Relationship between indoor/outdoor concentrations of particles: a critical review

The relationship between indoor and outdoor concentration levels of particles in the absenceand in the presence of indoor sources has been attracting an increasing level of attention.Understanding of the relationship and the mechanisms driving it, as well as the ability toquantify it, are of importance for assessment of source contribution, assessment of humanexposure and for control and management of particles. It became particularly important toaddress this topic when evidence came from epidemiological studies on the close associationbetween outdoor concentration levels of particles and health effects, yet with many studiesshowing that indoor concentrations could be significantly higher than those outdoors. Thispaper presents a summary of an extensive literature review on this topic conducted with anaim to identify general trends in relation to the I/O relationship emerging from studiesconducted worldwide. The review considered separately a larger body of papers published onPM10, PM2.5, as well as the smaller database on particle number and number or volume sizedistribution. A specific focus of this paper is on naturally ventilated houses. The conclusionfrom the review is that despite the multiplicity of factors that play role in affecting therelationship, there are clear trends emerging in relation to the I/O relationship for particlemass concentration, enabling more general predictions to be made about the relationship.However, more research is still needed on particle number concentration and size distribution.