What Grows in a Sauna (and Why the Bathers Bring It In)

Sauna microbial hazards come mainly from the people using them — skin, sweat, and organic matter shed onto benches and floors. What the science says lives on sauna surfaces, and why lower benches test dirtiest.

The most useful single source on sauna hygiene is a January 2026 rapid review by Canada’s National Collaborating Centre for Environmental Health (NCCEH), Rapid Review: Health Risks in Saunas. Its central finding is uncomfortable for the whole gym industry:

The contamination source is the bathers. Sauna microbial hazards come mainly from shedding of skin, fecal and non-fecal organic matter, and bodily fluids by the people using the room. E. coli has been found in sauna samples. In Finnish swimming-facility studies cited by the review, the highest total aerobic microbial counts were on the lower sauna benches — exactly where sweat and shed material drip and pool.

What has been found on sauna and wet-room surfaces

According to the NCCEH review and the dermatology trade literature it draws on:

• Viruses that transmit on shared surfaces, including molluscum contagiosum virus and papillomavirus (the plantar wart virus).
• Fungi such as Epidermophyton floccosum and Trichophyton species — the athlete’s foot and jock itch organisms.
• Pseudomonas aeruginosa, which cross-contaminates between pools, hot tubs, and wet rooms.
• Staphylococcus aureus, including MRSA, which can survive on sauna surfaces even at elevated temperatures. The review describes a 17-case MRSA outbreak associated with a shared sauna — covered in detail in Sweat, Benches, and the Under-Bench Problem.
• Mold. The species found in the Daytona founding case — Chaetomium, Aspergillus, and Nigrospora — are all recognized indoor, dampness-associated molds.

Dry sauna, steam room, infrared: the risk is not equal

An honest distinction, because credibility matters here:

• Dry Finnish-style saunas (high heat, low humidity) are the least mold-friendly of the wet-heat rooms. Heat alone is not the hazard.
• Steam rooms run around 110–116°F at roughly 100% humidity — permanently wet, and far more hospitable to growth. In an Iranian study of pool-facility wet areas cited by the NCCEH review, fungal contamination was found in about 97% of samples — mostly harmless species, which is precisely the point: the risk concentrates in persistently wet, nutrient-loaded, under-cleaned spaces, not in heat itself.
• Infrared saunas run cooler and are generally lower mold risk.

The pattern across all of it: moisture plus a food supply (sweat, skin, organic matter) plus no effective cleaning equals growth. Remove any one of the three and the picture changes — which is why the next article is about the cleaning that, in sealed-bench saunas, is physically impossible.

Sources

• NCCEH, Rapid Review: Health Risks in Saunas (Jan 2026) — PDF
• Summary coverage: Biology Insights — Are Saunas Dirty?
• Dermascope (dermatology trade): Steam and Sauna Bath Therapies
• CDC/NIOSH on mold health effects: cdc.gov/niosh/mold