Impacts of air conditioning on air quality in tiny homes in Hong Kong

The risk of developing sick building syndrome is known to be higher in air-conditioned than naturally ventilated spaces. In Hong Kong, air conditioning (AC) is commonly used in homes to relieve summer heat stress. This study aims to assess the air quality impacts of AC in tiny homes called SDUs (sub-divided units). Poor ventilation and stronger heat stress in such informal housing could necessitate the use of AC. Predicted mean vote (PMV), CO, CO₂, PM10, PM2.5 and VOCs were continuously monitored for 72 h in eight SDUs. PMV was ≥2 ('warm') in 75% of the SDUs at sleeping time (after 22:00), implying an 80% dissatisfaction among the occupants. During AC use, the mean concentrations of CO and CO₂ increased from 220 to 905 μg/m³ (+312%) and from 920 to 1711 mg/m³ (+86%) respectively. The highest CO₂ level (3758 mg/m³) was observed in a 3-person household (one more than other SDUs). The overall impacts on PM10 (+4%) and PM2.5 (+19%) were relatively insignificant. Reduced ventilation in air-conditioned homes facilitated the accumulation of VOCs (mean change: +22%). The findings could inform building design and modify AC usage practice to improve the indoor environment.