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Homes with dampness and mould show roughly 30–50% more respiratory symptoms (cough, wheeze, upper respiratory symptoms). … and 4 more sourced claims.

Explanatory diagramA standard filter has limits
A standard filter has limits
  1. Particles of different sizes move through the air.
  2. The filter retains some particles, depending on its type.
  3. A standard filter is not equivalent to HEPA filtration or gas removal.

Particles are symbolic and not to scale.

strong evidenceHomes with dampness and mould show roughly 30–50% more respiratory symptoms (cough, wheeze, upper respiratory symptoms).

The Fisk, Lei-Gomez and Mendell meta-analysis (Indoor Air, 2007) pooled dozens of studies and found statistically significant increases: upper respiratory symptoms OR 1.70, cough OR 1.67, wheeze OR 1.50. The LBNL summary describes 30–50% increases (some estimates 40–80%). An air conditioner with mould inside is a localised source of moisture and spores, in the very room where you sleep. Sanitising removes this source, but does not replace fixing leaks or wall condensation.

Sources (2)
strong evidenceThe US Institute of Medicine found sufficient evidence that damp/mouldy environments are associated with cough, wheeze, upper respiratory symptoms and asthma symptoms in asthmatics.

The 'Damp Indoor Spaces and Health' report (IOM, 2004) graded evidence: sufficient for association with upper respiratory symptoms, cough, wheeze and asthma symptoms in sensitised asthmatics; limited/suggestive for asthma onset and lower respiratory illness in otherwise healthy children; and association with hypersensitivity pneumonitis in susceptible people. The report found no support for vague symptoms like fatigue. For a sanitising service this means promises must be realistic: reduced exposure to mould and allergens, not 'cures'.

Sources (1)
strong evidenceMould odour in the home is associated with about 73% higher risk of developing asthma (OR 1.73), more than visible mould (OR 1.29).

The Quansah et al. meta-analysis (PLOS ONE, 2012) of 16 cohort and case-control studies found dampness raised new-asthma risk by 33%, visible mould by 29% and mould odour by 73%. Mould odour indicates active, often hidden microbial growth – exactly the situation of an air conditioner that 'smells like a cellar' at start-up with no visible mould on the walls. We cannot claim the unit causes asthma, but a mouldy smell from the unit is a signal worth acting on, especially in children's rooms.

Sources (1)
moderate evidenceThe filter of a daily-used air conditioner accumulates about 5 times more fungi than the carpet in the same room.

In Hamada & Fujita (2002), fungal contamination of AC filters was about 5 times that of the carpet, and daily-used units had much higher internal contamination than occasionally used ones. The filter traps dust, pollen, spores and mite fragments, which become growth substrate in humidity. Washing the filter every 2 weeks (as all manufacturers require) is the customer's simplest action, but does not replace cleaning the components behind the filter.

Sources (2)
limited evidenceSponge-type filters retain about 3–5 times more bacterial and fungal mass than polyester or HEPA filters, and microbial mass grows month after month.

A review chapter (IntechOpen, 2019) reports measurements by filter type: sponge accumulated 0.177 g bacteria and 0.257 g fungi, polyester 0.024/0.072 g, HEPA 0.037/0.047 g. Mean bacterial mass rose from about 0.061 g after one month to 0.101 g after three months. The average across 820 units was 1,252 CFU/m³ fungi. The authors' conclusion: filters must be cleaned regularly; washable ones washed, HEPA replaced. For a split, mesh filters are washed every 2 weeks and fine filters (deodorising, allergen) replaced per the manual.

Sources (1)

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