Research and analysis

Estimating health effects caused by indoor air pollution, using information from studies on outdoor air pollution – lay summary

Published 18 December 2025

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Most people in the UK spend most of their time indoors. This makes it important that the indoor air that they breathe is good quality. However, there is limited information on health effects of air pollution indoors, even for pollutants such as fine particles (PM2.5) and nitrogen dioxide gas (NO2) which have been well-studied in relation to exposures outdoors.

Concentration-response functions (CRFs) represent the relationship between a pollutant and health. Higher levels of a pollutant in the air will lead to greater damage to health than when the pollutant is present at lower levels. CRFs are used to estimate the scale of health effects caused by current levels of air pollution (burden estimates). They can also be used to predict health benefits if levels of pollutants are reduced. Because CRFs from studies of indoor air pollution are not usually available, the Committee on the Medical Effects of Air Pollutants (COMEAP) has been considering whether information from the large number of studies on outdoor air pollution can be used instead.

CRFs from studies on pollutants in outdoor air need to be adapted before they are used to estimate effects of pollutants indoors. COMEAP’s view is that using adapted CRFs based on outdoor air pollution studies is more suitable to estimate the effects of indoor PM2.5 and NO2 which has infiltrated from outdoors. These are likely to be less accurate if used to estimate health effects of PM2.5 and NO2 from indoor sources. There are a number of uncertainties that need to be acknowledged in these calculations. These include:

  • differences in the way in which levels of PM2.5 and NO2 act as indicators for sources or mixtures of pollutants outdoors compared with indoors
  • differences in the patterns, over time, of peaks and troughs of pollutant levels indoors and outdoors, which may be important for health
  • variations in how much time different people spend in different indoor and outdoor environments
  • differences in activities undertaken indoors and outdoors, which will affect factors such as breathing rates and the amount of pollutant inhaled
  • differences in particles from indoor and outdoor sources, for example particles from traffic exhaust compared with those produced during cooking

A further issue is that the health effects of outdoor air pollutants are usually estimated using CRFs from reviews (meta-analyses) that combine evidence from multiple studies, which may be from different countries or regions. Factors such as how much pollution infiltrates into buildings, or how much time people spend indoors and outdoors, may be different across these studies.

COMEAP has suggested further research that would help improve the methods used to estimate effects of indoor air pollution.

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