Bad smell from the air conditioner

Updated 2026-09-04 · reviewed by certified technician

In short The 'cellar', 'wet socks' or sour smell at start-up is not dust: it is the metabolic product of the bacteria and mould living in the biofilm on the evaporator and blower. They are called microbial volatile organic compounds and are exactly what you smell in the first minutes, when air passes over the damp layer.

Odours can have several sources — Deposits or moisture inside the unit.
The condensate drainage route.
Sources within the room, assessed separately.

Woman on the sofa noticing the unpleasant smell when switching the air conditioner on

What is really happening

The 'cellar', 'wet socks' or sour smell at start-up is not dust: it is the metabolic product of the bacteria and mould living in the biofilm on the evaporator and blower. They are called microbial volatile organic compounds and are exactly what you smell in the first minutes, when air passes over the damp layer.

'Fragrance' sprays cover the smell for a few days; the only thing that removes it is physically stripping the biofilm by pressure washing, followed by sanitising. If the smell returns within two or three weeks the cause is usually the drain pan or a hose that does not drain completely.

The evidence

What to do

Mechanical pressure cleaning of the evaporator, blower and pan inside a collection cover, then sanitising; filters washed every two weeks; room humidity kept between 40 and 60%; the unit dried on fan mode before winter. No sprays over dirt, no bleach in the unit, no refrigerant "top-ups".

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The information on this page is educational and based on cited sources; it does not replace medical advice. We do not do repairs, installation or refrigerant circuit work.