Mould

The 2009 WHO indoor air quality guideline reviewed hundreds of epidemiological studies and found sufficient evidence linking dampness/mould with respiratory symptoms, allergies and asthma, plus immune perturbation. WHO states that the most important means of prevention is avoiding persistent dampness and microbial growth on interior surfaces. A split air conditioner contains exactly such surfaces: the evaporator, the drain pan and the blower stay wet for hours. Periodic sanitising of the indoor unit is therefore a preventive measure, not just comfort.

WHO notes that mould fungi do not grow below about 80% relative humidity at the material surface, and for soiled materials the critical threshold drops to 75–80%. In an indoor AC unit, the evaporator works below the dew point, so it is covered in condensate (100% local humidity) whenever the unit is cooling. Dust settled on the fins and blower provides the food, and the moisture the growing conditions. This is why mould appears inside the unit even in a dry, clean home.

EPA states it is impossible to get rid of all mould spores indoors, since they float in the air and are found in house dust. Spores only become a problem when they find moisture and food – and a wet, dusty evaporator is the ideal place. The practical conclusion for a split unit: what matters is not how many spores enter, but how often dust and biofilm are removed from the wet surfaces. Mechanical cleaning of the fins and blower breaks this cycle.

EPA's mould remediation guide states that mould is allergenic whether dead or alive, and some moulds may be toxic. EPA therefore focuses on physical removal, not sterilisation. For an air conditioner, an 'antibacterial' spray through the grille may kill some organisms but leaves the biofilm layer, spores and allergenic fragments on the blower and fins, from where they are blown into the room. Proper sanitising means pressure washing and rinsing, then disinfecting.

Both the EPA home guide and the schools/commercial guide contain the same recommendation: do not run the HVAC system if it is mould-contaminated, since it can spread mould throughout the building. In a split unit the cross-flow fan pushes air directly over the evaporator and blower, so any mould colony becomes a spore source for the room. If you see black spots on the outlet louvres or smell mould at start-up, it is safer to switch the unit off and schedule sanitising. This matters most in bedrooms and children's rooms.

The Indoor Air study (Shiraishi et al., 2023) on Japanese household units measured a massive jump in airborne fungi immediately after switching the unit on: from a background of 40–920 CFU/m³ to 5,500–100,000 CFU/m³. This is exactly the 'burst' of odour and spores customers notice in the first minutes of operation. The main source is not the filter but the blower fan and the outlet vanes located downstream of the evaporator. Pressure washing these parts reduced fungal abundance by over 99%.

Fungal DNA analysis in Shiraishi et al. (2023) found high levels in all internal parts, but the blower fan was the most contaminated, followed by the air outlet vanes – both downstream of the heat exchanger where air is cold and saturated. Dominant taxa were Cladosporium, Toxicocladosporium, Aureobasidium, Aspergillus and Rhodotorula. The authors even recommend sampling the blower rather than the filter to assess contamination. In practice, a 'cleaning' limited to filters and the evaporator face leaves the main spore source untouched.

In the Indoor Air (2023) study, the sanitising procedure – applying alkaline detergent, brushing and rinsing internal parts with pressurised water – reduced fungal abundance by more than 99% across all components, including the blower. This confirms that the mechanical method (pressure washing with a cleaning solution, then rinsing) is effective, unlike spraying an aerosol through the grille. For a wall-mounted split, the procedure is done in place with a collection cover/bag, without removing the unit. The study provides the scientific backing for what we call 'complete sanitising'.

Hamada and Fujita (Atmospheric Environment, 2002) measured airborne fungi in Japanese homes and found that during AC operation contamination was about twice that of rooms without AC, peaking at start-up and decreasing over time. The unit's filter was about 5 times more contaminated than the carpet, with Cladosporium and Penicillium dominant. Interestingly, by drying the air the AC reduced mould in carpet dust to about one third – so a clean unit is beneficial, a dirty one is a source. Conclusion: daily use calls for more frequent cleaning.

A Building and Environment study (Liu et al., 2025) sequenced fungal communities in residential and office split units. Dominant genera were Cladosporium (31.6%), Aspergillus (9.5%), Acremonium (8.1%) and Alternaria (8.0%). About 64% of the genera identified include potentially pathogenic or allergenic species, with risk of asthma, allergic reactions and, in immunocompromised people, invasive infections. Alternaria and Cladosporium are among the most common triggers of allergic asthma in children. For families with allergy sufferers, sanitising the unit is an exposure-reduction measure as important as HEPA vacuuming.

Bakker et al. (Indoor Air, 2020) sequenced cooling coils from buildings in different climates and showed that coils are consistently colonised, with communities shaped by outdoor climate, humidity and building factors. In other words, evaporator contamination is the rule, not an accident, and hot humid summers (as in Bucharest) accelerate it. A coil that dries quickly after shut-off and is cleaned periodically carries a much lower load. The study supports coil inspection and cleaning as a standard practice, not an exception.

In the moisture-control chapter, WHO explicitly states that some microorganisms can grow in cooling coils and drip pans, as well as in humidifiers, causing respiratory diseases or symptoms. WHO stresses that 'it is important to keep all surfaces of an air-handling system clean' and that hygiene, commissioning, operation and maintenance are key to reducing the negative effects of HVAC systems. Although the text targets large systems, the principle is identical for a split: clean evaporator, pan, blower and condensate hose.