Purpose
This study investigates the interplay between Indoor/Outdoor Air Quality (IAQ/OAQ), occupant behaviour, and building Air Permeability (AP) aiming to identify the key Indoor Air Pollutants (IAPs) and develop effective mitigation strategies to simultaneously address energy efficiency, IAQ, and occupant health in social housing.

Design/methodology/approach
A mixed-method approach was applied across social housing dwellings in the London Borough of Newham. Physical monitoring of PM₁₀, PM₂..₅, CO, CO₂, TVOCs, temperature, and humidity was conducted using data loggers in three phases of: a) baseline conditions; b) air purifier intervention; and c) behavioural interventions. CONTAM multizone modelling was adopted to assess indoor/outdoor air pollutant correlations and dispersion at four AP levels (1, 5, 10, and 15 m³/m²·h@50Pa), while occupant behaviour and behavioural interventions were assessed through questionnaire surveys.

Key Findings
Daily average pollutant levels, particularly for CO and TVOCs, significantly exceeded the WHO and CIBSE guidelines (by up to 16 and 57 times, respectively), largely driven by occupant activities. Air purifiers reduced PMs however had limited impact on other air pollutants. Behavioural interventions showed limited effectiveness, with improvements not sustained over time. Higher AP generally increased concentration of IAPs, while outdoor pollutants such as PM₂..₅ and NO₂ showed limited sensitivity to AP levels. CO₂ concentration levels were found to be less reliable as an indicator of the overall air quality in residential buildings. A combination of technical and behavioural interventions should be considered to simultaneously improve IAQ and public health.

Practical implications
The findings emphasise the need for integrated IAQ strategies in the context of retrofit and social housing, combining ventilation, air filtration, and occupant engagement. Housing providers should prioritise ventilation strategies while . retrofit programmes should mandate effective natural/mechanical ventilation whenever significant airtightness improvements are planned. Improving occupant awareness and behaviour along with development of technological interventions, such as smart ventilation systems and VOC-targeted monitoring and controlling technologies, are essential to improve public health. The findings suggest that poor IAQ may poses much higher risk to health than outdoor air. The governmental policies should therefore shift attention towards IAQ by developing practical guidelines on ventilation rates and limiting harmful indoor air pollutants from household materials as well as on the effects of retrofit and their implications on IAQ and health.

Originality/value
This research provides a comprehensive real-world assessment of IAQ in social housing by integrating empirical monitoring, behavioural analysis, and simulation modelling within a unified framework. It contributes to the understanding of how socio-technical interactions between occupants and building systems influence IAQ and offers actionable insights for improving health outcomes in vulnerable residential environments