The following presentation takes a look at Legionnaires’ disease in the UK from a health and safety perspective dating back to 1991. It considers Legionella bacteria and where it is commonly found, how Legionella infection occurs, legionellosis, Legionnaires’ disease outbreaks and common sources of contamination including hot and cold water services and cooling towers.
Legionnaires’ disease – A Perspective from 1991
In 1976 a group of ex-service men attended a reunion of the America Legion. 182 of them suffered severe pneumonia, 29 died… the name ‘Legionnaires’ disease’ was coined. There is a milder illness called Pontiac Fever. This and another disease caused by the bacteria of the Legionella family are called legionellosis.
Research has shown how the disease spreads and how it can be controlled.
Water is a basic substance to life; it contains millions of microscopic organisms one of which can be the bacteria Legionella. In factories and hospitals and on the roof tops of hotels and offices we can create the right conditions for Legionella to multiply, from industrial processes, hot water systems and cooling towers.
Where do you find Legionella?
It’s not known exactly how systems can become seeded with Legionella – there is some evidence that the organism is washed into pipe work during building construction. It may also enter in low numbers through the mains water supply.
Airborne droplets or particles containing Legionella may contaminate reservoirs, open tanks and cooling ponds. Once the system is contaminated and given the right temperature range, intermittent use or stagnation. Legionella will multiply and colonise that system. The rate at which is multiplies is determined by water temperature. Between 20 and 45 degree Celsius growth may be rapid, but above 50 degrees the organism will begin to die.
Towers like this are commonly used for industrial cooling systems and air conditioning and have been associated with outbreaks of Legionnaires’ disease. Within a square mile of any city centre there may be hundreds of units like this on roof tops.
There may be a health hazard if any of them are producing spray containing Legionella. This may be carried long distances before becoming dilute enough to become harmless.
How does Legionella infect people?
To contract Legionnaires’ disease one must inhale the organism and it must penetrate deep into the lowest part of the lungs and to do so the organism must be contained in a particle of less than 3,000 of a millimetre or 3 micrometers in diameter.
Now particles of that size can remain suspended in air for prolonged periods of time, and scientifically we have a term for such a suspension which is an aerosol. Aerosols can be generated from water in a variety of ways -simply turning on a tap may generate an aerosol, a shower, and rainfall or in the case of cooling towers the spray and gutter distribution system.
An aerosol produces a range of droplet sizes – larger ones fall to earth fairly rapidly but smaller ones stay airborne and continue to evaporate down to an inhalable size that can cause infection.
What is Legionnaires’ disease?
Legionnaires’ disease is an infection caused by Legionella bacteria. Its main feature is pneumonia so it presents with fever, chills, headaches, aches and pains. A cough soon develops, usually a dry cough first then the patients may get difficulties with breathing and have chest pains and they are usually admitted to hospital towards the end of the first week of illness.
Diagnosis of Legionnaires’ disease relies on laboratory testing to distinguish it from other forms of pneumonia.
Confirming an outbreak takes time for water samples from the suspected source to be cultured. This is a culture after 48 hours, and this after 72 hours.
Outbreaks of Legionnaires’ disease
Most cases of Legionnaires’ disease appear to be sporadic, in other words no link is established with any other cases of Legionnaires’ disease. Most sporadic cases are indigenous, in other words contracted in this country but each year up to a third of cases are associated with travel abroad.
Now each year in this country we detect between 4 and 6 outbreaks of Legionnaires’ disease – the most common source of an outbreak of Legionnaires’ disease is a hot water system in a large building such as a hotel or a hospital.
No cases of Legionnaires’ disease have been attributed to domestic water systems .
Hot Water Services
In large buildings hot water services are one of the main sources of Legionnaires’ disease. Legionella can multiply in various parts of the system, for example in storage tanks – if they are in a warm place like a roof space which is heated by the sun.
If the temperature range is right – Legionella can multiply in calorifiers, the large hot water cylinders, and in the pipes if water stands for long periods when the system is used intermittently. The best way to avoid risk in a hot water system is to store and circulate the water at the right temperature.
Water should be stored at 60 degrees Celsius and reach the taps at 50 degrees – the system should be used frequently so that the water doesn’t stand. The storage tanks and calorifiers of hot water systems should be inspected annually – it is good practise to drain water and other deposits from the bottom of calorifiers frequently to reduce the collection of sediment.
If there is a build up of slime, scum or other deposits they should be drained cleaned and disinfected.
Cooling towers are the second most frequent source of outbreak of Legionnaires disease. A cooling tower is a heat exchange system – warm water enters the tower at the top and is sprayed and splashed onto the packing material.
As the water falls air is blown or drawn upwards through the packing material cooling the water, the air escapes from the top of the cooling tower, taking with it some warm water as aerosols. In order to reduce the escape of aerosols drift eliminators are used. The cooled water is collected in the pond and re-circulated to the air conditioning system or industrial process where it acquires heat. Then it’s returned to the top of the cooling tower.
Although there is a lot of spray here in an induced draft cooling tower, there is little history of these towers causing Legionnaires’ disease. Cooling towers produce aerosols and aerosols must be contained within the cooling tower because the conditions there are ideal for Legionella to multiply – water is at about 30 degrees Celsius, it collects dust from the incoming air, leaves and other materials may settle in the pond providing food for the bacteria and rust and corrosion products may provide more food.
A slimy layer called a biofilm may build up on the surface of the tower which may shelter these organisms. Because of this regular cleaning and disinfection is necessary.
Before scrubbing or using a hose the operator must put on suitable respiratory protective equipment then everything is cleaned to remove sludge, scale, rust, biofilms and debris. Cleaning is done twice a year or more frequently on particularly dusty industrial processes.
On a regular basis chemicals are added either manually or automatically to ensure the continued safe running of the system. Tests are performed to monitor the water quality and levels of chemicals and bacteria.
Approved Code of Practice
Outbreaks of Legionnaires’ disease can affect people at work or the general public so there is great media interest. Several committees of enquiry have been set up by parliament to investigate various aspects of Legionnaires’ disease and an approved code of practice has been published by the Health and Safety Commission [now the Health and Safety Executive] to give advice.
Legionnaires’ disease – A Controllable Risk
Most of the time the risk of contracting Legionnaires’ disease is low but given the right circumstances this can change rapidly to become a high risk. The risk is controllable by following a regular programme of cleaning and maintenance – the threat of Legionnaires’ disease can be contained.
Legionnaires’ disease (1991) – The Health & Safety Commission
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