The World Health Organization (WHO) has published new health and safety guidelines for swimming pools and hot tubs, providing a comprehensive, evidence-based resource to help facility operators and public health officials maintain healthy conditions for swimming pool users and employees. The May 2006 Guidelines for Safe Recreational Water Environments, Volume 2: Swimming Pools and Similar Environments (Guidelines) took over 10 years to develop, and involved the participation of numerous institutions and more than 60 experts from 20 countries worldwide.

The Guidelines discuss the available knowledge of key health and safety hazards associated with swimming pools, hot tubs and similar facilities, and present a broad range of strategies covering design, construction, operation and management to minimize these hazards. This document also serves as a framework for policy makers and regulators.

Where possible, WHO provides numerical guideline values as indicators of safety or good management practices. These values were developed using a risk-benefit approach, considering not only potential risks, but also the significant benefits - including aerobic exercise - related to use of these facilities. According to WHO, when a guideline value is exceeded, this should prompt an investigation to determine the cause of the failure and the likelihood of recurrence, and to identify corrective actions needed to address the immediate hazard and prevent similar conditions in the future.

This Water Quality and Health article summarizes several key elements of the Guidelines related to air and water quality, particularly those focused on preventing the spread of waterborne illnesses.

The Guidelines address three main water and air quality challenges, ranked in order of public health importance:

1) Maintaining water clarity to minimize injury hazard

2) Protecting water quality to prevent the transmission of infectious disease

3) Controlling potential hazards from chemical compounds

All of these challenges can be addressed through a combination of the following:

• Swimmer hygiene, including pre-swim showering, and preventing accidental fecal releases into the pool;
• Water treatment, including filtration (to remove particulates and other contaminants) and disinfection (to remove/inactivate infectious microorganisms and oxidize other contaminants during pool use);
• Pool hydraulics to ensure effective distribution of disinfectant throughout the pool, good mixing and removal of contaminated water;
• Addition of fresh water at frequent intervals to dilute substances that cannot be removed from the water by treatment);
• Cleaning to remove biofilms from surfaces, sediments from the pool floor and particulates adsorbed to filter materials; and
• Adequate ventilation of indoor pools.

The Guidelines describe a variety of microorganisms that can be found in swimming pools and related environments (see figure below). The risk of illness associated with swimming pools is primarily linked to fecal contamination of the water. However, a number of disease causing microorganisms can be introduced through other routes.

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Potential Microbial Hazards in Pools and Similar Environments (Figure 3.1)

For each type of microbial hazard, the Guidelines discuss known disease outbreaks and provide information on risk assessment and risk management. WHO reports that swimming pool-related outbreaks of illness are relatively infrequent, and can largely be prevented through proper disinfection, well-operated filters, swimmer hygiene, and appropriate response to accidental fecal releases. Good management practices are particularly important for chlorine-resistant microorganisms such as Giardia and Cryptosporidium.

Many of the known outbreaks related to swimming pools have occurred because disinfection was not applied or was inadequate. Disinfection is part of the treatment process whereby disease causing microorganisms are inactivated by chemical or physical means.

Chlorine-based disinfectants are the most widely-used, and include chlorine gas, sodium hypochlorite, calcium hypochlorite and chlorinated isocyanurates. Other processes use bromine-based disinfectants, ozone, and ultraviolet (UV) radiation. The Guidelines discuss the characteristics and uses of each, and considerations for choosing a disinfectant and application system. A combination of technologies may be used to take advantage of the strengths of various technologies. For example, ozone and UV are each effective against Cryptosporidium, but do not provide a residual disinfectant in the pool and, therefore, must be used in conjunction with a chlorine- or bromine-based disinfectant.

The Guidelines note that chlorination practices vary widely around the world, as do recommended free chlorine levels. The Guidelines state that adequate disinfection should be achieved with a free chlorine level of 1 mg/l throughout the pool. They further recommend that levels not exceed 3 mg/l in pools and 5 mg/l in hot tubs. The range of 1- 3 mg/l for pools is somewhat lower than the range of 2-4 mg/l recommended by the Association of Pool and Spa Professionals in the U.S. [and endorsed by the Water Quality & Health Council]. However, WHO recommends that acceptable levels of free chlorine continue to be set at the local level, and that these levels must always be consistent with satisfactory microbial quality. WHO also notes that chlorine levels well above the recommended range may not be of health significance, and that the primary issue would be acceptability to swimmers.

The Guidelines also discuss "shock dosing" with chlorine as part of a strategy of proper pool management. Shock dosing is used to control a variety of pathogens and nuisance microorganisms and to destroy organic contaminants and chloramine compounds (discussed more fully below). As a preventive measure, routine shock dosing typically involves raising free chlorine levels to at least 10 mg/l for between 1 and 4 hours. Addressing a specific water quality problem (such as an accidental fecal release) may involve raising the free chlorine residual to 20 mg/l for an 8 hour period while the pool is unoccupied.

The Guidelines assess hazards from disinfection byproducts (DBPs), compounds formed by the reaction of disinfectants with various organic compounds. Most information available relates to the reactions of chlorine, while less is known about byproducts of other disinfectants.

WHO's Guidelines for Drinking-water Quality can be used as a screening tool for potential risks associated with DBPs in swimming pools, while making appropriate allowance for the much lower quantities of water ingested, shorter exposure periods and proportionally higher non-ingestion exposure for pools. According to WHO, DBP concentrations below the drinking water guideline values can be achieved consistently in well managed pools. WHO also notes that since the drinking water guideline values reflect tolerable risks over a lifetime, this provides an additional level of reassurance.

Overall, WHO concludes, "The risks from exposure to chlorination byproducts in reasonably well managed swimming pools would be considered small and must be set against benefits of aerobic exercise and risks in the absence of disinfection."

The Guidelines give particular attention to chloramines and bromamines, which are formed when chlorine and bromine disinfectants react with ammonia and nitrogen containing substances in the water. Two of these compounds, nitrogen trichloride and nitrogen tribromide, are volatile and can give rise to significant eye and respiratory irritation in swimmers and pool attendants. Nitrogen trichloride has a strong and unpleasant odor [often mistakenly attributed to an excess of chlorine]. The Guidelines discuss studies that suggest a number of health symptoms associated with exposure to the atmosphere of indoor swimming pools, including respiratory symptoms likely to be particularly pronounced in those suffering from asthma. Various authors have suggested these were associated with nitrogen trichloride in particular, but the studies were not able to confirm this.

WHO recommends a provisional guideline value of 0.5 mg/m³ for nitrogen trichloride in the atmosphere of indoor pools and hot tubs, noting that additional data are needed to draw firm conclusions on health risks.

The Guidelines emphasize the need to optimize the management of indoor pools to reduce chloramines exposure, particularly for competitive swimmers and other heavy users of pools. Elements of a management strategy may include:

• Minimizing the introduction of precursors through bather hygiene (e.g., pre swim showering);
• Removing precursors through filtration and other treatment;
• Periodic shock dosing of pool water - raising free chlorine levels to at least 10 times the level of chloramines; and
• Ventilating the air with at least 10 liters of fresh air/s/m2 of water surface area.

To address the hazards discussed earlier, it is necessary to implement effective strategies at multiple levels. The final chapter of the Guidelines outlines responsibilities for various stakeholders, including those in charge of:

• design and construction of facilities, which will impact the ability to ensure safe operations once the pool is in use;
• operation and management, including the preparation of and compliance with a comprehensive pool safety plan;
• providing public education and targeted information to pool users; and
• regulatory requirements and enforcement at the national and/or local level.

The Guidelines provide a series of examples of good practices in each area. WHO emphasizes that successful implementation of these practices will require suitable capacities and expertise, as well as a coherent policy and legislative framework to assign responsibilities and ensure accountability.

Water Quality & Health articles are published periodically by the Water Quality & Health Council, an independent, multidisciplinary group that promotes science based practices and policies to enhance water quality and health by advising industry, health professionals, policy makers and the public.