Pool chlorination and closure guidelines

What is Free Available Chlorine (FAC)?

Chlorine and chlorine compounds have a high reactivity and oxidative capacity which is crucial in the degradation of organic compounds and microorganisms. There are a number of factors which impact the disinfecting efficiency of chlorine in swimming pools: type of chlorine, pH of the pool water, contaminants in pool water, water temperature and duration of contact. Disinfection by products (DBP), such as trihalomethanes can also be produced with the addition of chlorine, depending on the organic material; generally, the production of chloramines is a more important issue in maintaining pool water quality.

When chlorine is added to water, hypochlorous acid (HOCl), and the hypochlorite ion (OCl^-) are formed. Hypochlorous acid is the most effective antibacterial species formed by chlorine.

Cl₂+ H₂O ↔ HOCl + H⁺+ Cl^-
HOCl ↔ H⁺+ OCl^-

A measure of chlorine in these two chemicals is known as Free Available Chlorine (FAC). These compounds are available to oxidize contaminants in pool water. Hypochlorous acid can react with contaminants containing ammonia (NH₃), often found in pool water due mainly to perspiration and urine, to form chloramines. Chloramines are the cause of chlorine smell or pool odour and can be a source of irritation to eyes, lungs and skin of swimmers. The strong odour caused by chloramines usually means there is too little free chlorine (hypochlorous acid), rather than too much.

Chloramine formation occurs when hypochlorous acid combines with ammonia to form monochloramine (NH₂Cl). Monochloramine can combine further with hypochlorous acid to form dichloramine (NHCl₂), and again to form trichloramine (NCl₃) as shown below.

HOCl + NH₃ ↔ NH₂Cl + H₂0 monochloramine

HOCl + NH₂Cl ↔ NHCl₂ + H₂0 dichloramine

HOCl + NHCl₂ ↔ NCl₃ + H₂0 trichloramine

Monochloramine is a weaker disinfectant than hypochlorous acid, but more stable; therefore, often used as a disinfectant in the distribution lines of water treatment systems.¹ Monochloramine is also the most abundant chloramine when the water is a pH of 7 or higher. Combined Available Chlorine (CAC) refers to chloramines or compounds formed when free chlorine reacts with organic nitrogen-containing compounds produced by bathers. While both FAC and CAC have disinfecting properties, CAC (as monochloramine) has less disinfecting power than FAC.^2,3 Total chlorine simply refers to all the FAC and CAC combined in a swimming pool. To be effective, total chlorine should be maintained between 1-3 ppm with the bulk of this as FAC.¹

Shock, superchlorination or breakpoint chlorination, is a technique used to control an excess of chloramines by adjusting the pH to 7.5 or lower and by adding sufficient chlorine to achieve a FAC concentration ten times (10-20 ppm) the CAC concentration.^4,5 Oxidation of the chloramines occurs which converts to gaseous nitrogen.¹ Adding 10 times the level of combined chlorine or chloramines in the water achieves so-called breakpoint chlorination when there is enough extra chlorine to consume the nuisance chloramines which cause odours.

The concentration of hypochlorous acid and hypochlorite ions in chlorinated water will also depend on the water's pH. A higher pH (above 7.5) facilitates the formation of more hypochlorite ions and results in less hypochlorous acid (which is a more effective disinfectant) in the water.¹ Therefore disinfection is more efficient at a lower pH (with large quantities of hypochlorous acid in the water). Most recommendations specify a pH range of 7.2 to 7.8 to maintain efficient disinfection but also to prevent eye and skin irritation.⁶

What are the regulations and guidelines around chlorine levels in swimming pools?

Swimming pool chlorination guidelines and regulations vary across provinces and territories in Canada. Some jurisdictions require minimum free chlorine concentrations, but not maximums; others specify a free available chlorine (FAC) range. The FAC ranges stipulated by these jurisdictions range from 0.8 ppm to 5.0 ppm (see Table 1).

Table 1. Swimming pool and whirlpool FAC and CAC requirements in Canada

Province/Territory	Free Available Chlorine (ppm)	Combined Available Chlorine (ppm)	Notes/ Reference
British Columbia	min 0.5 if ≤ 30°C min 1.5 if > 30°C	< 1.0	British Columbia 2010⁷
Ontario	min 0.5 min 1.0 if cyanuric acid is used min 5 for spas	none	Ontario 2007⁸ Ontario 2005⁹
Saskatchewan	min 2.0 for swimming pools min 3.0 for whirlpools	none	Saskatchewan 2006¹⁰
Yukon Territory	min 0.5 if ≤ 30°C min 1.0 if > 30°C	< 1.0 in swimming pools <1.5 in whirlpools	Yukon Territory 1989¹¹
Nunavut	Show a residual as close to 1.2 as possible, should fall between 1.0 and 1.5 inclusive	Swimming pools: < 2.5 Spa pools: < 0.5	Free chlorine level should reach 10 ppm at least once per day; Nunavut 1990¹²
Northwest Territories	Show a residual as close to 1.2 as possible, should fall between 1.0 and 1.5 inclusive	Swimming pools: < 2.5 Spa pools: < 0.5	Free chlorine level should reach 10 ppm at least once per day; Northwest Territories 1990¹³
Newfoundland and Labrador	(Guideline) 1.5 for indoor pools (min 0.5) 3.0 for outdoor pools (min 1.0) 2-3 for whirlpools, spas Should not be greater than 5 when bathers present	(Guideline) < 0.5	Legislation only stipulates “…safe and satisfactory bacteriological and chemical quality”; Newfoundland and Labrador 2004¹⁴
Alberta	min 1.0 if ≤ 30°C min 2.0 if > 30°C	none	Alberta 2006¹⁵
Prince Edward Island	1.0-3.0 for swimming pools min 3.0 for whirlpools	none	Prince Edward Island 2005¹⁶
Quebec	0.8-2.0 for indoor pools 0.8-3 for outdoor pools	< 0.5 for indoor pools < 1.0 for outdoor pools	Quebec 2006¹⁷
Manitoba	1.0-5.0 for pools and whirlpools	< 1.5	Manitoba 1997¹⁸
New Brunswick			No guideline or legislation accessed
Nova Scotia	(Guideline) 1.0-2.0	Not legislated	Nova Scotia 1998¹⁹

Looking beyond Canada, a Department of Health swimming pool regulation in San Diego, CA mentions that the FAC level be no more than 10 ppm.²⁰ Also, the U.S. Centres for Disease Control (US CDC) is currently developing Model Aquatic Health Code (MAHC) which may address this issue. However, in email correspondence with the US CDC concerning maximum FAC levels, it was mentioned that although it is common practice to close at 10 ppm, the correspondent was not aware of any “definitive information” to support this level.²¹ The Department of Health in Sydney, Australia published a swimming pool guideline that recommends all pools, including swimming pools and spa pools, should have a maximum total available chlorine concentration of 10 ppm.²²

The WHO indicates that during shock chlorination the pool should be closed to patrons but swimming can resume after the chlorine level has been reduced to < 5.0 ppm; most likely tied to drinking water standards.²³

What are the health effects of chlorine and chlorination by-products?

Although chlorine has been used for pool disinfection since the 1920s (due largely to its inexpensive cost and excellent disinfecting properties), there has been an ongoing debate over its safety and its adverse health effects on humans.

Animal studies

A number of dose-response studies, involving animals, discuss the irritating effects of chlorine on eye conjunctiva, skin, and nasopharyngeal mucosa. A study by Mood et al (1951)²⁴ showed that irritant effects on eye conjunctiva began at a concentration of about 8 ppm of FAC, whereas another study²⁵ showed no reaction at 0-8 ppm FAC, uncertain reaction at 16 ppm FAC, and a clear reaction at 20 ppm FAC. With chloramines, on the other hand, the irritant effects began at a lower concentration of 2 ppm²⁴ and 3 ppm²⁵ with severe reaction occurring at 5 ppm.²⁵ Severe lysis, or breakdown of membrane cells occurred at a concentration of 9.8 ppm free chlorine, and 6.3 ppm of combined chlorine.²⁶

Ingestion

Exposure to chlorine and chlorination by-products can occur through ingestion of pool water; some drinking water guidelines provide maximum acceptable concentrations (MAC) for chemicals in water.²⁷ However, risk assessment methods used to set these levels are based on long-term exposure to a standard volume of water (usually 2 litres/day in a healthy adult). Pool patrons do not normally ingest 2L of pool water while at the pool on a given day. The average bather may ingest approximately 10 millilitres (mL) of pool water while swimming and 3.7 mL while wading/splashing.²⁸ Therefore exposure through the ingestion of swimming pool water is likely to be much lower.

Health Canada does not specify a MAC guideline for chlorine in water, due to insufficient evidence for health effects associated with ingestion of chlorine in drinking water.²⁹ On the other hand, the WHO established a guideline value of 5 ppm in drinking water.²⁷ Therefore, at recommended swimming pool FAC levels (ranging from 0.8-5.0 ppm according to swimming pool guidelines and regulations across Canada), ingestion of pool water does not have adverse health effects on bathers. In fact, past incidents have shown that short-term consumption of water with chlorine concentration as high as 50 ppm posed no ill effects, and consumption of water with greater than 90 ppm of chlorine resulted in momentary constriction of the throat and irritation of the mouth and throat.²⁹

Inhalation and dermal exposure

A number of outbreaks involving the inhalation of aerosolized chlorine by-products at improperly chlorinated pool facilities or accidental release of chlorine gas have been documented.^30-33 In addition, there are also reports of occupational asthma in pool workers as a result of aerosolized chlorine by-products, notably nitrogen trichloride (trichloramine). Common symptoms of chlorine inhalation include cough, throat irritation, wheezing, difficulty breathing, sneezing, headache, rash, nausea, and vomiting. Occupational asthma can also result from prolonged exposure to chloramines in indoor swimming pool air. Two such case studies were documented where exposure to airborne nitrogen trichloride resulted in occupational asthma, which improved when the employees were away from the swimming pool atmosphere.^30,34

Chlorine by-products, including trihalomethanes (THM) and nitrogen trichlorides (trichloramines), have been mentioned as culprits for health effects in bathers. THMs are volatile, and their air concentration depends on several factors: concentration in the pool, temperature, amount of splashing and surface disturbance, ventilation, size of facility, and air circulation.²³ Nitrogen trichlorides (trichloramines) are also volatile, therefore, more likely to be found in the atmosphere of swimming pools than monochloramines and dichloramines, which are more likely to be aerosolized through splashing and surface disturbances.³⁴

A study conducted by Hery et al. (1995)³¹ monitored the atmosphere in 13 swimming pools, and interviewed lifeguards and other pool workers regarding irritation symptoms, in order to establish a recommended maximum allowable airborne concentration of nitrogen trichlorides. They found that the first irritation complaints registered at around 0.5 ppm of nitrogen trichlorides in the air, and at 0.7 ppm, all individuals reported irritation symptoms.³¹ No information is available on the concentrations of FAC or CAC in water that may relate to these air concentrations.

Kaydos-Daniels (2007)³² and Safranek (2006)³³ describe two incidences involving exposure to chloramines. The first incident occurred in West Virginia in 2002. Guests at a hotel attended a birthday party at an indoor swimming pool and reported symptoms consistent with chloramine exposure. Further investigation revealed that while FAC levels were within the acceptable range of 1.0-5.0 ppm, both combined chlorine and pH were beyond the acceptable range prescribed by the city; CAC was found to be ≥ 0.7 ppm, and pH of the water was found to be ≥ 8.5.³² The symptoms were strongly attributed to exposure to pool water and atmosphere in the pool room. The second incidence occurred in Nebraska in 2006.³³ Guests reported symptoms including burning eyes, sore throat, watery eyes, coughing, sneezing, burning inside the nose, chest tightness, and shortness of breath. Inspection of the pool found the FAC to be 0.8 ppm; the CAC was 4.2 ppm, and the pH was 3.95.³³ All of these levels were in violation of the Nebraska health code.

So where did the common practice of closing pools at 10 ppm FAC come from?

No literature was found to provide concrete evidence for the closure of pools at a free chlorine level of 10 ppm; however, based on Eichelsdörfer’s animal study (1975)²⁵ on eye irritation, 8 ppm appears to be the NOAEL (no observable adverse effect level) for FAC on eye conjunctiva. Additionally, Erdinger et al. (1998)²⁶ showed that severe lysis of membrane cells occurred at 9.8 ppm of FAC. Therefore, these two studies could potentially provide a plausible reason for the 10 ppm FAC limit.

The literature does provide some information concerning disinfection and health effects at different levels of free available chlorine (FAC) and combined chlorine CAC or chloramine levels. Most provincial regulations in Canada specify a minimum level or range of FAC and a maximum level of combined chlorine CAC or chloramines but do not specify a maximum level of FAC at which a pool should be closed. Manitoba, for instance, has a FAC range from 1 to 5 ppm which was the highest specified range of the provinces and territories reviewed. San Diego Department of Health guidelines recommend that there be no more than 10 ppm FAC.²⁰ While the US CDC recognises this limit is used elsewhere in practice, they were not aware of any definitive evidence as to why. The city of Sydney, Australia is the only known jurisdiction that actually specifies 10 ppm as the upper limit for chlorine.

If CAC rises beyond acceptable levels, generally above 0.2 ppm, the pool can be superchlorinated by adding excess chlorine up to 10 times the CAC level to reduce chloramines (references sometimes specify 10 ppm).²² This level may be where the 10 ppm limit originated.

Health effects of free chlorine and chloramines indicate that irritation can occur at levels below 10 ppm but are generally for combined chlorine levels or chloramines. The World Health Organization (WHO) does report that swimming can resume once the FAC reaches 5 ppm but this may be based on ingestion (WHO drinking water guidelines specify that FAC levels should be below 5 ppm).²³ Since exposure via ingestion during swimming is significantly lower, this does not provide necessary evidence for setting a limit.

In conclusion, while there is no specific evidence to support the 10 ppm level used as an upper limit for pool closures based on health effects through ingestion, inhalation, and dermal exposure, animal studies on eye irritation do provide some evidence for the 10 ppm FAC limit. Important factors to consider are the combined chlorine levels (chloramines), to which most of the health impacts are attributed during swimming, and maintaining appropriate pH levels, which affect chloramine production and effectiveness of disinfection.

Acknowledgement

We would like to thank the following individuals for their valuable input and review of this document: Tina Chen and Sylvia Struck for content; Nelson Fok, Tom Kosatsky, and Ralph Stanley for input and review.

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