The main threat is from legionnaires’ disease, which causes serious breathing difficulties. There have been five reported outbreaks linked to spa pools in the past five years, leading to four deaths. The flu-like Pontiac fever is also a risk.

The HPA has produced a 124-page safety guide for the pools, but its findings make grim reading. Other possible hazards include the presence of the amoeba Naegleria fowleri, which, in rare cases, can cause a fatal form of meningitis. Acanthamoeba species can also thrive, which can trigger keratitis, a severe eye infection.

The herpes virus will also survive in spa baths without regular disinfecting, as will Cryptosporidium, a waterborne disease that can lead to heavy bouts of sickness and stomach pain.

The main message to pool users is to keep the hair above water and to avoid swallowing water. Spa pools are easily infected because people tend to crowd into a small area, ensuring a high concentration of organic matter — mucus from the nose, saliva from the mouth, perspiration, dead skin, residues of suntan lotion, cosmetics, shampoo or soap, hair, as well as urine and faecal matter — in the water.

The HPA report is available free on its website: www.hpa.org.uk.

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