Where does the 21% asthma statistic actually come from?

Mudarri, D. and Fisk, W.J. (2007), 'Public health and economic impact of dampness and mold,' published in the peer-reviewed journal Indoor Air (Wiley), a study conducted with EPA affiliation. The 21% figure (95% confidence interval 12-29%) represents the estimated proportion of U.S. current asthma cases attributable to dampness and mold exposure in the home — not a claim that mold causes all asthma, but a population-level attributable-fraction estimate from modeling epidemiological data.

Is dampness only a residential problem?

No — the same underlying research found water damage and moisture problems are widespread in commercial and institutional buildings too: historical water damage has been documented in the large majority of U.S. office buildings, and plumbing or roofing problems are common findings in school facility surveys. This matters because occupants spend meaningful time in these buildings as well as at home.

How often do U.S. homes actually experience water damage?

U.S. Census American Housing Survey data found roughly 9.9% of U.S. homes had exterior-leakage water damage and roughly 8.1% had interior-leakage water damage in a given survey year — figures that illustrate how common the underlying moisture events that lead to mold actually are, not a mold-specific measurement.

Mold and Indoor Air Quality: Key Statistics, Sourced and Verified

Numbers get repeated online until their original source is lost. This page traces every statistic back to where it actually came from — a peer-reviewed study, a government report, or a named survey — so you can verify it yourself rather than take our word for it.

How common is dampness and mold in U.S. buildings?

The U.S. EPA has stated that roughly one-third to one-half of all structures in the United States have damp conditions that may encourage mold development. A more specific, frequently cited figure comes from a 2007 peer-reviewed study: Mudarri, D. and Fisk, W.J., “Public health and economic impact of dampness and mold,” Indoor Air, 2007 — an EPA-affiliated analysis that estimated approximately 47% of U.S. homes have dampness or mold problems.

The underlying moisture events are documented separately by the U.S. Census Bureau’s American Housing Survey, which found in a representative year that 9.9% of U.S. homes reported water damage from exterior leakage and 8.1% reported water damage from interior leakage — real, common events that are frequently the precursor to a mold problem, whether or not mold itself was assessed in that survey.

The asthma burden attributable to dampness and mold

The same Mudarri and Fisk (2007) study modeled the population attributable fraction — the proportion of existing disease burden statistically attributable to a specific exposure — for asthma and home dampness/mold exposure in the United States. Their central estimates:

21% of current U.S. asthma cases (95% confidence interval: 12-29%) are estimated to be attributable to dampness and mold exposure in the home.
Of the roughly 21.8 million Americans reported to have asthma at the time of the study, this translates to approximately 4.6 million cases (range 2.7-6.3 million) attributable to home dampness/mold.
The estimated national annual cost of asthma attributable to home dampness and mold exposure: $3.5 billion (range $2.1-4.8 billion).

Separately, the 2004 Institute of Medicine report (see our full evidence-tier summary) found that dampness and mold exposure is associated with an estimated 30-50% increase in the occurrence of a range of respiratory problems — a broader finding than the asthma-specific attributable-fraction estimate above, covering multiple respiratory outcomes at once.

Beyond the home: commercial and institutional buildings

Moisture problems aren’t confined to residential properties. Building-science surveys cited in this same body of research found historical water damage in a large majority of U.S. office buildings, with a meaningful share reporting currently active leaks at the time of survey. School facility condition surveys have separately found roughly 30% of U.S. schools reporting plumbing problems and roughly 27% reporting roofing problems — both direct pathways for the kind of moisture intrusion that leads to indoor mold growth in buildings where children and staff spend significant time.

Water damage and mold insurance claims

For the financial side of this picture — how often these events turn into insurance claims, and what they typically cost — see our companion page, Water Damage and Mold Insurance Claims: Key Statistics, in our insurance section.

Why we cite it this way

A number without its source is just a number. Every figure on this page traces to either a named peer-reviewed study, a U.S. government survey, or an EPA/CDC/WHO publication — we’ve named each one specifically so you (or anyone else, including a researcher or journalist) can go verify it directly rather than trust a mold-remediation company’s word for it.

[Sources: U.S. EPA (epa.gov/mold); Mudarri, D. & Fisk, W.J. (2007), “Public health and economic impact of dampness and mold,” Indoor Air, Wiley — DOI 10.1111/j.1600-0668.2007.00474.x; Institute of Medicine (2004), Damp Indoor Spaces and Health, National Academies Press; U.S. Census Bureau, American Housing Survey.]