household activities.
In the work, published in the journal Atmospheric Environment , it was clear that the entry of outdoor particles was important. However, the influence of indoor activity often overwhelmed this background. Cleaning and general movement throughout the house stirred up dust that produced peaks of PM10, particles less than 10 microns in diameter.
Vast quantities were also produced by cooking and smoking, in addition to a large amount of tinier bits of organic carbon.