Factors shaping fungal contamination emissions from cooling coils: An experimental study across hot-humid and hot-dry climates in Shanghai

Abstract

Cooling coils in air-conditioning systems can compromise indoor air quality by fostering fungal growth and dispersing spores, posing health risks to occupants. However, the climate and operational factors shaping coil-induced airborne fungal contamination remain unclear, impeding the development of effective mitigation strategies. This study developed a lab-scale system with three coil rows to examine fungal contamination dynamics during the transition from the hot-humid plum rain season to the hot-dry heat wave period in Shanghai. A 55-day experiment was conducted following a standard office-hour schedule (9:00 to 17:00), with daily fungal sampling from air, coil surfaces, and condensate. Results showed that coil-induced fungal contamination depends on climate, system's operation duration, and coil row position, with condensate often heavily polluted. Humid climates amplified fungal emissions, particularly upon morning coil activation. Fungal level in downstream air increased by 24.5% on hot-humid mornings but decreased by 36.3% on hot-dry mornings. Hot-humid conditions also enriched allergenic species, such as Aspergillus and Malassezia, in downstream air. Additionally, continuous coil operation heightened fungal exposure risks, emitting respirable fungal particles into downstream air at nearly triple the upstream levels after three consecutive hot-humid mornings. Rear-row coils were the dominant emission sources, as evidenced by a “J-shaped” fungal distribution on coil surfaces: front-row coils cleared fungal deposits via condensate drainage, while rear-row coils retained moisture, promoting fungal proliferation and spore dissemination. These findings underscore cooling coils as substantial indoor fungal sources in hot-humid climates upon activation, emphasizing the need for targeted rear-row coil maintenance to mitigate fungal emissions.