Public hot tubs on cruise ships might gross some people out, but paying for a private spa may not actually reduce exposure to dangerous, water-dwelling pathogens.

A new report from the US Centers for Disease Control and Prevention (CDC) warns that hot tubs on the balconies of cruise ship rooms may be even riskier when it comes to disease spread – not just for those using the hot tub, but for anyone breathing in the steam.

According to a recent inquiry, private hot tubs on cruise ships are the "likely source" for two recent US outbreaks of Legionnaires' disease – a serious lung infection, spread via steam, that can cause a life-threatening form of pneumonia, especially among the elderly and those who are immunocompromised.

Between 2022 and 2024, the CDC tallied 12 cases of Legionnaires' disease on just two cruise ships. Ten of these cases were so serious, they resulted in hospitalization. One cruise ship alone was responsible for eight cases of the disease – the largest outbreak the CDC has investigated on a single ship since 2008.

For decades now, scientists have connected hot tubs on cruise ships to outbreaks of Legionnaires' disease. The temperatures of water are just right for Legionella bacteria to grow, which means if the tubs are not drained between use, cleaned, and hyperchlorinated, there is every chance a pathogen could take over.

Unlike public hot tubs on cruise ships, however, current CDC guidelines do not require private hot tubs to be cleaned or operated with the same stringency.

These newest outbreaks of Legionnaires' disease show why that is a mistake.

Because private hot tubs are often located on balconies, the vapor and splashes from these heated pools could easily slip into the lungs of unsuspecting neighbors, including those above and below, carrying pathogens with them.

The first outbreak was recorded in December of 2022, when five people who had boarded a cruise ship in November fell ill with pneumonia. Upon sampling the ship's cabins, heat exchangers, potable water tanks, decorative fountains, and public hot tubs, however, officials found no trace of Legionella bacteria.

The next year, in August, two additional cases of Legionnaires' disease were reported among guests on the same ship, this time among travelers who had their own private hot tub.

Only then did officials at the CDC request all 10 private balcony hot tubs on the ship be closed and sampled. Six of the tubs were home to Legionella bacteria, and four had very high concentrations.

"The hot tubs remained closed until their operation and maintenance protocols were modified and non-detectable Legionella sampling results were obtained," officials at the CDC write.

Even when sampling showed no bacteria, however, one more guest on the cruise ship fell ill with Legionnaires' disease in March after spending time at sea.

Flash forward to 2024, and four more confirmed cases were reported to the CDC from a different ship.

This time, the CDC requested that all hot tubs on the cruise be immediately closed and sampled, private and public alike. Legionella pneumophila species were detected in all eight private balcony hot tubs aboard.

Luckily, no one died in either of these outbreaks, but if officials at the CDC are right, and private hot tubs were the source of infection, then current government guidelines are insufficient to prevent future disease, and need to be updated.

"Adapting public hot tub maintenance and operations protocols for use on private outdoor hot tubs can reduce the risk for Legionella growth and transmission," concludes the official report.

The CDC report can be found here.