When I first started carrying out Legionella Risk Assessments a decade ago, when I arrived at a site and introduced myself, I was regularly met with a puzzled look from the receptionist.

'Legionella, whats that??'

These days, most people I meet on site are more aware of Legionella but there are still lots of people unaware of the disease and the danger it poses.

What is Legionnaires Disease?

Legionnaires disease is a type of pneumonia which is caught from inhaling droplets of water, or vapour, generally from a water system which has been contaminated with Legionella bacteria. The symptoms of the disease begin very similar to flu, but can develop into much more serious states. Around 300 cases of the disease are reported in the UK every year and around 10% are fatal.

However, as the symptoms are similar to flu, most cases are never diagnosed as Legionnaires Disease. It is believed that the actual figures could be 10-20x higher than those reported, meaning the disease could be far more common.

Most healthy people will fight off the disease before it gets very serious, but people aged over 40, or anyone with a reduced immune system or respiritory disease are far more at risk of the serious symptoms.

How do you catch Legionnaires Disease?

The disease is caught by breathing in contaminated vapour. It cannot be passed from person to person, and cannot be caught from drinking contaminated water. 

The vapour can be produced from a large range of sources, basically anytime the surface of water is broken, vapour is produced. So taps and showers are a risk, but anything that produces large amounts of vapour produce a much greater risk, such as pressure washers, water fountains and especially, cooling towers.

How does water get contaminated with Legionella bacteria?

Nearly all natural water sources contain low concentrations of the bacteria, but the danger is when higher concentrations build up. The bacteria will multiply when it is given the right conditions, which are a favourable temperature range, a food source and stagnation.

The favourable temperature range is around 25-45° Celsius, basically luke warm water. Food sources can be scale, sediment, corrosion or slime, all things found in a dirty water system. Stagnation is commonly a problem in water systems that are not used frequently, and can occur in storage tanks and water heaters and even within pipework.

All these conditions are frequently found in building water systems, and even the best designed systems can breed the bacteria if they are not looked after properly.