What Your Air Conditioner Is Growing

The smell started in the dining room of an aged care facility. Musty, close to the tables. A couple of residents were back with chest complaints. Maintenance said the filters had been changed the month before.

I followed it to the plant room and opened the access panel on the air handling unit. The coil looked clean from there. It never is.

Why a coil grows things

Warm, humid air hits cold metal fins and water condenses out. That water carries dust, skin cells, pollen and other organic particles, and they settle on the fins. What you get is a thin film of moisture and nutrient on metal held at a steady temperature, in the dark, enclosed. That is biofilm habitat.

What the research shows

A Yale research group sampled 40 large commercial units, 25 in humid southern Connecticut and 15 in hot, dry Sacramento [1]. The median coil carried around 12 million bacterial cells and 760,000 fungal spore equivalents per square metre. Two factors stood out: how good the upstream filter was, and how wet the coil stayed. On the humid-climate coils, fungal loading even differed between the top and bottom halves of the same coil. A follow-up on the same units found that lower-efficiency MERV 8 filters went with more pathogenic bacteria and medically relevant fungi than MERV 14 [2].

Collecting dust and growing on it is one thing. Putting it back into the air is another. The same group measured five units at the end of the cooling season [3]. The coils caught most of the particles passing through them. But three of the five were net emitters of biological particles. When a unit was emitting, it accounted for roughly a third of the biological particles and nearly three quarters of the airborne fungi in the air it supplied. The emitting coils carried Cladosporium, Penicillium, Aspergillus, Stachybotrys and Malassezia. The two that were not emitting ran at a higher cooling load.

A Shanghai laboratory study looked at when and where it happens [4]. On hot, humid mornings, fungal levels in the downstream air rose 24.5 percent when the coil switched on. The rear rows of the coil were the main source. Front rows were flushed by condensate draining off. Rear rows held moisture, grew fungi and released spores.

Why it matters in care settings

Cladosporium and Penicillium are common environmental moulds, often allergenic. Aspergillus includes species that cause invasive disease in people with weakened immune systems. Stachybotrys is the black mould linked to water-damaged buildings. Malassezia is a skin-associated yeast.

None of these are exotic. They are the ordinary flora of damp indoor environments. An operating coil is a very effective way of moving them into the rooms where residents eat and sleep.

What the industry does now

The baseline is still alkaline foam cleaner. Spray, dwell, rinse. It cuts grease and grime. It is not sporicidal.

UV lamps are the common add-on in hospitals and larger commercial systems. One controlled study found UV cut surface microbial loading on a wet coil by about half, but it also loosened clusters of bacteria into the downstream air [5]. Coil coatings exist, and so do probiotic treatments.

None of it is standardised. There is no accepted bioburden threshold for a coil surface. A facility can spend serious money and still not know whether it worked.

How I was taught to do it

Back to that plant room. Clean from the panel, never clean once you open it.

Soft nylon brush, with the fin direction, never across. The fins are thin aluminium and you bend them going sideways. Then detergent to strip the soil. Agitate. Remove the film and residue completely, because anything left behind uses up the next chemical before it reaches the surface you care about. Then the disinfectant: quaternary ammonium, with the full dwell time. Swab. Test. Verify.

That was the sequence, and it was the best the industry had in the early 2000s.

The layer you cannot see

Dry surface biofilm is what survives after the visible moisture is gone. It is not the slimy layer you brush off. It is a thin, dehydrated layer worked into the surface texture, protected by its own polymer matrix, and it persists long after the surface looks dry.

An Australian team grew this kind of biofilm in the lab and hit it with sodium hypochlorite at up to 20,000 parts per million [6]. Plate counts dropped sharply. Live cells remained anyway, and they regrew. The bacteria had not mutated. The resistance came from the biofilm state itself.

That is the lesson for coils. Chemistry alone, even a strong oxidiser, does not get there. Mechanical disruption has to come first, so the chemistry can reach what is underneath.

On a coil, the outer fins stay wet from condensate during dwell. Inside, it dries. A mist penetrates but evaporates before it soaks deep, and the rear rows dry unevenly, in patches. In my experience that is why repeated light applications, or a true dry-vapour fog with smaller droplets, reach further. It is also why the verification swab that matters is the one taken after the next run, not the one taken straight after treatment.

The gap is a protocol

The gap is not a missing product. It is a missing protocol.

Assess before you treat. Remove soil before you disinfect. Kill with a defined concentration and dwell. Swab and verify.

That sequence is what separates a cleaned coil from a coil that looks clean.

Whether any specific product, at a given concentration, reliably kills rear-row coil biofilm in occupied buildings is a question the published literature has not yet answered directly. That is the study that still needs to be done.

References

Bakker A, Siegel JA, Mendell MJ, Peccia J. Building and environmental factors that influence bacterial and fungal loading on air conditioning cooling coils. Indoor Air. 2018;28(5):689-696. doi:10.1111/ina.12474

Bakker A, Siegel JA, Mendell MJ, Prussin AJ, Marr LC, Peccia J. Bacterial and fungal ecology on air conditioning cooling coils is influenced by climate and building factors. Indoor Air. 2020;30(2):326-334. doi:10.1111/ina.12632

Luhung I, Bakker A, Angel DM, Sheu R, Gomez O, Siegel JA, Mendell MJ, Gentner D, Hernandez M, Peccia J. Fungal emissions from air conditioning cooling coils. Indoor Environments. 2026;3(2):100162. doi:10.1016/j.indenv.2026.100162

Gu Y, Yang W, Zhong K, Yang Z. Factors shaping fungal contamination emissions from cooling coils: an experimental study across hot-humid and hot-dry climates in Shanghai. Building and Environment. 2025;275:112820.

Luongo JC, Miller SL. Ultraviolet germicidal coil cleaning: decreased surface microbial loading and resuspension of cell clusters. Building and Environment. 2016;105:50-55. doi:10.1016/j.buildenv.2016.05.024

Almatroudi A, Gosbell IB, Hu H, et al. Staphylococcus aureus dry-surface biofilms are not killed by sodium hypochlorite: implications for infection control. Journal of Hospital Infection. 2016;93(3):263-270. doi:10.1016/j.jhin.2016.03.020