# A Review of Indoor Air Quality in Social Housing Across Low- and Middle-Income Countries
## 1 Introduction  
Countries around the world are increasingly implementing social housing programs to provide affordable homes for vulnerable populations, including those with low socio-economic status. However, research indicates that such housing often faces problems related to comfort, sustainability and, especially, indoor air quality (IAQ) [1, 2]. Residents of social housing are frequently exposed to higher levels of pollutants and temperature variations, leading to poor health outcomes [3, 4, 5, 6]. Key indoor air pollutants commonly found in social housing include particulate matter (PM2.5 and PM10), carbon monoxide (CO), nitrogen oxides (NOx), ozone (O3), volatile organic compounds (VOCs) such as benzene and formaldehyde, and biological pollutants like allergens, fungi, and bacteria. PM2.5 and PM10 represent a mixture of fine and coarse particles capable of penetrating deepinto the respiratory system, causing significant health risks. Vulner groups, including children, the elderly, and individuals with disabilities, are particularly affected, as they are overrepresented in social housing populations [7; 8]. For example, PM2.5 poses higher risks due to its ability to enter the bloodstream causing serious health outcomes, such as cancer, type 2 diabetes, low birthweight and gestational age, and chronic respiratory and cardiovascular diseases [9]. The World Health Organization (WHO) recommends PM2.5 exposure limits of 10 \(\upmu\)g/m\({}^{3}\) annually and 25 \(\upmu\)g/m\({}^{3}\) for 24 h to minimize health risks, but many indoor environments in social housing exceed these standards, revealing health disparities. Several factors contribute to these disparities including the use of low-cost building materials that may release harmful pollutants, inadequate ventilation systems [10], and limited public awareness about practices that could improve IAQ [11], such as effective ventilation and pollutant source control. These factors contribute to health issues such as respiratory problems, fatigue, and headaches [12; 13; 14; 15]. In multi-family buildings, a common typology for social housing, pollutants often spread between units because of shared air pathways and increased infiltration from outdoor sources [16; 17]. Behavioral factors, like smoking, are more prevalent among low-income households and significantly elevate indoor PM2.5 concentration levels [18; 19]. For example, PM2.5 concentrations in Canadian social housing units were reported to be two to three times higher in smoking units compared to non-smoking units [7]. Additionally, social housing is often located in urban areas with poor outdoor air quality, such as near major highways and industrial zones [18; 19; 20]. Energy retrofits that aim to improve energy efficiency have shown mixed results for IAQ, with some studies reporting increased PM2.5 levels following such interventions due to airtightness measures [21]. IAQ problems are even more critical in developing countries, where socio-economic barriers and infrastructural limitations often lead to increased health inequalities [22; 23; 24]. Reliance on solid fuels like wood, charcoal, and dung for cooking and heating exposes approximately 3.8 billion people globally to household air pollution (HAP) [25]. HAP generates high concentrations of PM2.5 and CO that not only degrade indoor air but also contribute to outdoor air pollution as emissions escape into the ambient environment. Also, overcrowding in smaller homes is common among low-income households and further increases exposure to biological pollutants and particulate matter [18; 26]. Despite a growing body of literature addressing air pollution globally, studies specifically addressing IAQ in social housing within developing countries are still limited. Existing reviews largely focus on high-income countries creating a significant knowledge gap in understanding IAQ within low- and middle-income countries (LMICs). For example, [PERSON] et al.