Hot tubs and spas may look like simple vessels of warm, relaxing water, but in practice they function as complex miniature ecosystems. Along with heat and agitation, they routinely contain sweat, skin cells, oils, cosmetics, hair, and dirt. When those organics combine with warm temperatures, low-flow zones, and intermittent sanitation, conditions become ideal for biofilm — a structured community of microorganisms that attach to plumbing surfaces, surround themselves in protective slime, and in some cases make people sick.
In hot tubs, biofilm does not float in the water where it can be easily tested or filtered. It forms inside pipes, jets, filters, pumps, and other hidden surfaces. Biofilm consists of bacteria and other microorganisms embedded in a self-produced matrix of sugars and proteins. Once established, it adheres tightly to surfaces, traps nutrients, and shields microbes from disinfectants. What service professionals often see as “sludge” or “gunk” is usually just the visible edge of a much larger and more resilient microbial structure.
For the industry, the risks tend to appear in three overlapping categories: Skin and ear infections most commonly associated with Pseudomonas aeruginosa; respiratory illness caused by Legionella bacteria, including Legionnaires’ Disease and Pontiac fever; and less common but serious infections involving nontuberculous mycobacteria (NTM) and free-living amoebas. Over the past two decades, public-health investigations have repeatedly identified poorly maintained hot tubs — including display models and short-term rental spas — as recurring sources of disease.
Pseudomonas aeruginosa is familiar to most service professionals as the cause of hot tub rash and swimmer’s ear. Infections typically occur as folliculitis, nodular rashes, ear infections, or eye irritation within 24-48 hours of exposure. While many cases resolve within one to two weeks, infections can persist or worsen in immunocompromised individuals.
Outbreaks continue to occur. In March 2023, 23 people developed ear pain, rashes, and nodular skin lesions after using a hotel pool and spa in Maine. A CDC Morbidity and Mortality Weekly Report linked the outbreak to equipment failures and inadequate disinfectant control, including a missing disinfectant feeder. Investigators noted that even when disinfectant levels appear adequate, “the extracellular matrix of the biofilm can protect P. aeruginosa and other pathogens from disinfectants.”
Legionella pneumophila presents a far more serious risk. Transmitted through inhalation of contaminated aerosols, it can cause Legionnaires’ Disease — a severe pneumonia with a typical incubation period of two to 14 days — or the milder Pontiac fever. Hospitalization is common, and fatalities occur, particularly among older adults, smokers, and people with chronic lung disease or weakened immune systems.
Hot tubs are a well-documented source of Legionella exposure. The CDC has repeatedly stated that hot tubs can support Legionella growth when inadequately maintained. Recent investigations linked cases to private hot tubs on cruise ship balconies and to display spas, prompting updated CDC guidance between 2022 and 2024 with direct relevance for residential and shortterm rental hot tubs.
NTM represent a slower moving but stubborn threat to spa users. Species such as Mycobacterium avium complex and Mycobacterium abscessus are highly adapted to life inside biofilms and have been associated with skin and lung infections after hot tub exposure. Symptoms may not appear for weeks or months, and treatment often requires long courses of antibiotics.
Free-living amoebas add another layer of complexity. While Naegleria fowleri (brain-eating amoeba) is primarily associated with untreated water, other amoebas commonly occur in spa biofilms. These organisms can harbor bacteria such as Legionella, effectively protecting them and contributing to rapid rebound after disinfection failures.
Biofilm Matters
The CDC defines biofilm as “a primarily polysaccharide matrix produced by microbial cells and in which bacteria are embedded.”
This matrix anchors organisms to surfaces, traps organic debris, and provides shelter from disinfectants. Protozoa within biofilms may ingest bacteria and later release them in more disinfectant-resistant forms. Once established, biofilm continuously sheds microorganisms into the water — even when test results look acceptable.
In practical terms, a spa can test “clean” while mature biofilm behind the jets quietly repopulates the system between shocks.
Chlorine and Biofilm
Chlorine is highly effective against free-floating organisms, but its ability to penetrate established biofilm is limited. Studies show chlorine reacts quickly with the outer layers of biofilm, neutralizing surface organisms while leaving deeper bacteria viable.
A frequently cited 1994 study concluded that limited chlorine penetration is a key reason for reduced efficacy against biofilms.
Alternative disinfectants (such as monochloramine or chlorine dioxide) used in municipal systems can penetrate biofilms more effectively, but taste, odor, regulatory, and byproduct concerns make them impractical for consumer hot tubs. In the field, the operational reality remains unchanged: Physical removal of biofilms paired with validated chemical treatment.
Controlling Biofilm
CDC investigations and toolkits are consistent. Effective biofilm control requires both mechanical and chemical action: Removing the tub from service when needed; hyperchlorinating; draining; scrubbing all accessible surfaces; cleaning or replacing filters; flushing plumbing; refilling; and closely monitoring sanitizer residuals. Disinfectants alone cannot reliably eliminate established biofilm. Are Biofilm Removers Regulated?
Many products claim to “eliminate” biofilm. Most rely on enzymes or surfactants that may loosen debris but do not kill embedded pathogens. In the U.S., products making antimicrobial claims generally require EPA registration.
Hot Tub Serum Total Maintenance (EPA Reg. No. 84409-2) includes label claims related to non-publichealth biofilms. Professionals should always verify EPA registration and follow label directions exactly.
Biofilm makes hot tubs fundamentally different from pools. Routine testing and shock treatments are necessary — but not sufficient once biofilm is established.
Effective control depends on consistent mechanical cleaning, validated chemistries, and disciplined operations.
Outbreaks persist not because the science is unclear, but because maintenance drifts. The professional’s role is to stop that drift before microorganisms thrive.
