Biological Pollution

Biological pollution is any pollution that is a living organism. Examples below:

  • Bacteria

o    e coli

o    legionella pneumophila

o    pseudomonas aeruginosa

  • Protozoa

o    cryptosporidia

  • Algae

Biological pollution is particularly hazardous to bathers as it can cause a number of infectious diseases, some of which can be fatal. It is introduced into the swimming pool water in a number of ways. A few examples are given below:

  • On bathers bodies (skin)
  • Faecal matter
  • Blood
  • Mucus
  • Via the source water
  • On the bottom of outdoor footwear
  • Canoes/scuba gear which has not been cleaned

Biological pollution needs to be dealt with via a process of disinfection. The most common way of disinfecting swimming pool water in the UK is by the addition of a chlorine-based disinfectant to the pool water circulation system. Chlorine kills most (but not all) types of biological pollution.

Infectious Agents


Amoebae have been linked to spa pools and can contaminate contact lenses and cause corneal ulcers.


Viruses causing pharyngoconjunctival fever in under-chlorinated pools.


Cryptosporidiosis is an infectious diarrhoeal disease caused by a waterborne protozoan parasite. It is a disease of humans and animals, including cattle and sheep. Cryptosporidiosis cases have been declining in the UK for many years, but there are still around 4000 recorded cases each year in England and Wales.

Cryptosporidium most commonly affects young children and the immunocompromised, but can affect anyone. Cryptosporidium is found in the gut of man and animals (particularly cattle and sheep). It is also found in water contaminated with faeces.

It can be transmitted via contact with infected animals, by drinking or swimming in contaminated water and by eating contaminated food, e.g. salad vegetables. It can be spread from person to person where there is poor hygiene.

Occupations where there may be a risk of occupationally acquired cryptosporidiosis include workers in outdoor leisure industries in contact with water.

The incubation period is 2–10 days (average 7 days). The main symptom is watery diarrhoea, but symptoms can also include fever, stomach cramps and vomiting. Anyone with severe symptoms should seek medical attention. There is no treatment apart from rehydration therapy and most people recover within one month.

Cryptosporidiosis is a predominantly waterborne disease with infections caused by contaminated drinking water, swimming pools, water features, natural waters, or acquired by animal and human contact and a range of other routes. Cryptosporidium is a particular problem for swimming pools and drinking water because the oocysts are resistant to chlorine based disinfectants.

Swimming pool contamination is likely to occur all year round, but outbreaks are more common in the late summer period; this may be as a result of people using swimming pools more and also linked to holiday travel. Swimming pool outbreaks result from contamination of the water with cryptosporidium oocysts, usually from young swimmers.

Swimmers need to make sure they:

  • shower before swimming
  • do not swim if they have diarrhoea
  • try not to swallow pool water

Cryptosporidia is a parasite that is of particular concern for pool plant operators because it is not killed by chlorine. The parasites live inside a protective shell called an oocyst which protect them from the chlorine in the swimming pool or spa water. If these oocysts are ingested by swallowing contaminated water, the cryptosporidia with hatch out of the shells and reproduce, causing a gastro-intestinal illness. When the newly-created oocysts are expelled from the body via the faeces, the whole cycle starts again.

As chlorine is an ineffective defence, the pool plant operator must use other methods. The key operational defence is keeping cryptosporidium out of the swimming pool in the first place. Anyone diagnosed as having been infected with cryptosporidia should not go swimming until they are symptom-free for at least 14 days.  Signage is required at reception areas as well as in the changing rooms etc. You also need to ensure the effective coagulation and filtration of the oocysts. The thing to bear in mind is that without the addition of a coagulant (such as poly-aluminium chloride) to the circulation system at the correct dosing rate, the oocysts will pass through the sand in a commercial swimming pool filter. This is because the oocysts are about 3-5 microns in diameter, whereas the gaps between the sand grains are about 10 microns in width in a ripened sand filter. The addition of a coagulant will cause the minute particles of pollution (including the cryptosporidium oocysts) to clump together to form what are known as 'flocs'. These flocs are large enough to not pass through the sand filter and end up in the swimming pool.

Ultra violet radiation and ozone disinfection have been found to eliminate cryptosporidia, but even when using these types of disinfection processes, the use of a coagulant is still recommended.

It is vital that pool plant operators keep their sand filters clean and well-maintained. This means that for swimming pools, the sand filters should be backwashed at least weekly, or according to the filter manufacturer’s instructions. Spa pool filters should be backwashed every day.

Pool operators can help to prevent Cryptosporidium incidents by:

  • discouraging babies under the age of six months from using public pools;
  • encouraging all bathers to shower thoroughly before using a pool;
  • providing good, hygienic nappy changing areas;
  • discouraging anyone ill with diarrhoea (up to 14 days previously) from swimming.

If you end up with loose, runny stool in the swimming pool, you will need to assume that cryptosporidia is present and clear the pool and keep it closed for 6 turnover cycles. While you’re closed down, backwash the filters, get the chlorine up at the high end of the acceptable range and get the pH at the low end of the acceptable range. Also, scrub, sweep, brush, squeegee, net, and vacuum the whole area before re-opening.

Pool operators should:

  • ensure that filters are operating well and with coagulation
  • ensure there is sufficient water replacement (particularly in periods of high bather load)
  • conduct filter backwashing after the pool has closed at night
  • encourage pre-swim showering
  • ensure people do not use the pool if they have had within the last 48 hours (extended to 14 days if the diarrhoea was caused by cryptosporidia infection)
  • close the pool for a period equivalent to 6 turnover periods in the event of a liquid faecal release into the pool
  • ensure that very young children do not enter the pool unless wearing special swim nappies, designed to retain runny faecal matter

Dermatophyte Fungi

Can cause athletes foot from contact with contaminated surfaces.

Escherichia Coli

VTEC (verotoxigenic E. coli – also known as Vero-cytotoxin producing E. coli) are a group of bacteria that cause diarrhoeal disease in humans. The disease can range from mild gastroenteritis to severe bloody diarrhoea and in some can develop in to a serious, potentially fatal illness.

VTEC is a common cause of diarrhoeal disease in the UK. It is easily spread because very few bacteria are required to cause infection. It is spread via consumption of undercooked, infected meat and meat products and via faeces-contaminated water. It can also be spread via contaminated vegetables and other ready to eat foods or via contact with contaminated soil. It can be spread via direct or indirect contact with infected animals. It can be spread from person to person by faecal-oral transmission.

Occupational exposure to VTEC may occur in those who:

  • are in contact with infected animals or humans
  • are in contact with materials or products from infected animals
  • are in contact with contaminated water or soil

The incubation period is usually 3–4 days (can be 1–8 days). Symptoms range from mild diarrhoea to severe bloody diarrhoea with fever and stomach cramps. In rare cases VTEC can cause serious illness, including kidney damage and blood clotting disorders. In rare cases the disease can be fatal.

Anyone with severe symptoms should seek immediate medical attention. There is no specific treatment available for VTEC apart from rehydration therapy and most patients will recover within two weeks. Patients with kidney damage or blood clotting disorders will require specialist treatment in hospital.

The following control measures reduce the risk of infection:

  • Good occupational hygiene practices should be followed, especially washing with warm water and soap
  • Avoid swallowing water when participating in swimming activities
  • A suitable disinfectant should be used


Includes poliovirus, echovirus and coxsackieviruses A and B and linked to pools with insufficient chlorination.


Protozoa with similar characteristics to cryptosporidium.

Hepatitis A Virus

Spread via contaminated and underchlorinated water.

Legionella Pneumophilia

Legionellosis is a collective term for diseases caused by legionella bacteria including the most serious:Legionnaires’ disease, as well as the similar but less serious conditions of Pontiac fever and Lochgoilhead fever. Legionnaires’ disease is a potentially fatal form of pneumonia and everyone is susceptible to infection.

The risk increases with age, but some people are at higher risk, e.g.

  • people over 45
  • smokers and heavy drinkers
  • people suffering from chronic respiratory or kidney disease
  • diabetes, lung and heart disease 
  • anyone with an impaired immune system

The bacterium Legionella Pneumophila and related bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers. They may also be found in purpose-built water systems, such as cooling towers, evaporative condensers, hot and cold water systems and spa pools. If conditions are favourable, the bacteria may multiply, increasing the risks of Legionnaires ’ disease, and it is therefore important to control the risks by introducing appropriate measures.

Legionella bacteria are widespread in natural water systems, e.g. rivers and ponds. However, the conditions are rarely conducive for people to catch the disease from these sources. Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where water is maintained at a temperature high enough to encourage growth, e.g. cooling towers, evaporative condensers, hot and cold water systems and spa pools used in all sorts of premises (work and domestic).

Legionnaires’ disease is normally contracted by inhaling small droplets of water (aerosols), suspended in the air, containing the bacteria. Certain conditions increase the risk from legionella if:

  • The water temperature in all or some parts of the system may be between 20–45 °C, which is suitable for growth
  • It is possible for water droplets to be produced and if so, they can be dispersed
  • Water is stored and/or re-circulated
  • There are deposits that can support bacterial growth, such as rust, sludge, scale, organic matter and biofilms

It is important to control the risks by introducing measures which do not allow proliferation of the organisms in the water systems and reduce, so far as is reasonably practicable, exposure to water droplets and aerosol. This will reduce the possibility of creating conditions in which the risk from exposure to legionella bacteria is increased.

Identification and assessment of the risk

Before any formal health and safety management system for water systems is implemented, the duty holder should carry out a risk assessment to identify the possible risks. The purpose of the assessment is to enable a decision on:

  • the risk to health, i.e. whether the potential for harm to health from exposure is reasonably foreseeable, unless adequate precautionary measures are taken;
  • the necessary measures to prevent, or adequately control, the risk from exposure to legionella bacteria

The risk assessment also enables the duty holder to show they have considered all the relevant factors, and the steps needed to prevent or control the risk.

The duty holder may need access to competent help and advice when carrying out the risk assessment. This source of advice may not necessarily be from within the person’s organisation but may be from a consultancy, water treatment company or a person experienced in carrying out risk assessments. Employers are required to consult employees or their representatives about the arrangements for getting competent help and advice.

The duty holder under, with the help of the appointed responsible person, make reasonable enquiries to ensure that organisations such as water treatment companies or consultants, and staff from the occupier’s organisation, are competent and suitably trained and have the necessary equipment to carry out their duties in the written scheme safely and adequately.

Few workplaces stay the same, so it makes sense to review regularly what you are doing.

Carrying out a risk assessment

Consider the individual nature of the site and system as a whole, including dead-legs and parts of the system used intermittently. These should be included because they can create particular problems, as microbial growth can go unnoticed. When they are brought back online, they can cause heavy contamination, which could disrupt the efficacy of the water treatment regime.

A schematic diagram is an important tool to show the layout of the plant or system, including parts temporarily out of use and should be made available to inform the risk assessment process. These are not formal technical drawings and are intended to be easy to read without specialised training or experience. While providing only an indication of the size and scale, they allow someone unfamiliar with the layout of a system to understand the relative positions and connections of the relevant components quickly. They also help the person who carries out the assessment decide which parts of the water system, e.g. which specific equipment and services, may pose a risk to those at work or other people.

There are a number of factors that create a risk of someone acquiring legionellosis, such as:

  • the presence of legionella bacteria
  • conditions suitable for growth of the organisms, e.g. suitable water temperature (20 °C–45 °C) and deposits that are a source of nutrients for the organism, such as sludge, scale, rust, algae, other organic matter and biofilms
  • a means of creating and spreading breathable droplets, e.g. the aerosol generated by cooling towers, showers or spa pools
  • the presence (and numbers) of people who may be exposed, especially in premises where occupants are particularly vulnerable, e.g. healthcare, residential and nursing homes

The following list contains some of the factors to consider, as appropriate, when carrying out the risk assessment:

  • the source of system supply water, e.g. whether from a mains supply or not;
  • possible sources of contamination of the supply water in the premises before it reaches the cold water storage tank, calorifier, cooling tower or any other system using water that may present a risk of exposure to legionella bacteria;
  • the normal plant operating characteristics
  • unusual, but reasonably foreseeable operating conditions, e.g. breakdowns
  • any means of disinfection in use
  • the review of any current control measures
  • the local environment

Where there are five or more employees, the significant findings of the assessment must be recorded but in any case, it may be necessary to record sufficient details of the assessment to be able to show that it has been done. Link the record of the assessment to other relevant health and safety records and, in particular, the written legionella control scheme.

Employers must consult employees or their representatives on the identified risks of exposure to legionella bacteria and the measures and actions taken to control the risks. Employees should be given an opportunity to comment on the assessment and control measures and the employer should take account of these views, so it is important for employers to publicise to employees that a legionella risk assessment has been performed.  Employers may wish to involve employees and/or safety representatives when carrying out and reviewing risk assessments as a good way of helping to manage health and safety risk. 

It is essential to monitor the effectiveness of the control measures and make decisions about when and how monitoring should take place.

If the risks are considered insignificant and are being properly managed to comply with the law, the assessment is complete. It may not be necessary to take any further action, but it is important to review the assessment periodically, in case anything has changed.

The record of the assessment is a living document that must be reviewed to ensure it remains up-to-date. Arrange to review the assessment regularly and specifically whenever there is reason to suspect it is no longer valid. An indication of when to review the assessment and what to consider should be recorded. This may result from, e.g.:

  • changes to the water system or its use
  • changes to the use of the building in which the water system is installed
  • the availability of new information about risks or control measures
  • the results of checks indicating that control measures are no longer effective
  • changes to key personnel
  • a case of legionnaires’ disease/legionellosis associated with the system

Managing the risk: Management responsibilities, training and competence

Inadequate management, lack of training and poor communication are all contributory factors in outbreaks of Legionnaires’ disease. It is therefore important that the people involved in assessing risk and applying precautions are competent, trained and aware of their responsibilities.

The duty holder should specifically appoint a competent person or persons to take day-to-day responsibility for controlling any identified risk from legionella bacteria, known as the ‘responsible person’. It is important for the appointed responsible person to have sufficient authority, competence and knowledge of the installation to ensure that all operational procedures are carried out effectively and in a timely way. Those specifically appointed to implement the control measures and strategies should be suitably informed, instructed and trained and their suitability assessed. They must be properly trained to a level that ensures tasks are carried out in a safe, technically competent manner; and receive regular refresher training. Keep records of all initial and refresher training. If a duty holder is self-employed or a member of a partnership, and is competent, they may appoint themselves. The appointed responsible person should have a clear understanding of their role and the overall health and safety management structure and policy in the organisation.


The duty holder should also ensure that all employees involved in work that may expose an employee or other person to legionella are given suitable and sufficient information, instruction and training. This includes information, instruction and training on the significant findings of the risk assessment and the appropriate precautions and actions they need to take to safeguard themselves and others.

This should be reviewed and updated whenever significant changes are made to the type of work carried out or methods used. Training is an essential element of an employee’s capability to carry out work safely, but it is not the only factor: instructions, experience, knowledge and other personal qualities are also relevant to perform a task safely.

Implementation of the control scheme

Monitor the implementation of the written scheme for the prevention and control of the risk. Supervise everyone involved in any related operational procedure properly. Define staff responsibilities and lines of communication properly and document them clearly.

Make arrangements to ensure that appropriate staff levels are available during all hours the water system is operating. The precise requirements will depend on the nature and complexity of the water system. In some cases, e.g. where there is complex cooling plant, shift working and arrangements to cover for all absences from duty, for whatever reason, may be necessary. Appropriate arrangements should be made to ensure that the responsible person, or an authorised deputy, can be contacted at all times.

Also, make call-out arrangements for people engaged in the management of water systems which operate automatically. Details of the contact arrangements for emergency call-out personnel should be clearly displayed at access points to all automatically or remotely controlled water systems.

Communications and management procedures are particularly important where several people are responsible for different aspects of the operational procedures. For example, responsibility for applying control measures may change when shift work is involved, or when the person who monitors the efficacy of a water treatment regime may not be the person who applies it. In such circumstances, responsibilities should be well defined in writing and understood by all concerned. Lines of communication should be clear, unambiguous and audited regularly to ensure they are effective. This also applies to outside companies and consultants who may be responsible for certain parts of the control regime.

Employing contractors or consultants does not absolve the duty holder of responsibility for ensuring that control procedures are carried out to the standard required to prevent the proliferation of legionella bacteria. Dutyholders should make reasonable enquiries to satisfy themselves of the competence of contractors in the area of work before they enter into contracts for the treatment, monitoring, and cleaning of the water system, and other aspects of water treatment and control. An illustration of the levels of service to expect from Service Providers can be found in the Code of Conduct administered by the Legionella Control Association (LCA). 

Preventing or controlling the risk from exposure to legionella bacteria

Once the risk has been identified and assessed, a written scheme should be prepared for preventing or controlling it. In particular, the written scheme should contain the information about the water system needed to control the risk from exposure. However, if it is decided that the risks are insignificant and are being properly managed to comply with the law, you may not need to take any further action. But it is important to review the risk assessment regularly and specifically if there is reason to suspect it is no longer valid, for example changes in the water system or its use. The primary objective should be to avoid conditions that allow legionella bacteria to proliferate and to avoid creating a spray or aerosol. It may be possible to prevent the risk of exposure by, e.g., using dry cooling plant. Where this is not reasonably practicable, the risk may be controlled by minimising the release of droplets and ensuring water conditions that prevent the proliferation of legionella bacteria. This might include engineering controls, cleaning protocols and other control strategies. Make decisions about the maintenance procedures and intervals, where relevant, on equipment used for implementing the control measures.

Legionella bacteria may be present in low or very low numbers in many water systems, but careful control will prevent them from multiplying.

The written scheme should give details on how to use and carry out the various control measures and water treatment regimes, including:

  • the physical treatment programme – e.g. using temperature control for hot and cold water systems
  • the chemical treatment programme, including a description of the manufacturer’s data on effectiveness, the concentrations and contact time required
  • health and safety information for storage, handling, use and disposal of chemicals
  • system control parameters (together with allowable tolerances); physical, chemical and biological parameters, together with measurement methods and sampling locations, test frequencies and procedures for maintaining consistency
  • remedial measures to take in case the control limits are exceeded, including lines of communication
  • cleaning and disinfection procedures
  • emergency procedures

The written scheme should also describe the correct operation of the water system plant, including:

  • commissioning and recommissioning procedures;
  • shutdown procedures;
  • checks of warning systems and diagnostic systems in case of system malfunctions;
  • maintenance requirements and frequencies;
  • operating cycles – including when the system plant is in use or idle

Review of control measures: Monitoring and routine inspection

The frequency and extent of routine monitoring will depend on the operating characteristics of the water system. Testing of water quality is an essential part of the treatment regime, particularly in cooling systems. It may be carried out by a service provider, such as a water treatment company or consultant, or by the operator, provided they have been trained to do so and are properly supervised. The type of tests required will depend on the nature of the water system.

The routine monitoring of general bacterial numbers (total viable count) is also appropriate as an indication of whether microbiological control is being achieved. This is generally only carried out for cooling tower systems, but it is also recommended for spa pools. The risk assessment will help identify if you need to conduct routine monitoring in the specific system. Periodic sampling and testing for the presence of legionella bacteria may also be relevant to show that adequate control is being achieved. However, reliably detecting the presence of legionella bacteria is technically difficult and requires specialist laboratory facilities. The interpretation of results is also difficult; a negative result is no guarantee that legionella bacteria are not present in the system. Conversely, a positive result may not indicate a failure of controls, as legionella are present in almost all natural water sources.

A suitably experienced and competent person should interpret the results of monitoring and testing. Carry out any remedial measures promptly, where needed.

Record keeping

To ensure that precautions continue to be applied and that adequate information is available, where there are five employees or more, you must keep a record of the assessment, the precautionary measures, and the treatments. All records should be signed, verified or authenticated by those people performing the various tasks assigned to them.

The following items should normally be recorded:

  • names and positions of people responsible, and their deputies, for carrying out the various tasks under the written scheme;
  • a risk assessment and a written scheme of actions and control measures;
  • schematic diagrams of the water systems;
  • details of precautionary measures that have been applied/implemented including enough detail to show that they were applied/implemented correctly, and the dates on which they were carried out;
  • remedial work required and carried out, and the date of completion
  • a log detailing visits by contractors, consultants and other personnel
  • cleaning and disinfection procedures and associated reports and certificates
  • results of the chemical analysis of the water;
  • results of any biological monitoring
  • information on other hazards, e.g. treatment chemicals;
  • training records of personnel
  • the name and position of the person or people who have responsibilities for implementing the written scheme, their respective responsibilities and their lines of communication;
  • records showing the current state of operation of the water system, e.g. when the system or plant is in use and, if not in use, whether it is drained down
  • either the signature of the person carrying out the work, or other form of authentication where appropriate.


Small protozoa that are thought to cause diarrhoea in immunocompromised people.

Molluscum Contagiosum Virus

Poxyvirus that causes a skin rash. Caused by skin-to-skin contact, or sharing towels, rather than through the swimming pool water.

Mycobacterium Marinum

Bacteria that infects the skin. More resistant to chlorine than other bacteria.

Mycobacterium Avium

Bacteria that infects immunocompromised people, causing respiratory symptoms. Has been linked to spa pools and sometimes referred to as ‘hot tub lung’.

Papilloma Virus

Causes verrucas by contact with contaminated surfaces.

Pseudomonas aeruginosa

There have been numerous outbreaks of folliculitis caused by P. aeruginosa associated with pools and hot tubs. The folliculitis presents as a red rash and involves infection of the hair follicles. Disease is related to the duration of pool immersion as well as the degree of contamination of the water, and children and young adults are most susceptible.

Well-operated pools should not normally contain P. aeruginosa. If the count is over 10 P. aeruginosa per 100ml, repeat testing should be undertaken. Where repeated samples contain P. aeruginosa the filtration and disinfection processes should be examined to determine whether there are areas within the pool circulation where the organism is able to multiply.

There is a risk of an outbreak of folliculitis when the count exceeds 50cfu/100ml so the pool should be closed, remedial action taken and the water resampled.

High numbers of pseudomonas aeruginosa can also cause the condition otitis externa, which is an infection of the outer ear. This condition is sometimes known as swimmers ear and can also be caused by the wetting, dewaxing and degreasing of the outer ear.

Pseudomonas aeruginosa can readily infect plastic and inflatable items of pool equipment, such as blow-up play features, armbands, flotation aids etc. Equipment such as this should be cleaned regularly with a 10mg/l chlorine solution. Inflatables should be dried out prior to being folded up and staked away in order to avoid moist areas where the bacteria can proliferate.


Bacteria causing dysentery in badly-run swimming pools. Closely related to salmonella.

Staphylococcus Aureus/MRSA

Causes boils, abscesses and infected wounds. Can be resistant to methicillin and other antibiotics (Methicillin Resistant Staphylococcus Aureus MRSA). No evidence to suggest that ii is transmitted through use of swimming pools and therefore no justification for excluding MRSA carriers unless they have contaminated wounds.

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