Bacteria

Sun et al. (Frontiers in Public Health, 2024) measured bacteria on components of household split units in Chengdu: drain pan 12,296 CFU/cm², filter 9,128 CFU/cm², evaporator fins 6,595 CFU/cm² and air outlet 5,042 CFU/cm². All exceeded the Chinese 500 CFU/cm² standard. Opportunistic genera such as Acinetobacter, Stenotrophomonas and Corynebacterium were identified. Professional cleaning substantially reduced bacterial and fungal counts on all components, showing that washing the pan and condensate hose is mandatory in a proper sanitising job.

Schmidt et al. (Current Microbiology, 2012) tested aluminium and copper heat exchangers in a full-scale HVAC rig. On aluminium – the fin material of all split units – 11,411–47,257 CFU/cm² bacteria and about 378 CFU/cm² fungi accumulated, with Methylobacterium dominant; on copper, almost none. Biofilm is a sticky film that shields bacteria from drying and disinfectants and re-grows quickly unless mechanically removed. This is why sanitising must start with pressure washing, with disinfectant applied to the already-cleaned surface.

Park et al. (mSphere, 2020) analysed automotive AC evaporators from five countries and found Methylobacterium dominant (21.7–63.6% of bacteria). Nine of twelve species tested were 'extreme' or 'high' biofilm formers, and biofilm confers resistance to UV, desiccation, starvation and heat/cold shock. The same bacteria dominate aluminium HVAC evaporators (Schmidt 2012). In practice, 'antibacterial sprays' and UV lamps alone cannot remove a mature biofilm; mechanical action (pressure + detergent) followed by disinfectant is needed.

A Building and Environment study (2024) characterised pathogenic bacteria in split units and the environmental factors tied to their pathogenicity, while Sun et al. (2024) found genera implicated in eye, wound and respiratory infections in vulnerable people. A bedroom metagenomics study (2023) showed AC operation changes the pathogenic potential of the air microbiome. For most healthy people the risk is small, but for infants, the elderly, chronically ill or immunocompromised, daily exposure – especially in the bedroom – is worth reducing. Annual sanitising and a clean filter are the simplest measures.

A review in Building and Environment (2025) surveyed the literature on antibiotic-resistant bacteria in HVAC systems and concluded that wet coils, filters and ducts can host and spread such strains. Biofilm favours exchange of resistance genes. Data on household units are still limited, but the mechanism is identical: wet surface, dust, no cleaning. It is an additional argument for proper sanitising in homes with vulnerable people and in medical offices or crowded workplaces.

A Journal of Aerosol Science study (2024) compared the microbial load of several air-conditioning system types under different operating conditions and applied quantitative microbial risk assessment (QMRA). Results show that heavily used units in humid conditions release bioaerosols in amounts that matter for occupant exposure. These data support scaling cleaning frequency to usage intensity: a unit running daily in summer and winter needs sanitising more often than one used occasionally.