Investigating the impact of relative humidity and air tightness on PM sedimentation and concentration reduction

Exposure to particulate matter (PM) has a negative impact on human health. The hygroscopicity of PM varies with the chemical composition and particle size, and the sedimentation trends leading to decreased PM concentrations also vary. However, PM hygroscopicity has mostly been considered in terms of outdoor air quality such as visibility and radiation force. Thus, in this study, the relationship between PM and ambient humidity was evaluated through two experiments based on chemical composition and PM particle size analysis, which are representative PM classification methods. Firstly, six aerosols commonly found in indoor and outdoor atmospheres were selected, and the reduction rate of PM number concentrations by particle diameter and mass concentrations (PM10 and PM2.5) was evaluated at different relative humidity(R.H) values. The results indicated that the PM concentration was effectively decreased when indoor R.H was >70%, and the spread of PM was effectively prevented when indoor R.H was >50%. Secondly, measurements were obtained from four houses with different levels of airtightness to clarify the relationship between PM and R.H in actual living environments. Higher airtightness corresponded to lower PM concentrations due to higher R.H, but this effect was not observed when the air change rate was high. Therefore, increasing airtightness to increase R.H may not be a practical solution for reducing indoor PM concentrations in all cases. Our findings suggest that indoor air quality management strategies should consider the complex interactions between PM and ambient humidity in specific building contexts. [Display omitted] •Lab scale test measurement of 6 aerosol reduction rate at 30–90% R.H.•At least 50% R.H is recommended to prevent PM spread.•Relative humidity has a strong nonlinear effect on indoor PM2.5•Combined analysis of actual indoor PM and humidity measurements in 4 houses with different airtightness.•Indoor PM management should consider both humidity and airtightness performance.