Why does the air conditioner smell sour when you switch it on?

When the air conditioner runs, water condenses in the indoor unit and the filter stays wet. Mould and bacteria grow on the wet filter; a study in Japan found that the mould load in air-conditioner filters was about 5 times that in carpets. The smell is strongest when the unit is first switched on.

How do I use it for air conditioner smells?

With the air conditioner switched off, take out the indoor unit's filter, spray it from 20-30 cm away and leave it to dry. The smell comes from a filter that stays damp, so the filter also needs washing regularly.

Air conditioning · 4 min read · 3 October 2026

Why does the air conditioner smell bad?

The source of the sour smell when you switch on the air conditioner, mould in the filter, and the maintenance you can do at home.

MVOC

Mould-derived volatile compounds

In short

1When the air conditioner runs, water condenses in the indoor unit and the filter and coils stay wet.
2In one study, the mould load in air-conditioner filters was about 5 times that found in carpets.
3Washing and drying the filter regularly is the maintenance that does most to reduce the smell.

A cooling device, a damp environment

As the air conditioner cools the air, the moisture in it condenses on the coils in the indoor unit. This water drains away through the drain pipe, but the filter and coils stay wet for a long time. When dust and moisture come together, they create favourable conditions for mould and bacteria to grow.

A study in homes in Japan found that the mould load in air-conditioner filters was on average about 5 times that in carpets. Cladosporium and Penicillium were the dominant species. In units used every day, contamination increased faster.

Worst when first switched on

The same study measured about twice as many mould spores in room air while the air conditioner was running as in rooms without air conditioning. The highest value was seen when the unit was first switched on, and it fell over time. That sour smell you notice when you switch on the air conditioner is this build-up from the filter getting into the air.

Maintenance you can do at home

On most models, the indoor unit's filter can be removed by hand. Follow the instructions in the maintenance manual.

01Switch off the air conditioner, unplug it and remove the filter.
02Wash the filter in lukewarm water and let it dry completely in the shade.
03Once the filter is dry, apply a probiotic spray to it from 20-30 cm away and put it back.
04Before switching the unit off, run it in fan-only mode for a few minutes so the inside dries.
05Have the indoor unit professionally cleaned at least once a year.

Questions

Why does the air conditioner smell sour when you switch it on?: When the air conditioner runs, water condenses in the indoor unit and the filter stays wet. Mould and bacteria grow on the wet filter; a study in Japan found that the mould load in air-conditioner filters was about 5 times that in carpets. The smell is strongest when the unit is first switched on.
How do I use it for air conditioner smells?: With the air conditioner switched off, take out the indoor unit's filter, spray it from 20-30 cm away and leave it to dry. The smell comes from a filter that stays damp, so the filter also needs washing regularly.

Sources

All literature →

01
Effect of air-conditioner on fungal contamination
Hamada N, Fujita T (2002). Atmospheric Environment 36(35): 5443-5448
www.aspergillus.org.uk/article_database/effect-of-air-conditioner-on-fungal-contamination/
Finding · Field studyOn average, the mould load in air conditioner filters was about 5 times that of carpets; Cladosporium and Penicillium were dominant. With the air conditioner running, mould spores in the room air were about twice the level of rooms without air conditioning, and the highest level was measured when the air conditioner was first switched on.
02
Microbial volatile organic compounds
Korpi A, Järnberg J, Pasanen AL (2009). Critical Reviews in Toxicology 39(2): 139-193
doi:10.1080/10408440802291497
Finding · ReviewMould and bacteria release volatile organic compounds (MVOCs) as part of their metabolism. The review looks at 96 typical MVOCs and the 15 most often measured in damp-damaged buildings; the most notable effect is irritation of the eyes and upper airways.

These studies were not carried out with Air Biotics. What we say about the product is limited to what is written on its packaging; Air Biotics is not a disinfectant.

More guides

Full guide →

DampC₈H₁₆OWhere does the damp smell come from?The mould-derived volatile compounds behind the smell of closed wardrobes, basements and bathrooms, and how to tackle damp.Kitchen(CH₃)₃NWhy won't fish and frying smells go away?Trimethylamine, oxidised oil and vapour settling on curtains. The chemistry of the smell that spreads from the kitchen to the living room.AllergenDer p 1House dust mite allergy: what's happening in your bed?Why mites live in your bed, what actually causes the allergy, how it disrupts sleep and what you can do in the bedroom.