An air conditioner brings together, by design, the three conditions fungi need to grow: constant moisture, total darkness and a mild temperature. Add organic matter — dust, pollen, fibres, grease — and you have a culture medium running inside the wall of your living room.
It is not an exotic or rare problem. It is the normal state of any unit that has never been opened.
What grows inside
The genera most often identified in air conditioning units are Aspergillus, Penicillium and Cladosporium. They grow on the heat exchanger fins, on the fan barrel blades and on the walls of the drain tray, forming biofilm — that dark, sticky layer that does not come off with running water.
The drain tray deserves special attention. It is where the standing water is, especially when the drainage is partly blocked. Standing water and organic matter at room temperature is the combination that most quickly produces active colonies.
When the fan starts, the air passes through all of this before entering the room.
Effects on health
Exposure to fungal spores indoors is associated with respiratory and allergic symptoms: cough, wheezing, nasal congestion, and irritation of the throat and eyes. In people with asthma or allergic rhinitis, continued exposure tends to worsen existing symptoms.
Two signs help show whether the problem is in the indoor air and not elsewhere. The first is inverted seasonality: symptoms that get worse indoors and improve when you go out, rather than the other way round. The second is coincidence: several people in the same home or office with complaints at the same time, in the same period of the year.
None of these observations replaces a medical assessment. But if the musty smell appears when you switch the unit on and the symptoms go with it, it is worth opening the unit before looking for more complicated causes.
What Portuguese law requires
This part concerns mainly hotels, holiday rentals run as businesses, restaurants, shops, offices and spaces serving the public.
Decree-Law no. 101-D/2020 establishes that all commercial and service buildings in operation are subject to indoor air quality requirements. Ordinance no. 138-G/2021 puts them into practice, setting protection thresholds and reference conditions that include microbiological pollutants — that is, bacteria and fungi — and providing for periodic assessment.
Ordinance no. 138-I/2021 goes further on maintenance: it requires that ventilation systems have easy access for inspecting and maintaining filters, coils and heat exchangers, condensate trays, air handling units, fans and the duct network.
Put directly: the law does not treat air conditioning cleaning as a cosmetic extra. It treats it as part of the building's compliance.
This is general information and does not replace legal advice or confirmation with the competent authority. Each case depends on the type of building, the area and the activity.
Why this matters more in hotels and holiday rentals
Beyond compliance, there is the commercial side, and it is measurable.
An analysis of more than two and a half million online reviews, across almost six thousand hotels in twenty cities, identified smells and air conditioning among the categories with the most negative mentions. Complaints about air conditioning appeared in twice as many reviews as problems with internet.
The guest comes into the room, switches on the air conditioning and smells damp in the first few seconds. From then on, everything else in the room is judged through that lens — however impeccable the visible cleanliness. It is one of the few hotel problems that is entirely avoidable with scheduled servicing.
What does not fix it
Air fresheners and sprays. They mask the smell for a few hours and do not touch the cause. In hotel rooms they have the opposite effect to the one intended: the guest realises something is being hidden.
Washing the filters. It should be done, but it holds back the coarse dust. The biofilm is on the heat exchanger, the fan barrel and the tray, behind the filters.
Replacing the unit. A new unit under the same conditions of use reaches the same state in a few years. What changes the outcome is maintenance, not the date of manufacture.
What does fix it
Open the unit, remove the fan barrel, wash the heat exchanger fin by fin, clean the tray, clear the drainage and apply a sanitising product made specifically for air conditioning equipment. Then test and confirm the airflow.
That is what we do, and it is all we do. In Almancil, Quinta do Lago, Vale do Lobo, Vilamoura, Quarteira, Loulé, Albufeira and Faro.
For hotels, condominiums and property managers, we plan the job by wing, by floor or by property, in low season, with a record of what was done on each unit.
