The suspension of very small liquid or solid particles dispersed in air or gas.

Once aerosols are suspended in the atmosphere it is possible that the may be dispersed over an area of several square kilometres.

A microscopic life form consisting of single celled organisms. Although the human body is naturally host to many species of symbiotic bacteria (over 1000 species found in a healthy gut alone), many others are the cause of infections and disease including Legionellosis.

The word Bacteria is used to indicate plural whereas a bacterium indicates a single unit.

An aggregation of bacteria and other microorganisms to form a mat or colony adhering to a surface or each other.

Once protected within a biofilm, bacteria are much harder to eradicate than individual isolated bacteria.

A vessel used for the transfer of heat to water from a separate primary source.

Commonly applied name to the agent used in the chemical disinfection of water systems.

However, to refer to the agent as Chlorine is a misnomer as the agents most commonly employed are Calcium hypochlorite (powder or tablet form) or more usually Sodium hypochlorite (liquid form).

Dead Leg/Dead End:
An isolated or blanked off section of pipework spurring off a water supply system. Deadlegs are usually the result of redundant outlets being isolated or improperly removed from a system.

Such items create areas of stagnant water, allowing nutrients to collect and bacteria to proliferate. Redundant pipework should always be completely removed and cut back to the main supply branch including the supply tee.

A means of preventing the presence or proliferation of pathogenic organisms such as bacteria in a water system. Many different disinfection techniques are available to keep systems under control.

They may take the form of long term management strategies utilising temperature control or continuous low level dosing of chemical agents such as Chlorine Dioxide or using Copper-Silver Ionisation technology.

Emergency or shock disinfections can utilise chemical agents such Sodium Hypochlorite to quickly neutralise organisms in a system.

A system undergoing shock disinfection will need to be withdrawn from use whist the process takes place.

Escherichia coli is a bacteria commonly found in the lower intestine of most warm blooded animals.

There are many strains of E coli, some are beneficial to its host helping to produce important vitamins.

Other strains, most notably one known as ‘E coli 0157’, can cause severe health complications such as urinary tract infection, sepsis/meningitis, and enteric/diarrheal disease.

These problems are especially dangerous for infants; the elderly and those with a suppressed immune system.

Outbreaks are usually associated with cases of food poisoning, especially under-cooked meat although it is also possible to come become infected through drinking contaminated water.

Flow and Return:
A circulation circuit found on most large domestic hot water systems. Hot water is taken from the top of a Calorifier/water heater and constantly circulated around a building and then returned back to source via a circulation pump.

Outlets or draw off points are taken off the circuit therefore reducing hot water delivery time to the outlet. Single run pipework spurred off the circulation circuit should be kept as short as possible.

Stored hot water and flow and return systems should be monitored monthly to ensure that delivery temperatures are within the correct parameters (stored temperature >60°C, return temperature >50°C).

Legionella Pneumophila:
One of the causative bacteria of Legionnaires’ disease, responsible for 90% of recorded infections.

The incidence of infection of those exposed (morbidity rate) to Legionella pneumophila is typically 5%, although this is greatly dependent upon the susceptibility of the exposed population.

Legionella Risk Assessment:
A detailed survey of water systems identified as potential sources of Legionellosis.

The assessment identifies potential areas concern and recommends remedial actions to remove, reduce or manage the risk. The survey must be carried out by competent and experienced assessors.

Under HSE guidelines the risk assessment must be reviewed every two years or following any major modifications to the system.

Any illness caused by exposure to Legionella bacteria.

Legionnaires Disease:
A severe form of pneumonia caused by Legionella bacteria. The incubation period is between 2-10 days (usually 3-6 days).

Not everyone exposed will develop symptoms of the disease and those that do not develop the severe disease may only present with a mild flu-like infection.

Lochgoilhead Fever:
A non pneumonic disease caused by the species of bacterium ‘Legionella micdadei’. The resulting infection is a non fatal upper respiratory illness from which the patient will recover without medical intervention.

The incidence of infection of those exposed (morbidity rate) to Legionella micdadei is typically 90%, much higher than other strains if the bacteria.

A unicellular or multicellular organism including bacteria, fungi, algae and viruses not visible to the naked eye.

Food source for bacteria and other micro-organisms. Nutrients found in a typical man made water system include natural rubber gaskets; hemp seals; silt; rust and external contaminants found in poorly maintained or sealed storage tanks.

The disinfection of a system by raising the temperature to 70°C throughout the whole of the system for an appropriate length of time.

Capable of causing disease.

‘Pseudomonas aeruginosa’ is a commonly occurring bacterium found in soil and ground water and therefore can be found in many man-made environments.

It can cause severe illness in susceptible populations, however it is most commonly associated with a condition known as ‘Hot tub rash’.

Hot tub rash is an infection of the hair follicles resulting in an itching rash, the rash will usually clear up without the need for treatment but more persistent infections can be treated with antibiotics.

Pontiac Fever:
A non pneumonic upper respiratory illness less severe than Legionnaires’ disease caused a species of Legionella bacteria.

Incubation time is much shorter at 12 to 48 hours, and the patient will recover from the resulting infection without medical intervention.

The incidence of infection of those exposed (morbidity rate) is typically 70 to 90%.

Parts per million.

Sentinel Outlet:
In a hot water circulating system the sentinel outlets are identified as the first and last outlets on the circuit.

In a cold water system or a non-circulatory hot system the sentinel outlets are those that are nearest and farthest from the storage tank or vessel.

As part of a monitoring programme these outlets are checked every month to monitor the correct operation of the system.

Other outlets identified as posing additional risks may also be included as sentinels.

Serratia Marcescens:
A bacterium responsible for the pink or orange discolouration often found around hand basin/bath or shower wastes and shower cubicles, curtains and grout.

It is responsible for a small percentage of nosocomial (hospital or healthcare acquired) infections, although studies have shown that many cases have their onset in the community.

Serratia marcescens can cause a wide range of respiratory, urinary and eye infections.

A mud-like layer found at the bottom of storage vessels (especially calorifiers) and unused pipe runs.

These deposits form the ideal environment for micro-organisms to proliferate.

Thermostatic Mixing Valve (TMV):
A mechanical device that when fitted to the hot and cold water supply of a hand basin, bath or shower, blends the hot water to a safe, usable temperature.

The TMV will supply water at a constant preset temperature (usually 43°C although this is adjustable) below that of the incoming hot supply (>50°C). A failsafe mechanism within a TMV ensures that in the event of a failure of the cold water supply the valve will automatically shut off the hot supply and so prevent scalding.

As the temperature of the water delivered by the TMV falls within the ideal range for proliferation of Legionella, the TMV should be sited as close to the outlet as possible, no greater than 2 metres, and should not serve excessive multiple outlets.

TMVs’ should be positioned where they are accessible for maintenance, and should be serviced, calibrated and tested for correct fail-safe operation at least annually.

Total Viable Count (TVC):
A count of bacteria cultured from a water sample, the count is nonspecific and does not identify the type of bacteria found.

TVC is useful for monitoring the general health of a system with high levels indicating that any control measures in place may not be effective. (Also known as Total Aerobic Count, TAC).

Written Scheme:
A document detailing the specific measures taken to control or reduce the risk posed by Legionellosis from a system.

The written scheme will explain the correct conditions required to ensure the safe operation of the system.

It will also detail when and how checks are carried out to monitor the efficacy of the controls in place and also identify the remedial actions required should the systems control measures fail.