Estimating the health effects attributable to PM2.5 and NO2 indoors, based on outdoor air pollution evidence: current approaches and uncertainties

Milner, James, Clark, Sierra N, Bartington, Suzanne E, Birchby, David, Evangelopoulos, Dimitris, Forastiere, Francesco, Hansell, Anna L, Holland, Mike, Katsouyanni, Klea, Khreis, Haneen, Lee, Duncan, Shaddick, Gavin ORCID: https://orcid.org/0000-0002-4117-4264, Stedman, John R, Walton, Heather A, Mitsakou, Christina and Gowers, Alison M 2026. Estimating the health effects attributable to PM2.5 and NO2 indoors, based on outdoor air pollution evidence: current approaches and uncertainties. Environmental Research Health 4 (3) , 033002. 10.1088/2752-5309/ae8051

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Abstract

The ability to quantify the impacts of indoor air pollutants on health can advance understanding and communication of their importance to public health and allow the effects of future interventions or policies affecting indoor air quality to be evaluated. However, epidemiological evidence from indoor environments, suitable as the basis for such estimates, is lacking for many pollutant-outcome pairs. To address this issue, the expert scientific committee which advises the UK Government on the health impacts of air pollution (the Committee on the Medical Effects of Air Pollutants, COMEAP) was asked to consider whether the epidemiological literature linking health endpoints with concentrations of air pollutants outdoors might be used to quantify effects attributable to exposure indoors (COMEAP 2025) and to consider different approaches used to modify concentration response functions (CRFs) to make them more applicable to indoor environments. COMEAP’s consideration and subsequent statement focused on fine particles (PM2.5) and nitrogen dioxide (NO2), which are important pollutants for health and emitted from sources both outdoors and indoors. This piece summarises key points from COMEAP’s statement, proposes future research directions, and puts these recommendations into context for a broader audience, with a focus on the evidence from high-income countries (due to similar housing and pollution sources). Using modified CRFs to estimate effects of pollutants infiltrating indoors from outdoor sources carries the least uncertainty, whereas applying them to pollutants generated solely indoors carries the greatest uncertainty. Key factors and uncertainties that need to be considered include: assumptions of equal toxicity, variation in time activity patterns as well as activity-levels and behaviours which affect inhaled dose (when spending time indoors vs outdoors), and whether the data used to modify CRFs reflects the conditions and context where the CRFs were derived.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Professional Services > Vice Chancellors Office
ISSN:	2752-5309
Date of First Compliant Deposit:	22 July 2026
Date of Acceptance:	12 June 2026
Last Modified:	22 Jul 2026 11:00
URI:	https://orca.cardiff.ac.uk/id/eprint/188396

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