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Environmental Health

Disinfection of Public Pools & Management of Fecal Accidents 
Reprinted from Journal of Environmental Health, Vol. 58, No. 1, pp 8-12, ISSN 0022-0892, July/August 1995]


The association between the use of swimming pools and illness has long been recognized. Discovery of hardier varieties of pathogens and increased use of spa pools and hot tubs has resulted in higher recommended levels of disinfectants. Chlorine is the most widely used disinfectant in pools. When maintained at proper levels, chlorine can inactivate or control most pathogens in swimming pool water. The exception is Cryptosporidium oocysts which are extremely resistant to most disinfectants and require high levels of chlorine for long periods of time to achieve deactivation. With the prevalence of Cryptosporidium infection estimated as high as 4.5% in the general population and the recent occurrence of five outbreaks of Cryptosporidiosis at public pools. more extreme measures must be taken to protect the health of pool users. Depending on the circumstances, when a fecal accident occurs at a public pool, it is prudent to consider closing the pool for up to a day so that the pool water can be properly disinfected and filtered.

 The association between the use of swimming pools and disease has been well documented in literature (1). A variety of illnesses ranging from minor and self-limiting to life-threatening has been reported as a result of using improperly maintained swimming pools, spa pools, and hot tubs. Reported illnesses which have increased over the past few decades can most likely be attributed to improved reporting techniques and to the increased popularity of spa pools and hot tubs. One of the most important factors in preventing outbreaks of illnesses at pools is adequate disinfection. Proper filtration and chemically balanced water are also essential in maintaining a healthful pool. All three factors work together to control the proliferation of pathogens and prevent disease. Disinfectants oxidize organic matter and inactivate pathogens. Filtration removes both organic and inorganic matter from pool water eliminating materials in which pathogens can be sheltered from disinfectants. Filtration also aids in decreasing disinfectant demand, thereby reducing the possibility of disinfectant depletion during heavy pool use and making disinfection more cost effective. Proper levels of pH, alkalinity, calcium hardness, total dissolved solids, and cyanuric acid are essential to assure adequate disinfection, bather comfort, and prevention of corrosion or scaling of equipment.

Chlorine is the agent most commonly used to disinfect pools (2). All types of chlorine disinfectants dissociate when added to water to produce hypochlorous acid as well as other by-products (3). Hypochlorous acid, or "free chlorine," is what effectively oxidizes organic matter and destroys bacteria, viruses, yeasts, and protozoa. The percentage of hypochlorous acid in solution is directly dependent upon the pH and temperature of pool water. Therefore, it is essential that the pH of the pool water be maintained between 7.2 and 7.8. Of equal importance in providing proper disinfection is controlling the buildup of chloramines. Overtime, hypochlorite ions combine with nitrogen and ammonia products deposited by pool users to form chloramines. Chloramines are weak disinfectants and contribute to eye burning, mucous membrane irritation, and objectionable chlorine odors. Chloramines can be eliminated by super-chlorination of the pool. Super chlorination or breakpoint chlorination can be achieved by adding additional chlorine to the pool until there is an abrupt decrease in chloramines and an increase in free chlorine residual.

There has been a great deal of controversy as to what level free chlorine residuals should be maintained in pool water to promote bather comfort and at the same time ensure adequate disinfection. Chlorine residual is commonly measured in parts per million (ppm) or milligrams per liter (mg/L). Several decades ago, before the discovery of chlorine-resistant organisms and prior to the popular use of spa pools and hot tubs, free chlorine residuals were generally kept below 1.0 mg/L in pool water.

A second method by which disinfectant activity can be measured is by the oxidation-reduction potential (ORP). ORP is measured by an ORP probe and readings are expressed in millivolts (mV). A reading of 650 mV or greater indicates satisfactory disinfectant activity. A major advantage of using an ORP reading is that it indicates the actual level of disinfection taking place and takes into account pH, water temperature, chloramines, and other factors.

Increased use of spa pools and hot tubs and the discovery of more resistant strains of bacteria, viruses, and protozoa have led to recommendations of higher levels of disinfectant(4,5). In spa pools, elevated water temperature, turbulent water, and heavy bather load lead to the rapid depletion of disinfectants. Of the three, bather load is the most significant factor in the depletion of disinfectant. Fifty people using a 1,000-gallon spa pool at a health club during the course of a day would be the equivalentof5,OOOpeople using a 100,000-gallon swimming pool. Bather loads of this size require constant monitoring of disinfectant levels and the use of automatic chlorinators and pH/ORP controllers to. assure a steady supply of disinfectant to the pool.

The hardiness of an organism when exposed to a specific disinfectant can be quantitatively expressed by the formula CT, where C is the concentration in mg/L of the disinfectant and T is the time in minutes of exposure (6). This value typically represents the concentration of a particular disinfectant and the time required to inactivate 99.9% of the organisms. Depending on the organism and disinfectant, this can vary from low to high levels of disinfectant for short to long periods of exposure time.

One of the most frequently isolated organisms found most often in spa pools, is Pseudomonas aeruginosa (7). These hardy thermophilic bacteria are encapsulated with a slimy coating that makes them more resistant to disinfectants. P. aeruginosa is most often responsible for outbreaks of folliculitis and skin dermatitis but can also cause otitis, pneumonia, and urinary tract infections (8,9,10,11,12,13). Symptoms of folliculitis usually last about seven days and include malaise, fatigue, fever, and a papulopustular rash. The onset of the illness is usually within two days to two weeks following exposure, and most cases are self-limiting requiring little or no medical treatment. The amount of time spent in the contaminated water is an important factor in determining whether or not a person will become ill (14). Most outbreaks can be traced to lack of proper disinfection, inadequate filtration, and/or lack of proper pool maintenance and cleaning practices (8,9,11,12,13). Many outbreaks have been associated with spa pools at health clubs, hotels, and motels (9,10,11,12,13). Undoubtedly, a large number of cases go unrecognized. Although testing has shown that P. aeruginosa is susceptible to low concentrations of free chlorine, it has been recovered from spas containing free chlorine residuals of 2 mg/L or greater (7). Exact numbers at which these organisms need to be present to cause illness is not known. It appears, however, that a level of 2 mg/L of free chlorine prevents the proliferation of the bacteria. Outbreaks in the past suggest that relatively large numbers of P. aeruginosa are needed to cause illness, and there have been few, if any, outbreaks observed at pools with adequate disinfectant levels (7,9).

Another group of bacteria that are commonly found in swimming and spa pools are Staphylococci. These bacteria originate from the pool user's skin and oral and nasal tracts. The coagulase positive varieties, e.g. Staphylococcus aureus, can cause serious skin infections as well as conjunctivitis, odds, and upper respiratory and urinary tract infections (15). Relatively low levels of free chlorine (<1.0 mg/L) are adequate to inactivate the bacteria in a short time (15).

In 1994, an outbreak of Legionnaires' disease was reported, but superchlorination and proper cleaning of filter devices successfully ended the outbreak. A Vermont study in 1983 indicated that Legionella pneumophila was isolated from three out of seven whirlpool spas located at local inns (17). In 1982, 14 out of 23 Michigan church group members became ill with flu-like symptoms after using a spa pool (18). The illness was identified as Pontiac Fever, a milder form of Legionnaires' disease. Legionella, which have been identified as causing Legionnaires' disease and Pontiac Fever, were isolated from the spa water.

Legionella can be found in both raw and treated water. These bacteria are thermophilic and can survive temperatures up to 500C. Legionella may often be found in the plumbing systems of hospitals with no apparent cases of Legionnaires' disease, indicating that some strains are more virulent than others. The United States Environmental Protection Agency (EPA) recommends free chlorine residuals of 8 mg/L to control Legionella in hot water plumbing systems. The EPA also reports that in some cases, a level of 1.5-2.0 mg/L is sufficient to control the organism (19).

Shigellosis is an acute bacterial illness involving the large intestine and is characterized by diarrhea, fever, nausea, and sometimes, vomiting, cramps, and toxemia. It is caused by bacteria of the genus Shigella. The illness is usually self-limiting, and lasts four to seven days. Mild and asymptomatic infections do occur. An outbreak of Shigellosis occurred at a recreational swim area in Los Angeles County in 1985(20). The swim area was limited to a small section (150 x 700 ft.) of an artificially constructed sand bottom lake measuring approximately 70 acres. The water for the lake was supplied by a deep underground well that was used to replace water lost through evaporation and drainage. Bather loads during weekends were typically high and the swim area lacked any appreciable circulation. The recreational swim area had a chlorination system in place at the time of the outbreak, but it was not in use. Within one week following exposure at the recreational site, 68 persons had onset of diarrheal illness, including seven persons who required hospitalization. It was theorized that the outbreak was caused by direct bather contamination of the swim area which was caused by heavy bather load, inadequate restroom facilities, poor water exchange, and the absence of disinfection. Since the occurrence of the outbreak, the swim area has been chlorinated during periods of heavy use and no further outbreaks of illness have been reported. In the setting of a swimming poo1 with continuous filtration and chlorination, Shigella are quite susceptible to low levels of chlorine disinfectants and can be inactivated at levels less than 1.0 mg/L.

A variety of viruses have been implicated in disease outbreaks at public pools including adenovirus, enterovirus, and Hepatitis A virus. Adenovirus type 4 was identified as causing an outbreak of pharyngoconjunctival fever at a swimming pool in Georgia and a summer camp in North Carolina (21,22). Pharyngoconjunctival fever is characterized by fever, conjunctivitis, sore throat, headache, and chills. A major factor in both outbreaks was inadequate chlorination. Laboratory tests have shown that low levels of free chlorine (0.2 mg/L) are effective in inactivating adenovirus type 3 and type 4(21). In a 1981 study, municipal and wading poo1s in the Houston area were tested for viruses (23). Viruses, including enterovirus, echovirus, coxsackievirus, adenovirus, and poliovirus, were isolated in 10 out of 14 samples. Two of the pools that contained enterovirus had free chlorine residuals that exceeded 0.4 mg/L. The study suggested that viruses were generally more resistant to chlorine than coliform bacteria. Coliform bacteria are commonly used as indicator organisms to determine water quality. The study concluded that a higher free chlorine residual should be considered for public pools.

In 1989, an outbreak of Hepatitis A associated with swimming pool use occurred at a Louisiana campground (24). Twenty people became ill approximately one month after swimming in one of the campground's swimming pools. It was theorized that hot weather and heavy bather loads depleted the free chlorine residual in the swimming pools. It is speculated that the pools were contaminated by either fecal contamination from one of the pool users or by sewage from a cross-connection. The former is more likely since several people reported that children wearing diapers were allowed to swim in the pools, and the campground management reported that fecal accidents resulting in fecal contamination were not uncommon. Depending on pH, Hepatitis A virus is inactivated at CT values between 10 and 15(3).

Of great concern in the last decade have been outbreaks of illness caused by species of Giardia and Cryptosporidium. Both organisms are protozoans and are transmitted from person-to-person through oral-fecal route. Giardiasis is caused by several species of Giardia that infect the upper portion of the small intestine. While frequently asymptomatic, it may cause diarrhea, abdominal cramps, fatigue, and weight loss. The prevalence of stool samples that test positive for Giardia in the general population may range from 1% to 30% (25). The disease is spread by ingestion of cysts from the feces of infected individuals. An outbreak of Giardiasis occurred at a public swimming pool in New Jersey in fall 1985 (26). The source of the contamination was most likely a handicapped child who had a fecal accident in the pool. Records indicated that no chlorine reading had been taken on the day of the contamination, and the following day the chlorine level was zero. Giardia cysts are somewhat resistant to chlorine, especially at colder water temperatures (<100C). At temperatures normally maintained in pools (>200C), inactivation of Giardia cysts occurs at free chlorine concentrations of 1.5 mg/L for 10 minutes or a CT value of 15 (26,27).

Cryptosporidiosis is primarily a disease of animals and has not been recognized as a human disease since 1976 (28). The illness is characterized by prolonged diarrhea, abdominal cramps, malaise, and fever. Cryptosporidiosis is usually a self-limiting illness but can be life-threatening to immunocompromised individuals, such as persons with AIDS or persons receiving chemotherapy. In developed areas such as the United States or Europe, prevalence of infection with Cryptosporidium is found to be between 2.2% and 4.5% (25,28,29,30,31,32,33). In underdeveloped countries the prevalence is between 3% and 20%. There is usually a significantly higher prevalence in children than in adults. Asymptomatic infections in children are common, and Cryptosporidium can be excreted in stools for up to two weeks after resolution of diarrhea (29,30). Since most states do not routinely report cases of Cryptosporidiosis to health officials and many health professionals do not include Cryptosporidiosis in the differential diagnosis of patients with diarrhea, many cases undoubtedly go unrecognized. Infectivity is high, with as little as 10 oocysts causing illness (34). A single fecal accident in even a large pool is sufficient to cause illness in a great number of people (35,36). It has been estimated that 1 ml of feces can contain as much as 5 x 10 oocysts (34). If we assume that a child has a loose bowel movement of 150 ml into a 1 00,000-gallon swimming pool, this would result in a concentration of 20 oocysts/mll of pool water {(5x107).( 15)I(1x105).(29.5).( 128)). If we further assume that a swimmer swallows 10 ml of water, he or she would ingest 200 oocysts, a dose capable of causing infection.

In 1988, an outbreak in Los Angeles County involved 60 cases of Cryptosporidiosis resulting from individuals swimming in a 100,000-gallon swimming pool in which there was a single fecal accident (35). The overall attack rate was 73%. The illness was observed in several separate groups of people with no common link other than using the swimming pool. Length of exposure and immersing the head under water were risk factors in contracting the disease. In a second outbreak reported in British Columbia, 66 clinical and 23 confirmed cases of Cryptosporidiosis were shown to have resulted from swimming in a 70,000-gallon swimming pool (36). The children's pooi was closed when it was found to be the probable source of infection. The pool in question had experienced an increase in the number of fecal accidents from the usual one or two per month to one or two per week, with three known diarrheal episodes. A third outbreak of Cryptosporidiosis occurred at a swimming pool in Dane County, Wisconsin, in 1993 (37). A fourth outbreak of Cryptosporidiosis occurred at a wave pool in Lane County, Oregon, in 1992 (38). A fifth outbreak of Cryptosporidiosis occurred at a swimming pool in Great Britain in 1988 (39).

It has been demonstrated that rapid and high rate sand filters cannot be relied on to effectively remove oocysts. While diatomaceous earth filters can remove oocysts, it generally takes at least three turnovers of pool water to eliminate 95% of the pollutants (40,4 1). Cryptosporidium oocysts are extremely resistant to chlorine, and it has been reported that a CT value of 9,600 is required to inactivate them (28). Another study indicated that exposure of oocysts to 80 mg/L of free chlorine at 250C for 90 minutes produced a 99% inactivation rate resulting in a CT value of 7,200 (6).

Many illnesses, some very serious, can be transmitted through improperly maintained swimming pools and spa pools. With the exception of Cryptosporidium, current information indicates that most pathogens in pool water can be inactivated or at least controlled with a minimum free chlorine residual of 2.0 mg/L or an ORP of 650 mV. It is apparent that chlorine levels, needed to rapidly deactivate Cryptosporidium 00-cysts, cannot be maintained while bathers are using the pool. Consequently, it is difficult to decide what course of action to take when feces contaminates a pool. Fecal accidents occur rather frequently at public pools, and given the many variables involved, it is often difficult to determine the extent of the threat to public health. The size of the pool, the type of filtration system, whether or not the individual who contaminated the pool is infected, how much fecal material was released, and whether it dissolved in the water are all factors to consider in deciding on whether to close a pool and how long to keep it closed. Closing a large municipal pool or water theme park pool every time there is an incidence of fecal contamination can be extremely disruptive and the risk of leaving the pool open must be weighed against the threat to public health.

The following guidelines can be used to reduce the potential for transmission of waterborne diseases at public pools. The recommended disinfectant levels here are for chlorine. Free chlorine levels are recommended with the assumption that all other chemical parameters are at their proper levels. An ORP measurement can be used to confirm disinfectant efficacy. Other disinfectants may be used provided they afford the same effectiveness as chlorine. Supplemental disinfectants may be used in conjunction with chlorine, e.g., ozone is excellent at inactivating Cryptosporidium oocysts (CT of 4.1 mg. min/liter) (42).

1) All persons maintaining or operating public pools should be properly trained. A good example of a training program is the Certified Pool/Spa Operator training offered by the National Swimming Pool Foundation. Personnel should keep up-to-date with new technologies and developments in pool care.

2) The recirculation and filtration system should be maintained to provide maximum filtration at all times. Gauges and flow meters should be frequently monitored and filters promptly cleaned when required. Backwash water and filtering media should be disposed to a sanitary sewer or in another approved manner.

3) Pool water should always be kept in chemical balance. The pH should be maintained between 7.2 and 7.8, the alkalinity between 80 and 150 ppm, and calcium hardness between 200 and 400 ppm. Stabilizer or cyanuric acid, if used, should be kept at or below 40 ppm. Free chlorine residual and pH should be tested at least twice daily, and in heavily used pools, hourly. A log should be kept of all chemical tests and maintenance procedures performed.

4) The free chlorine residual in swimming pools should be continually maintained at a minimum of 2.0 mg/L, and in spa and wading pools at 3.0 mg/L. ORP levels should be maintained at or above 650 mV.

5) All pools using chlorine as a disinfectant should be super chlorinated when combined chlorine levels exceed 0.5 mg/L. Chlorine levels should be increased to 10 times the combined chlorine level to achieve breakpoint chlorination. This should be done while the pool is not in use.

6) Instructors, lifeguards, and the general public should not use the pool if they are suffering from a diarrheal type illness or other communicable disease. Diaper-age children or children who are not toilet-trained should be prohibited from using the pool. As an alternative to excluding non-toilet-trained children from using the pool, special "swimsuit diapers" may be used, although their effectiveness is questionable.

7) In the event of fecal contamination, the following procedures should be performed:

a. All pool users should be instructed to exit the pool, and the pool should be closed.

b. As much fecal material as possible should be removed from the pool. If the pool is vacuumed, waste should be directed to a sanitary sewer or other approved waste disposal system and not through the filtration system. The vacuum equipment should be cleaned and disinfected before reuse.

c. The free chlorine residual should be raised to 20.0 mg/L, and the pH adjusted to between 7.2 and 7.5. This chlorine level should be maintained for at least nine hours. This is equivalent to an approximate CT value of 10,000. A higher or lower chlorine residual can be used, provided a CT value of 10,000 is achieved.

d. The filtration system should be operated for a minimum of three to four turnovers. At public swimming pools, the turnover rate, or the amount of time it takes to filter all of the water in the pool, is usually six to eight hours; therefore, three turnovers can be achieved within 24 hours. In general, filters are more effective when they are slightly dirty. If the filter is not in need of backwashing at the time of the fecal accident, do not backwash the filter.

e. After three to four turnovers, thoroughly backwash the filter.

f. If the pool is a low volume pool, such as a spa pool or wading pool, drain the pool at this point.

g. Disinfect the filter tank and filter media with a 20:1 solution of sodium hypochlorite (20 parts water to I part 12% sodium hypochlorite).

h. Restart the filtration system. Neutralize any excessively high chlorine residual with sodium thiosulfate. Balance the water if necessary and reopen the pool.