Poorly-maintained whirlpool baths can be a breeding ground for potentially dangerous bacteria, research suggests.
All samples contained bacterial growth, the study found
A team from Texas A&M University tested 43 water samples from private and public whirlpool baths, and found all had some kind of microbial growth.
Bacteria derived from faeces were present in 95% of samples, while 81% had fungi and 34% contained potentially deadly staphylococcus bacteria.
Details are published in an online journal called PM Engineer.
Lead researcher Dr Rita Moyes said: "Whirlpool baths are almost always a prime area for potentially harmful microbes.
"The main reason is the lining of the pipes. They are full of inaccessible air, and water in these pipes tends to get trapped, often for long periods of time.
"When the jets are then switched on, this water with harmful bacteria gets blown into the tub where a person is soaking and then trouble can start."
Dr Moyes said a teaspoon of normal tap water contains about 138 bacteria, with many samples not having any bacteria at all.
A teaspoon of whirlpool tub water, on the other hand, contained an average of 2.17m bacteria.
Dr Moyes said such harmful bacteria could lead to numerous diseases, among them urinary tract infections, septicaemia, pneumonia and several types of skin infections
She said the aerosol mist created by the whirlpool action could force microbes into the lungs, or open cuts.
Dr Moyes says that as long ago as 1972, studies were done to test the bacteria levels in whirlpool baths and hot tubs, but evidence collected has often not shown sufficient reasons for concern.
"That's probably because a hot tub or whirlpool as a source of infection can't be clearly distinguished from other sources," she said.
She said this was similar to how a doctor might be able to tell you that you have a respiratory infection, but not how you got it.
Dr Moyes added: "The best way to prevent such bacteria from forming is to clean out the pipes.
"The pipes in a whirlpool bathtub need to be scraped and cleaned just like you need to brush your teeth with toothpaste.
"We also need to explore effective ways to prevent the growth of bacteria in whirlpool bathtubs through new cleaning methods and new technology in tub design."
Dr John Lee, of the Health Protection Agency, said people fitting whirlpool baths into their own homes should be aware of the need for regular cleaning.
"If you don't clean them regularly you often see discoloured or black bits coming out of the pipes, which is certainly a warning sign," he said.
He said most commercial and healthcare operators did clean their products on a regular basis.
Allen Brobyn, of the British and Irish Spa and Hot Tub Association, stressed the distinction between whirlpool baths, which are emptied every time they are used and not routinely sanitised, and hot tubs, where the water remains in the tub for up to three months and is treated to ensure safety.