In 1,717 U.S. buildings sampled indoors and out, the indoor mold count was at or below the outdoor count in 85% of them. That is why a mold test needs a same-day outdoor sample, and why one number on its own tells you very little.
If you are getting mold testing or indoor air quality testing in Portland or Vancouver, read the indoor count against outdoor air sampled the same day. In what its authors called the largest standardized survey of its kind to that date, indoor air usually carried far less culturable mold than outdoor air. An indoor reading above the outdoor one puts a building in the minority, and that is worth finding the source of. A reading below it is not an all-clear on its own.
85%:
share of 1,717 buildings whose median indoor mold count was at or below the median outdoor count. In half of them, indoors ran at 0.16 of outdoors or lower, about one-sixth (our arithmetic).
The study
Brian Shelton, Kimberly Kirkland, W. Dana Flanders and George Morris (PathCon Laboratories and Emory University’s Rollins School of Public Health) analyzed 12,026 fungal air samples, 9,619 indoor and 2,407 outdoor, from 1,717 buildings across the United States. The samples were collected from 1996 to 1998 during indoor air quality investigations, every building had at least one outdoor sample, and every sample was taken and analyzed the same way: a culture sampler, which counts the mold that grows on a lab plate, reported as colony-forming units per cubic meter of air (CFU/m3). The paper appeared in Applied and Environmental Microbiology in 2002. The authors call it the largest study of airborne indoor and outdoor fungi done with a standardized protocol to that date.
What they found
Indoors was usually lower. The median indoor concentration was about 80 CFU/m3 (82 in the table) and the median outdoor about 500 (540). For each building the authors divided the median indoor count by the median outdoor count. That ratio was 1 or lower in 85% of buildings, 0.16 or lower in half, and 2.8 or lower in 95%. The middle half of buildings sat between 0.049 and 0.45.
The ratio held across seasons. Outdoor and indoor counts were highest in fall and summer and lowest in winter and spring. But the indoor-to-outdoor ratio “did not vary substantially by season.” So a count that looks high in August and one that looks low in January can mean the same thing once the outdoor air is beside it.
The Northwest was different. Outdoor counts were lowest in the Northwest of all six regions, and the highest indoor-to-outdoor ratios were in the Far West and the Northwest. The results section does not say why. Our reading, not the authors’: when there is less mold in the outdoor air, whatever the house adds makes up a bigger share of the indoor sample. Either way, a rule of thumb built on national numbers is a poor fit for a Portland or Vancouver house. The same-day outdoor sample from your own property is the baseline.
The kind of mold matters too. Stachybotrys chartarum was found in indoor air in 6% of buildings and in outdoor air in only 1%. The most common molds, Cladosporium, Penicillium, Aspergillus and the nonsporulating fungi, were found indoors and outdoors in every region and season.
No limit to compare against.
The authors wrote in 2002 that there were “no government or industry standards that specify allowable or acceptable concentrations of indoor airborne fungi.” EPA says the same today (page updated January 14, 2026): “Standards or Threshold Limit Values (TLVs) for airborne concentrations of mold, or mold spores, have not been set.”
Why it works this way
The authors put the mechanism plainly: “Outdoor air often is the dominant source of indoor fungi.” Spores come in through doors, windows and air intakes. If a building stays wet long enough, mold grows and sporulates inside, adding to what came in. Our explanation beyond that: in a dry building, only part of the outdoor air’s mold makes it inside, which fits indoors normally running lower. A reading above outdoors, or a kind of mold indoors that the outdoor sample does not have, suggests the house itself is adding mold to the air. That last step is our inference from the paper’s numbers, not a test the authors ran.
The limits, stated plainly
This is descriptive, not a test of cause. The samples were sent in by investigators, not drawn at random. In a phone survey of 82 of 100 randomly chosen 1998 buildings, the most common reason for sampling was health complaints (45%), and only 16% were routine checks with no complaint. Homes were a small slice of that survey: 46% were office buildings, 18% schools, 13% hospitals and 4% residences. The data are from 1996 to 1998 and use culture plates, which count only mold that can still grow. Our note: spore-trap counts, which also include spores that will not grow, are a different measure and will give different numbers.
And the most important caveat for a homeowner: a ratio below 1 did not mean a clean building. In the small survey subset, buildings sampled because of visible mold had a median indoor count of 141 CFU/m3 and a median ratio of 0.29, still below outdoors. Routine checks with no complaint had 12 and 0.02. Those subsets were tiny (8 and 16 entries in the table), so treat the numbers as a direction, not a benchmark.
What survives the caveats: in a normal building, mold in the indoor air runs below the outdoor air, the comparison holds across seasons, and a single indoor number with nothing beside it cannot be read.
What happens when you fix it
This study did not follow any repairs. On the follow-through, EPA’s current guidance (page updated September 8, 2026) is that “Surface sampling may be useful to determine if an area has been adequately cleaned or remediated,” and that in most cases, if visible mold growth is present, sampling is unnecessary. Mold you can see gets fixed, along with the water behind it. Testing earns its place when the problem is a smell, a symptom or a question with nothing visible to point at, and after cleanup to confirm the work.
What gets lost in the mold test marketing
Mold tests are often sold as a single number with a “safe” or “elevated” label beside it. There is no federal limit for that label to come from. EPA: “Since no EPA or other federal limits have been set for mold or mold spores, sampling cannot be used to check a building’s compliance with federal mold standards.” A number with no same-day outdoor sample beside it, taken the same way, cannot be placed against anything. EPA also says sampling “should be conducted by professionals who have specific experience in designing mold sampling protocols, sampling methods, and interpreting results.”
What the data supports
Insist on an outdoor sample, same day, same method. Every building in this study had at least one. Without it, the indoor count has nothing to be read against.
Read the ratio and the kinds. Indoor above outdoor is uncommon (about 15% of buildings here, our arithmetic from the 85%). A mold indoors that is rare or missing outdoors, like Stachybotrys at 6% indoors versus 1% outdoors, is a flag even when the total looks modest.
Do not treat “below outdoors” as an all-clear when there is a musty smell, staining or a water history. Buildings sampled for visible mold still had a median ratio of 0.29.
Do not compare against a chart. Counts swing with the season and the region, and the Northwest’s outdoor air ran lowest of the six regions. Your own property’s outdoor air, on the same day, is the comparison.
If you can see mold, EPA says sampling is usually unnecessary. Fix the moisture and clean or remove the growth, then test to confirm.
Bottom line
Mold in indoor air normally runs well below the outdoor air, so a mold test is read against a same-day outdoor sample, not against a number on a chart. A reading above outdoors is the one to chase. A reading below it is not an all-clear if the house smells musty or has had water. See how our room-by-room testing works, or book indoor air quality testing in Portland or mold inspection in Vancouver, WA.
First step: if you already have a mold report, look for the outdoor sample and check it was taken the same day.
Indoor air quality testing and mold testing in Portland and Vancouver, room by room.
illumenair does indoor air quality testing and mold testing in Portland and Vancouver. We test and report. We do not do remediation.
971-363-5626 · info@illumenair.com · www.illumenair.com
Sources: Shelton BG, Kirkland KH, Flanders WD, Morris GK. Profiles of airborne fungi in buildings and outdoor environments in the United States. Applied and Environmental Microbiology 2002;68(4):1743-1753, doi:10.1128/AEM.68.4.1743-1753.2002 (PMC123871; read November 3, 2026). U.S. EPA. Mold Testing or Sampling. Last updated January 14, 2026; read November 3, 2026. U.S. EPA. Is Sampling/Testing for Mold Necessary? Last updated September 8, 2026; read November 3, 2026.