It is the most common situation in the Algarve and one of the least guarded against. The house is shut in October, the shutters are locked, everything is switched off. You come back in May, open the windows, switch on the air conditioning — and there is that smell.
The explanation is more interesting than it seems, and involves three different things happening at the same time.
1. The humidity does not leave
A house closed in the Algarve between October and April has consistently high relative humidity. It does not rain every day, but the air does not circulate, there is no heating drying the atmosphere, and the proximity of the sea keeps the vapour content high.
Inside the air conditioning unit, that means seven months of damp surfaces on the heat exchanger and the drain tray, in total darkness, at a mild temperature, with no air current at all.
It is important to understand that this is worse than during use. When the unit is running there is constant airflow, and many models have a drying function that keeps the fan running for a few minutes after switching off, precisely to dry the heat exchanger. Standing idle, there is nothing to counter the humidity.
2. The fungi take advantage
The conditions described above are, literally, the ones fungi need: humidity, darkness, a mild temperature and organic matter. The organic matter is already there — the dust, pollen and fibres that built up during the previous summer.
The genera most often found in air conditioning units are Aspergillus, Penicillium and Cladosporium. They form biofilm on the heat exchanger fins and on the fan barrel blades, and it is that biofilm that produces the compounds responsible for the smell.
When you switch on the unit in May, the fan starts and pushes seven months of growth into the room, concentrated in the first few minutes.
3. Dust mites move in — but not where you think
This part is poorly explained by everyone, so it is worth being precise.
Dust mites do not live inside the air conditioning. They live in textiles: mattresses, pillows, sofas, rugs, curtains. They feed on skin cells and need ambient humidity to survive.
The optimum conditions for their proliferation are between 20 and 30 °C with relative humidity between 65 and 80%. Below 50% relative humidity survival drops sharply, and there are studies recording total mortality of adult mites in four to eleven days at humidity below 40% and a temperature below 25 °C.
In other words: a house closed through the Algarve winter is exactly in the window in which mites thrive, and what is in the mattresses and sofas in May is not what was left there in October — it is considerably more.
The role of air conditioning here is twofold, and it is where the story crosses over.
First: a unit working well removes humidity from the air. Bringing relative humidity down to around 45 to 50% is the most effective environmental measure there is against mites. A unit with a blocked heat exchanger dehumidifies badly, because cooling capacity has fallen — and with it condensation.
Second: what mites leave behind is allergen in fine particles, which are lifted by air movement and circulate through the room. A unit with saturated filters and a dirty fan barrel retains nothing and blows everything out.
So: air conditioning is not where the mites live, but it determines whether conditions are favourable to them and whether the allergens stay in suspension.
What to do before reopening
Open the house before switching on anything. Windows open, a cross draught, a few hours. It lowers the accumulated humidity and avoids the unit's first start-up spreading everything around a closed house.
Do not switch on the air conditioning in a hurry. If it is going to be cleaned, clean it before the first start-up. Switching on first means distributing around the house what is inside.
Treat the textiles. Bedding washed at 60 °C, mattresses and sofas vacuumed, curtains washed. None of this is air conditioning work, but it is half the problem.
Book the cleaning for April or May. Not for August, with the house full. The right time is before the first use of the season.
And if the house is yours but someone else manages it
It is the case for most villas in Quinta do Lago, Vale do Lobo and Almancil. In those situations, the annual air conditioning clean should be part of the property's opening plan, alongside the pool, the gardening and the general clean — not requested in a hurry when someone complains about the smell.
For property managers, we plan by property and hand over a record of what was done on each unit.
