If sanitizers are designed to prevent disease, why do outbreaks still occur in professionally maintained pools?
The primary purpose of chlorine — and every sanitizer used in pools and spas — is simple: To prevent disease. That is the foundation of recreational water treatment. And yet, even in systems that are regulated, monitored, and professionally maintained, outbreaks continue to occur every year.
In many cases, the organisms involved are not especially difficult to kill.
Under typical pool conditions — approximately 1 ppm free chlorine at pH 7.5 and moderate temperature — disinfection is rapid for most pathogens:
• Many bacteria (such as E. coli) are inactivated in less than 1 minute.
• Giardia typically requires approximately 30-45 minutes.
• Hepatitis A virus may persist for 15-20 minutes.
These are manageable timeframes in a properly maintained system, and improvements in pool codes and water treatment have reduced the prevalence of some pathogens over time.
For example, Giardia was once a leading cause of waterborne illness, peaking in the 1980s at 21.7 cases per 100,000 people. In 2022, the CDC reported 13,835 cases — about 4.2 per 100,000 — reflecting a significant decline. While this includes all transmission pathways, swimming pools and waterparks continue to be identified as settings associated with Giardia outbreaks.
Today, Legionnaires’ Disease remains a serious risk in recreational water, particularly in hot tubs, where aerosolized water can transmit bacteria. In January 2025, a fatal case in South Carolina was linked to exposure to a contaminated hot tub. Later that year, two cruise passengers were diagnosed with Legionnaires’ Disease following travel, with investigators noting that exposure can occur through aerosolized water from hot tubs and similar features.
Health investigations have also identified growing concerns for Legionnaires’ Disease in vacation rental properties, where private hot tubs and pools may operate outside traditional regulatory oversight. In Minnesota in 2025, health authorities cited violations involving short-term rental properties with improperly maintained pools and hot tubs.
Then there is Cryptosporidium, which, unlike most organisms, is highly chlorine-tolerant. At standard pool levels (1 ppm free chlorine), it can survive for days — in some cases more than a week.
That difference is why Cryptosporidium remains the most persistent problem in recreational water.
In late 2025 into early 2026, a multifacility outbreak in Georgia linked to a water park system resulted in more than 100 confirmed and probable cases. The outbreak spread beyond the original facility to include a community pool, apartment complex pool, and splash pad, requiring repeated closures as transmission continued.
In Yellowstone County, Montana (summer 2024), a pool-associated outbreak resulted in more than 70 confirmed and probable cases, with public health officials noting the organism’s resistance to chlorine as a contributing factor.
These outbreaks occurred in operating, regulated facilities. Chlorine was present. Systems were functioning. And yet transmission still occurred — not because sanitation was absent, but because the system was not sufficient to address that particular organism.
A broader perspective comes from the CDC, which in a 2024 review of outbreaks from 1997 through 2022 documented 60 splash-pad-associated outbreaks resulting in 10,611 illnesses. These involved multiple enteric pathogens, including Giardia, Shigella, E. coli, and norovirus. These environments present ongoing challenges for maintaining consistent disinfection.
The issue is not simply whether chlorine is present, but whether it is consistently maintained and effectively distributed throughout the system. Pathogens can flourish in biofilms, which form where disinfectant is low or intermittent — such as inside plumbing or low-flow zones. Once established, biofilms can protect bacteria from normal sanitizer levels.
Maintaining an adequate residual is critical, but consistency over time is equally important. Circulation, turnover, temperature control, and system design all determine whether disinfectant actually reaches the areas where pathogens can grow.
Chlorine is extremely effective — when it is maintained continuously, at appropriate levels, and allowed sufficient contact time.
With the notable exception of Cryptosporidium — which can survive standard chlorine levels — most of these organisms are controllable under properly maintained conditions. When they persist or spread, it reflects a breakdown in maintaining those conditions over time.
