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ABOUT LEGIONNAIRES' DISEASE

What is Legionnaires’ disease? Current curative treatments are ineffective
It is a serious respiratory infection caused by a bacterium: - Chemical shocks: a chlorine injection 50 mg/L for
Legionella. The illness which follows, Legionnaires' disease, 1 hour (HSE guidance); ≥ 10 mg/L for 8 hours (WHO).
is fatal in 15 20% of cases. - Thermal shocks: circulating hot water at 60°C
for 60 minutes (in Europe: 70°C for 30 minutes).
Legionella is a system bacterium which develops in warm water in
pipework or cisterns. It only becomes dangerous when inhaled when • A short term effect:
it may spread into the lungs and rapidly develop in white blood cells. The system can be very quickly recolonised within 3 4 weeks.
The inside of the pipework is covered by a biofilm: a sort of gel
Legionnaires' disease is more likely to affect adults, especially older which bacteria stick to, feed on and proliferate in.
smokers and people with respiratory problems or weak immune systems. The biofilm protects them and enables them to withstand
temperature variations (thermal shocks) and biocide products
Legionnaires' disease is treated with antibiotics. (chemical shocks).
The World Health Organization (WHO) states that the alert level
for Legionella starts at 1,000 Colony Forming Units (CFU) per litre During curative treatments, the risk is not only that the bacteria will
of water (250 for hospitals). The UK alert level is 100 CFU/L. get used to the treatment and thus develop their resistance to it,
but above all will provoke detachment of bacteria aggregates which
Which installations are at risk? will be deposited closer to the water outlet and thus, to the user.
Legionellae develop in stagnant water between 25 45°C, with an
optimum temperature between 30 37°C. Humid areas with scale, • Negative effect in distribution networks and drainage systems
mud or which have been affected by corrosion are more prone They are not necessarily designed to withstand temperatures
to Legionella development. above 60°C or significant quantities of chlorine. They can cause
corrosion or premature ageing of the distribution and drainage
Environments at risk include hot water systems, cooling towers, system.
health resorts, water fountains, shower rooms, spas, etc.
• High risk for the user
During the treatment, the user is exposed to high risks:
Contamination source
- 3 degree burns in the case of a thermal shock.
unknown rd
- Poisoning in the case of chemical shock treatment.
61%
Buildings open
to the public 9% • Costly and difficult operation to undertake
(swimming pools, stadiums, etc.) Curative treatments are difficult to undertake fully at all points
4%
6% 1% 6% in the system (various deposits, dead-legs, heat loss, etc.).
9% 4%
They require the water supply to be out-of-service and the
mobilisation of maintenance staff.
Other
trips Hospitals
Care homes
Temporary for the elderly To avoid these treatments it is important to act on four levels:
residences Hotels, Thermal
(rented accommodation, B&Bs, etc.) - Avoid dead-legs and water stagnation.
campsites spas
- Regularly maintain the system to prevent corrosion or scaling.
Source: Public Health France 2022
- Produce and store hot water at a high temperature
that will prohibit bacterial proliferation.
Development of Legionella per temperature
- Maintain the temperature in the system with a circulating loop
and mix the water as close as possible to the point-of-use.
Temperature Development of Legionella
< 20°C Dormant Capacity and length of pipework
25 45°C Multiplies
Pipe length for a capacity of
Material Pipe dimension
50°C 90% die within 2 hours 3 litres 1 litre
15 1 22m 7m
60°C 90% die within 2 minutes
Copper 18 1 15m 5m
22 1 9m 3m
Source: CSTC Belgium November 2002
DN 15 17m 5m
Galvanised steel
DN 20 10m 3m
15 2.5 38m 12m
PEX/PER plastic
18 2.5 22m 7m
20 1.9 14m 4m
PR plastic
25 1.9 8m 2m

144 Thermostatic mixing valves