Filters turn black in 3 days in a clean bedroom – what is going on?
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Voluntari, new block, clean bedroom, no carpet, no pets. 9,000 BTU Gree, 3 years old. The filters turn black (not brown, black) within 3 days. I vacuum daily. I do not understand where so much dirt comes from. Could it be from the unit onto the filters, not the other way?
What is really happening
Room dust is brown-grey and accumulates slowly, in proportion to how much dust is in the air. A filter that turns uniformly black in 3 days, especially in the zone in front of the evaporator, is a filter on which Cladosporium is growing – the dominant genus on split-unit filters, ~32% in studies. Conditions are perfect: the mesh sits in air at nearly 100% humidity while the unit cools, and behind it the roller and evaporator, the most contaminated components, release spores that lodge on the damp mesh and germinate. Studies show the filter of a daily-used unit accumulates 5 times more fungi than the room's carpet. Washing the filter removes what grew on it, but the spore source – behind – remains and recolonises it within days. The filter is the symptom; the unit is the cause.
- Particles reach the air intake.
- Some remain on the filters.
- Internal deposits need assessment and appropriate cleaning.
- On filters of residential split units, Cladosporium (~32%), Aspergillus (~9%), Acremonium and Alternaria dominate – genera known as allergens. [Building and Environment, 2025]
- The filter of a daily-used air conditioner accumulates about 5 times more fungi than the carpet in the same room. [Atmospheric Environment, 2002]
- The blower (cross-flow fan) is the most fungus-contaminated component of a split unit, more than the filter or the front of the evaporator. [Indoor Air, 2023]
- Mould does not grow below roughly 80% surface relative humidity, but the cold surfaces inside an air conditioner constantly exceed this threshold. [WHO / NCBI Bookshelf, 2009]
What we do on site
- Collection cover; we photograph the roller and evaporator – where the spores come from.
- Cleaning solution, pressure wash in place of evaporator, roller, pan.
- Sanitising solution.
- Filters: washed in warm water and detergent, fully dried in the sun or in air, not refitted wet.
- Outdoor unit, washed in place.
- See current rates on the Prices page.
What you can do yourself
- Filters fully dry before refitting – a wet filter is a mould bed.
- 'Fan' 20–30 minutes before shut-off – the filters dry along with the evaporator.
- Hygrometer in the bedroom, under 55%.
What NOT to do
- Do not replace the filters with sponge 'more dense' – sponge filters hold 3–5 times more microbial mass.
- Do not spray 'anti-mould' on the filters – you breathe the product.
- Do not fit filters wet 'they will dry in the unit'.
Prevention
Filters that stay grey, not black, are your test that the unit is clean. Annual sanitising, daily drying, filters fully dried.
When to call us
0764 012 943, Monday–Saturday 08:00–20:00. Voluntari.
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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.