4 August 2012 Last updated at 00:10 GMT

How hot tubs can harbour Legionnaires' bacteria

Hot tubs are known to be effective mechanisms for spreading legionella infection, an official at the Health Protection Agency said this week.

That statement followed the death of one person from Legionnaires' disease and 18 further cases in Stoke-on-Trent since 24 July.

A hot tub displayed in a store in the town is thought to be the "probable" source after samples from it were found to match those taken from the patients.

Hot tubs or spa pools are popular in gyms, hotels and increasingly in people's back gardens - but experts say they can be a health risk if they are not looked after.

The water in hot tubs is kept at between 30 and 35 degrees, close to body temperature, which is the ideal environment for legionella bacteria to grow.

The bubbling and frothing of the aerosols in a hot tub can then throw the bacteria into the air for several yards around the tub.

So you don't have to be sitting in the hot tub to inhale the bacteria.

Fine spray

Professor Nick Phin, head of Legionnaires' Disease at the Health Protection Agency, says an outbreak of the disease in the Netherlands in 1999 illustrates how far the spray can travel.

"Over 140 people were affected at a Dutch flower show by two spa baths being exhibited. People were just passing through and breathing in the fine spray."

But not everyone who walked past the hot tubs was infected by the legionella bacterium, he says.

"You have to be susceptible to it. There was a very low attack rate of 0.2% in the Holland example."

People aged over 50 are more likely to be affected, as are people with existing health problems, particularly lung damage, and those who smoke.

"If your immune system is OK then you don't tend to get it," says Prof Phin.

Cleaning and maintaining the hot tub regularly is the only way to ensure that bacteria do not concentrate in the water and the spray, he explains.

As a result, the HPA has produced guidance on how to clean hot tubs and spa pools with chemicals which disinfect the water.

In general, cases of Legionnaires' disease have been increasing in number over the past 30 years. In the last 10 years, HPA figures show that cases have averaged 350 each year.

There is no knowing how many of these are due to hot tubs, because of the difficulty of finding the exact source of an outbreak.

Around half of cases are linked to overseas travel.

But there are some other potential problems with hot tubs if they are not maintained properly.

The amount of skin that is shed in hot tubs can lead to cases of Pseudomonal infection and some people have also had boils appear on their bodies.

"If spa pools are not looked after properly then they can cause other diseases," Prof Phin says.

Local authorities will carry out inspections and private owners should make sure they use the appropriate chemicals to clean tubs and disinfect the water too.