Air Conditioner Mould & Health Risks

Mould needs three things to thrive: moisture, food (organic dust) and a stable, dark surface. A split-system air conditioner running in coastal Far North Queensland delivers all three. The evaporator coil condenses litres of water out of humid air every cooling cycle, the airflow drags fine household dust onto the wet fins, and the inside of the indoor unit stays dark and undisturbed between services.

The result, after a couple of wet seasons, is a thin biofilm of mould and bacteria coating the coil and blower wheel. Every time the system runs, it blows microscopic fragments of that biofilm — spores, fragments, and volatile organic compounds — into the room you're sitting in.

The symptoms below are commonly reported by people living with a mouldy AC. They aren't proof on their own — colds, pollen and pet dander cause many of the same things — but a clear pattern that tracks with AC use is a strong signal worth acting on:

• Persistent sneezing, runny nose or itchy eyes that ease when the AC is off
• Sore throat, hoarse voice or a dry cough in the morning
• Asthma flare-ups or more frequent reliever use indoors
• Headaches or sinus pressure that build during the day at home
• A musty, "dirty sock" smell when the unit first starts
• Skin rashes or eczema flare-up.

If symptoms are severe, persistent, or you're managing asthma or another respiratory condition, see your GP. This guide isn't medical advice — it's about the equipment side of the problem.

• Children and infants — smaller airways react faster to irritants
• Older adults
• Anyone with asthma, hay fever or allergic rhinitis
• Immunocompromised people, including chemotherapy patients
• People who sleep in heavily air-conditioned bedrooms every night

If your household includes anyone in those groups, treat annual professional cleaning as routine maintenance — not optional.

• Musty odour for the first 5–15 minutes after start-up
• Black or dark-green specks on or around the front louvres
• Visible fuzz on the louvres, fascia or air outlet
• Water marks or drips below the indoor unit (blocked drain pan)
• Allergy symptoms that are worse indoors than outside
• It's been more than 12 months since the last professional clean

Routine homeowner care helps slow biofilm build-up between services:

• Rinse the filters every 4–6 weeks during cooling season — lukewarm water, a soft brush, dry fully before reinstalling.
• Run the system on fan-only for 10–15 minutes after a long cooling cycle. That dries the coil and starves the mould of the moisture it needs.
• Keep furniture and curtains clear of the indoor unit so airflow isn't disturbed.
• Wipe the front fascia and louvres with a damp microfibre cloth.

What homeowner care can't reach is the evaporator coil and the blower wheel, which is where the bulk of the biofilm actually lives. Supermarket "AC cleaner" sprays mist the coil but don't touch the blower wheel at all, and pulling the unit apart without training risks bent fins, water damage and voided warranties.

A proper indoor-unit clean is a 60–90 minute job per head. The short version:

• Electrical isolation where applicable and a wall & floor protection bib around the unit
• Fascia, filters and louvres removed and washed
• Antibacterial / antifungal coil treatment, allowed to dwell
• High-pressure flush of the coil into the catch bib
• Blower wheel cleaned in place — this is the step most cheap services skip
• Drain pan flushed; condensate line cleared so water exits cleanly
• System reassembled, run-tested, and the indoor air sniff-tested

Done well, the difference is obvious the first time you turn the unit on — no smell, stronger airflow, and faster cool-down.

• Rinse filters every 4–6 weeks during cooling season
• Run fan-only for 10–15 minutes after long cooling runs
• Book a professional deep clean every 12 months (every 6 if you have pets, allergies, or run the AC year-round)
• Keep indoor humidity below ~60% where possible
• Don't ignore a musty smell — it always gets worse, never better