What the health evidence base actually shows

The Institute of Medicine's Damp Indoor Spaces and Health, published in 2004, and the World Health Organization's guidelines on dampness and mould, published in 2009, are the reference points. Both worked from published epidemiology rather than case reports, and both classify findings by strength of evidence rather than asserting them flatly.

The IOM found sufficient evidence of an association between damp indoor environments and upper respiratory tract symptoms, cough, wheeze, and asthma symptoms in sensitised asthmatic people, and between visible mold and hypersensitivity pneumonitis in susceptible people. For most other outcomes it found the evidence inadequate or insufficient to determine whether an association exists — which is a statement about the research, not a finding of no effect.

The WHO review, drawing on later work, additionally found sufficient evidence for associations with asthma development, dyspnoea, current asthma and respiratory infections. Its headline framing is that occupants of damp or mouldy buildings face a substantially increased risk of respiratory symptoms and asthma.

Both reviews locate the exposure in dampness itself rather than in any named organism. That single point is the most consistently misreported thing in this field.

Mudarri and Fisk, writing from EPA's Indoor Environments Division in 2007, estimated the proportion of US current asthma cases attributable to dampness and mold exposure in the home at 21%, with a 95% confidence interval of 12% to 29%. Against the 21.8 million people then reported to have asthma, that worked out to roughly 4.6 million cases, with a range of 2.7 to 6.3 million.

Applying that fraction to the national annual cost of asthma gave an estimated $3.5 billion a year attributable to home dampness and mold, with a range of $2.1 to $4.8 billion.

A 2016 analysis by Mudarri extended the accounting beyond asthma and restated it in 2014 dollars: $3.7 billion for allergic rhinitis, $1.9 billion for acute bronchitis, $15.1 billion for asthma morbidity and $1.7 billion for asthma mortality. Note the base year. A figure from this paper quoted as current dollars without adjustment has been silently changed.

National asthma prevalence has not been rising. CDC's National Health Interview Survey puts current asthma among all ages at 7.4% in 2001, 8.5% in 2010, and 7.7% in 2021, with no significant change after 2010. Among children the series rose to 9.5% in 2011 and then declined significantly to 6.5% in 2021. Among adults it rose modestly, from 6.9% in 2001 to 8.0% in 2021.

This is stated here because it is true and because leaving it out would be the easy thing to do. A great deal of marketing in this industry implies a mold-driven epidemic of illness. The federal prevalence series does not show one.

The two findings are not in conflict, which is the part usually missed. An attributable fraction is a share of cases, and a share can hold steady while the total is flat. Twenty-one per cent of a stable number is still several million people, and the estimate is about how much of an existing burden is preventable rather than about whether the burden is growing.

What follows for a homeowner is unchanged by any of it: dampness is associated with respiratory outcomes, that association is the best-evidenced thing in the field, and controlling moisture is the intervention both reviews point to.

They are not a diagnosis and not a basis for one. An attributable fraction is a population statistic; it says nothing about whether any particular person's symptoms are caused by their house.

They are not species-specific. None of this work identifies an organism as the agent, and applying these figures to a page about a particular species would be a misuse of them.

And they are not a measurement of mold. Every estimate here starts from surveys of dampness and visible mold in homes and from asthma epidemiology. If the underlying dampness prevalence estimates are off, everything downstream moves with them.