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Odour and volatile compounds

The 'dirty sock' or musty smell from an air conditioner comes from volatile organic compounds produced by microbial biofilm on the evaporator and blower. … and 2 more sourced claims.

Explanatory diagramOdours can have several sources
Odours can have several sources
  1. Deposits or moisture inside the unit.
  2. The condensate drainage route.
  3. Sources within the room, assessed separately.
moderate evidenceThe 'dirty sock' or musty smell from an air conditioner comes from volatile organic compounds produced by microbial biofilm on the evaporator and blower.

Studies of automotive AC systems (Rose et al., Current Microbiology, 2000; Simmons et al., 1999) identified mixed bacterial-fungal biofilms on evaporators and linked the unpleasant smell to microbial volatile organic compounds (MVOC) they emit. Park et al. (2020) confirm microbial growth in the evaporator is a major cause of foul-smelling air. The same mechanism applies to wall splits: the smell is strongest at start-up, when air passes over the wet biofilm. Air fresheners mask, not solve; only removing the biofilm eliminates the source.

Sources (3)
moderate evidenceMould odour is treated by WHO and epidemiological studies as an indicator of active microbial growth, even when nothing is visible.

In the epidemiological literature, 'mould odour' is used as an independent exposure indicator and shows even stronger associations with asthma than visible mould (Quansah 2012: OR 1.73). WHO explains that active microorganisms produce volatile compounds perceived as an 'earthy' or 'cellar' smell. In a split, odour at start-up usually means biofilm on the blower or standing water in the pan. The right advice: no air fresheners, no sprays through the grille; book a sanitising and check the condensate drain.

Sources (2)
moderate evidenceManufacturers recommend running the unit in fan-only mode for several hours to dry the interior before long periods of non-use, precisely to prevent odour and mould.

The Daikin FTXF-D manual instructs the user to 'operate the unit in fan only mode for several hours to dry the inside' before a long idle period, and in the filter section notes that dirty filters cause odour. The logic is simple: a wet evaporator left switched off in a closed casing becomes an incubator. Practical tip: after the last cooling of the day, run 20–30 minutes on 'Fan' (many units have an auto-dry function), and in autumn, before the unit is retired for the season, run 2–3 hours on fan. This reduces odour but does not replace annual sanitising.

Sources (2)

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