Researchers at the University of Ljubljana swabbed nine user-accessible sites inside 30 household dishwashers, plus the tap water going in, the waste water coming out, the steam from an opened door, and the kitchens around them. The rubber seal was the worst spot in the machine.
Household mold advice concentrates on the places you would expect: a leaking supply line, a bathroom with no fan, a crawlspace, the north wall of a bedroom. It rarely mentions the one appliance in the kitchen designed to be hot, wet and full of detergent. That turns out to be a gap.
What they did
Jerneja Zupančič, Monika Novak Babič, Polona Zalar and Nina Gunde-Cimerman sampled 30 randomly selected household dishwashers in seven Slovenian cities in the autumn of 2013. The machines were between 1 and 8 years old, came from four manufacturers, and were used anywhere from once a week to twice a day.
Nine user-accessible sites were swabbed in each machine: rubber seal, door, interior side wall, cutlery rack, detergent dispenser, rinse-aid dispenser, drain, side nozzle and sprinklers. Sampling was done immediately after a wash cycle finished.
They then followed the organisms outward. The tap water feeding each machine was sampled, along with the waste water leaving it, the hot aerosol released when a door is opened before the load has cooled, the washed dishes themselves, and the kitchens: 30 kitchens with a dishwasher and 14 without, as a comparison. The paper appeared in PLOS ONE in 2016.
Across the whole study the team cultured 503 fungal strains, belonging to 10 genera and 84 species. Inside the dishwashers alone the count was 97 strains from 29 species.
What they found
The headline result, verbatim from the abstract: “Irrespective of the sampled site, 83% of the dishwashers were positive for fungi.”
The site-by-site ranking is the part a homeowner can act on, because it maps where water sits and never fully dries.
Rubber seal 57%. Seventeen of the 30 machines. Then side nozzles 50% , door 47% , drain 43% , and a steep drop to the interior wall at 23% , cutlery rack 13% , rinse-aid dispenser 10% and sprinkler 7% .
The seal wins for an unglamorous reason. It is flexible, it is folded, it traps a film of water and food residue, and it is the part of the interior that spends the least time at wash temperature.
The organism doing most of the colonizing is one very few people have heard of. Exophiala dermatitidis is a black yeast, and the paper found it in 47% of the machines, concentrated on the seals, at cell densities up to 106 colony-forming units per square centimeter. Black yeasts of the Exophiala group were the dominant contamination on rubber seals at 40%.
What makes it a survivor is a stack of tolerances: heat up to 60 or 80 degrees, wide swings in pH, detergent and surfactants, repeated drying, and the mechanical battering of the water jets. An earlier worldwide survey by the same Ljubljana group, quoted inside this paper, found Exophiala dermatitidis and its close relative Exophiala phaeomuriformis “in 35% of dishwashers examined around the world.”
It does not stay in the machine
Hot aerosol from an opened door was sampled from 29 of the machines. Aspergillus was present in 83% of those samples, Penicillium in 55%, and Trichoderma and Cladosporium in 21% each.
Exophiala dermatitidis itself could not be cultured from a 100 liter air sample. When the team pulled 1,000 liters through a cyclone collector and used species-specific DNA primers, they confirmed it was in the air. The paper describes that as the first report of this organism in air.
Then the comparison that gives the study its title. On kitchen surfaces, Exophiala dermatitidis appeared in 23% of homes with a dishwasher and 7% of homes without one. Kitchens with dishwashers also carried a wider range of fungal species, 53 against 27.
Within those kitchens, the drain was the most contaminated site at 78%, followed by the dish drying rack at 60%.
One honest complication belongs here, because it cuts against the easy reading. Essentially every kitchen in the study was contaminated with fungi, dishwasher or not, and the drain and drying rack were actually higher in the kitchens without a dishwasher, at 86% each. The dishwasher did not create fungal contamination in these kitchens. What it changed was the cast: black yeasts, and Exophiala dermatitidis in particular, in place of the white yeast Candida parapsilosis that dominated the kitchens without one.
The limits, stated plainly
Nobody got sick in this study. It counted organisms on surfaces and in air. It contains no health outcome at all, no symptom questionnaire, and no clinical follow-up. Anything about consequences is inference from other literature, not a finding here.
Exophiala dermatitidis is described in the literature as the third most frequent respiratory fungal pathogen infecting people with cystic fibrosis, and it can cause serious disease. But the people genuinely at risk are a defined group: cystic fibrosis, immune suppression, chemotherapy, transplant. A colonized seal in an otherwise healthy household is a housekeeping finding, not a diagnosis, and it should not be read as one.
Thirty machines, in one country, sampled once, in 2013. Culture-based detection also misses organisms, which is exactly why the black yeast in the air only showed up after the team switched to DNA primers and a ten-times larger air volume.
What survives the caveats is narrower and more useful: the warm wet parts of ordinary household appliances select for fungi that standard indoor mold advice never mentions, and those fungi measurably reach the kitchen around them.
Where it comes from, which changes the fix
The team traced the supply. With one exception, every species making up the core dishwasher community was also detected in the tap water feeding the machines.
That reframes the whole problem. This is not a housekeeping failure or a sign of a dirty kitchen. A dishwasher is a selective incubator sitting at the end of a water line: it receives whatever arrives, then applies heat, detergent and drying cycles that eliminate most organisms and enrich the few that can take it. Scrubbing a seal removes biofilm from a seal. It does not remove the supply, which is why the seal comes back.
One finding does point at a real lever. When the team ran a metagenomic analysis on the dry biofilm inside a 7 year old machine that had been sitting unused for six months, disconnected from the water, the result was a marked reduction in yeast species. Their explanation is straightforward: yeasts are far more sensitive to drying out than sporulating fungi, whose spores can wait out a dry spell and resume growth when conditions improve.
What the data supports
Pull the seal open and look. It is the highest-yield site in the machine at 57%, and it is the one part you can actually reach with a cloth.
Let it dry. Leave the door ajar after the cycle. Drying is the single condition in this paper that measurably reduced the yeasts.
Do not open the door into your face while it is still steaming. Aspergillus was in 83% of those steam samples. Cracking the door and walking away costs nothing.
Check the kitchen drain and the dish drying rack. In these homes the drain was the most contaminated kitchen site at 78% and the rack was second at 60%. Both are plastic, both stay wet, and both are easy to forget.
Keep it in proportion. For most households this is a cleaning detail. For a household that includes someone with cystic fibrosis or a suppressed immune system, it is worth raising with their clinician rather than acting on a blog post.
This study measured fungal colonization of appliances, surfaces and air. It did not measure illness, and it does not establish that dishwashers cause disease in healthy people. Air sampling would not detect a biofilm on a rubber gasket, which is a surface question rather than an airborne one. illumenair measures indoor air quality, moisture and mold in buildings. We do not diagnose illness, we do not treat it, and we do not perform remediation.
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Sources: Zupančič J, Novak Babič M, Zalar P, Gunde-Cimerman N. The Black Yeast Exophiala dermatitidis and Other Selected Opportunistic Human Fungal Pathogens Spread from Dishwashers to Kitchens. PLOS ONE, 2016;11(2):e0148166, doi:10.1371/journal.pone.0148166. Zalar P, Novak M, de Hoog GS, Gunde-Cimerman N. Dishwashers - a man-made ecological niche accommodating human opportunistic fungal pathogens. Fungal Biology, 2011;115(10):997-1007 (figures as quoted within the 2016 paper).