Hot tub rash gets the attention, but it isn’t the only germ problem tied to spas. Legionella causes a serious pneumonia, and hot tub mist is one way it reaches people.
This article covers what CDC says about the risk and what a 2024 outbreak report showed. It sits under our overview of hot tub benefits. It is general information, not medical advice.
Short answer: Legionnaires’ disease is a pneumonia caused by Legionella bacteria. CDC says the most common way to catch it is breathing in mist that contains them, and hot tubs make mist. Most healthy people exposed don’t get sick, but people over 50, smokers, and people with certain conditions are at higher risk. The defense is steady disinfection and maintenance.
What it is and how it spreads
CDC: “Legionnaires disease is a serious pneumonia caused by Legionella bacteria.” It is treatable with antibiotics. Symptoms usually appear 2 to 14 days after exposure.
On spread, CDC says Legionella “most commonly spreads through devices that use water and produce mist,” and lists hot tubs among the building water systems that can spread it. It says people generally don’t pass it to each other.
Who is at higher risk
CDC says most healthy people exposed don’t get sick. It lists higher risk for:
- People 50 or older
- Current or former smokers
- People with cancer, chronic lung disease, diabetes, kidney failure, or liver failure
- People with a weak immune system, including from medicines such as chemotherapy or drugs taken after a transplant
What a cruise ship outbreak showed
A CDC report in October 2024 traced two outbreaks, 12 cases from November 2022 to July 2024, to private balcony hot tubs on two cruise ships. Ten of the 12 people were hospitalized; all recovered.
On the first ship, Legionella was found in 6 of 10 private tubs tested. Those tubs held water at 77 to 113°F for days without draining and with no residual disinfectant. On the second ship, it was found in all 8 private tubs.
Here’s what the spec sheet won’t tell you. Those private tubs met their rules, which required only weekly shock chlorination and draining, or draining between guests. Public tubs must have continuous disinfectant dosing and pH monitoring. CDC’s authors concluded the lighter rules weren’t enough and recommended public-tub protocols for private outdoor tubs.
The report is about cruise ships, not home tubs. The lesson it draws is about water left warm without disinfectant.
What CDC tells owners
CDC’s guidance for hot tub owners:
- Follow the manufacturer’s instructions for cleaning, disinfecting, and maintenance.
- Check disinfectant and chemical levels regularly.
- Learn how to test the water before you use a hot tub.
Its guidance for hot tubs at events sets free chlorine at 3 to 10 ppm or bromine at 4 to 8 ppm, held continuously, and pH at 7.0 to 7.8. Test strips like the AquaChek 7-way read all three. Reading Water Test Strips explains each pad.
A related problem: hot tub lung
A 2006 case series from a U.S. tertiary medical center described 21 patients with “hot tub lung,” a lung reaction linked to Mycobacterium avium complex in hot tub water. All had ongoing hot tub exposure, and all had shortness of breath and cough. All improved after the exposure stopped. The authors concluded it likely represents hypersensitivity pneumonitis, an inflammatory lung reaction to the inhaled organisms.
Bottom line: Legionnaires’ disease is serious, though CDC says most healthy people exposed don’t get sick. Keep disinfectant and pH in range and follow the maker’s maintenance schedule. For the wider health picture, see the hot tub benefits overview linked above.
Sources
- CDC, Legionnaires’ disease overview page, August 6, 2025: cdc.gov
- CDC, “How Legionella Spreads,” June 9, 2025: cdc.gov
- CDC, Legionella control overview page, March 15, 2024: cdc.gov
- CDC, guidance for hot tub displays at temporary events, March 18, 2024: cdc.gov
- Lee S, et al., “Two Outbreaks of Legionnaires Disease Associated with Outdoor Hot Tubs for Private Use: Two Cruise Ships, November 2022–July 2024,” MMWR, October 24, 2024: cdc.gov
- Hanak V, et al., “Hot tub lung: presenting features and clinical course of 21 patients,” Respiratory Medicine, 2006 (abstract): pubmed.ncbi.nlm.nih.gov
- AquaChek 7-way test strips listing on Amazon, checked October 19, 2026