Introduction Exposure to secondhand smoke (SHS) is a health risk to non-smokers. Indoor particulate matter (PM_2.5) is associated with SHS exposure and is used as a proxy measure. However, PM_2.5 is non-specific and influenced by a number of environmental factors, which are subject to geographical variation. The nature of association between SHS exposure and indoor PM_2.5—studied primarily in high-income countries (HICs) context—may not be globally applicable. We set out to explore this association in a low/middle-income country setting, Dhaka, Bangladesh.

Methods A cross-sectional study was conducted among households with at least one resident smoker. We inquired whether smoking was permitted inside the home (smoking-permitted homes, SPH) or not (smoke-free homes, SFH), and measured indoor PM_2.5 concentrations using a low-cost instrument (Dylos DC1700) for at least 22 hours. We describe and compare SPH and SFH and use multiple linear regression to evaluate which variables are associated with PM_2.5 level among all households.

Results We surveyed 1746 households between April and August 2018; 967 (55%) were SPH and 779 (45%) were SFH. The difference between PM_2.5 values for SFH (median 27 µg/m³, IQR 25) and SPH (median 32 µg/m³, IQR 31) was 5 µg/m³ (p<0.001). Lead participant’s education level, being a non-smoker, having outdoor space and smoke-free rule at home and not using kerosene oil for cooking were significantly associated with lower PM_2.5.

Conclusions We found a small but significant difference between PM_2.5 concentrations in SPH compared with SFH in Dhaka, Bangladesh—a value much lower than observed in HICs.