4.2. Use of CYA/FC Ratio
The HOCl concentration is a more accurate predictor of disinfection rate, but pool operators and regulators are generally not prepared to calculate HOCl concentration from pH, FC, and CYA concentrations. A surrogate for HOCl is to specify an upper limit to the CYA/FC ratio instead of specifying independent limits for FC and CYA when CYA is present. The model indicates that reductions in infection risk are possible by reducing the allowed CYA/FC ratio. However, the reductions in infection risk need to be balanced with the practical operation of public pools. Restricting the ratio to too low of a value will effectively eliminate the use of chlorinated isocyanurates from public pools and result in greater chlorine consumption. Furthermore, if the ratio is too low, operators may have difficulty maintaining a chlorine residual.
The current MAHC
(3rd Edition) allows up to 90 mg/L CYA with as low as 2 mg/L FC in normal risk aquatic venues. resulting in a de facto ratio of 45:1 CYA:FC. Swimmers using pools with a CYA/FC ratio of 45 are approximately two and five times as likely to become infected with Giardia
and E. coli
O157:H7, respectively, as swimmers using pools with a CYA/FC ratio of 20 (Table 7
). The annual infection risk for E. coli
O157:H7 with a CYA/FC ratio of 45 is already very low (<0.001%), so the 5-fold difference in infection risk between ratios of 45 and 20 is an insignificant absolute reduction in infection risk. Although the absolute risk is low for E. coli
O157:H7, if a swimmer were to become infected with any of the three pathogens analyzed in this study, E. coli
O157:H7 presents the most danger. E. coli
O157:H7 has a greater ability to cause chronic sequelae and mortality compared to Giardia
. The burden of disease (DALY) for E. coli
O157:H7 is about 25 times that of Giardia
The higher burden of an E. coli
O157:H7 infection on human health, therefore, should be considered when interpreting the low absolute risk. If a single swimmer becomes infected with E. coli
O157:H7 from swimming in a pool, they could die or be disabled for the remainder of their life.
4.3. Risk Assessment
Given the sources of model error described in the Results section, it was not possible to develop a model that provides accurate estimates of absolute risk. However, the modeling approach used herein is believed to provide meaningful estimates of relative risk.
The 98% risk of Giardia infection, 1% risk of Cryptosporidium infection, and 0.5% risk of E. coli O157:H7 infection in the absence of chlorine emphasize the importance of maintaining a chlorine residual. The high rate of Giardia infection is not realistic but results from the assumptions used in this model: concentration of Giardia in feces, percent of population infected with Giardia, every visit to a pool with no chlorine and constant 24 h/day 7 day/week high bather load, and no filtration of cysts.
The effective risk for E. coli
) is significantly reduced by even small amounts of chlorine. Note that an infection may not result in symptoms, as there are asymptomatic carriers. Cryptosporidium
is only mildly chlorine susceptible. But its risk is generally lower than Giardia
due to the significantly lower Cryptosporidium
pathogen fecal concentration and percent infected values used in this model. There are limited data available that provide pathogen concentrations in feces, and even with the limited data available, the sensitivity analysis shows the differences in risk due to the shedding rate uncertainty are larger than the differences in risk between Giardia
. The discrepancy between risk calculations in this study and cryptosporidiosis outbreak statistics may be due to the limited data available for model calculations or due to any number of factors that are outside the scope of this model, such as the effect of AFRs on outbreak statistics (Supplementary Materials S20
), outbreak reporting bias, and pathogen removal by means other than chlorine disinfection.
4.4. Setting a Standard
The goal of this work was to provide a sound scientific basis for recommending a limit on the concentration of CYA in public swimming pools. Meeting this objective was difficult because no guidelines for acceptable risk exist for treated recreational water. We, thus, based our recommendation for a CYA/FC ratio in swimming pools on CYA/FC ratios calculated by analyzing several risk and pool operations scenarios. The remainder of this section outlines those scenarios and describes how each was considered in the determination of a CYA/FC standard for swimming pools.
The CYA/FC ratio of 45:1 was considered a starting point because this ratio corresponds to the highest CYA and lowest FC concentrations suggested in the current version of the MAHC. However, arguments for maintaining the current MAHC 45:1 ratio are based on the absence of documented cases where this or higher ratios have resulted in a demonstrated health hazard, such as the inability to adequately control pathogen concentrations in pool water. CYA is rarely measured during an outbreak, so there are no published data to confirm or deny that CYA has been a contributing factor in outbreaks or that lower ratios would prevent future outbreaks.
The maximum concentration of CYA recommended by the MAHC and maximum concentration of FC required by the U.S. EPA were considered in establishing a CYA/FC ratio. The FC concentration should be increased as the CYA concentration increases. Thus, as the CYA concentration nears the current MAHC limit of 90 mg/L, the FC concentration should also increase to the maximum allowed, which is 4 mg/L according to the U.S. EPA. This would result in a CYA/FC ratio of 22.5 (90/4). Based on this argument, 22.5 would be consistent with these limits. However, a ratio of 22.5 would be more difficult to administer than a rounded value. Furthermore, this argument is not based on the model risk calculations but rather is a logical argument based on current limits from EPA and the MAHC.
The lowest amount of CYA needed to achieve chlorine stability was considered. Arguments for the prohibition of CYA in indoor pools and using as little CYA as possible in outdoors pools assume that it is prudent to reduce risk wherever practical. It is certainly practical to add as little CYA as needed to obtain the desired amount of chlorine stability with unstabilized sanitizers such as sodium hypochlorite or calcium hypochlorite. A CYA/FC ratio of 5–10 provides most of the stabilization with diminishing returns in additional stabilization at higher CYA/FC ratios [69
]. The benefits of any incremental increase in stability should be balanced against reduced efficacy with additional CYA.
Since monochloramine is known to be a less effective (slower) disinfectant than free chlorine, the free chlorine efficacy should be maintained in pool water at least equivalent to that of monochloramine. Based on the Ct values for free chlorine and monochloramine for Giardia
at pool conditions (Appendix E in U.S. EPA [70
]), a limiting CYA/FC ratio of 15 would ensure the inactivation time for Giardia
in a stabilized pool is at least equivalent to 1 mg/L monochloramine (Supplementary Materials S21
). However, monochloramine and free chlorine have very different relative efficacies for different microorganisms, even for closely related strains. Rose [71
] and O’Connell [73
] compared the efficacy of monochloramine and free chlorine for several different bacteria. Based on their Cts at 25 °C for a 3-log reduction of twelve species/strains of bacteria, the median CYA/FC ratio that provides equivalent efficacy to monochloramine for bacteria is 99. Based on EPA’s Ct tables [70
] for 3-log reduction of viruses by free chlorine or monochloramine at 25 °C, the CYA/FC ratio that provides equivalent efficacy to monochloramine for viruses is 253. The ratio for Giardia
of 15 represents the ratio for the highest risk organism as seen in these model calculations and will provide better protection against bacteria and viruses.
Infection probabilities from this study were compared to the annual risk of infection for drinking water (1/10,000) and per-season risk for untreated recreational water (36/1000) acceptable to the EPA to estimate a CYA/FC ratio that corresponded to infection probabilities acceptable to the public (i.e., “acceptable risk”). It is not reasonable that swimming pools be required to obtain an annual infection risk of 1/10,000, since swimming is a voluntary activity and drinking water is not. There is no guidance in the MAHC
on acceptable microbial quality in swimming pool water, although maximum acceptable microbial concentrations are provided in the WHO Guidelines for pools [74
]. Swimmer exposures in treated and untreated recreational water (pool visit frequency, swim duration, and water ingestion) are more aligned than drinking water exposures. Therefore, the U.S. EPA acceptable risk of 36/1000 for untreated recreational waters is the best available comparator for establishing a CYA/FC risk-based standard for swimming pools. It is worth noting that U.S. EPA based untreated recreational water acceptable risk on indicator organisms (E. coli
) and NGI (National Epidemiological and Environmental Assessment of Recreational Water-GI Illness) rather than a specific etiological agent or gastrointestinal illness like giardiasis. If the U.S. EPA study [67
] were replicated in swimming pools, E. coli
, and fecal coliforms would likely be below detection limits in pools with acceptable FC levels. Therefore, a different set of pathogens (such as Giardia
) should be used to define risk-based water treatment standards for treated swimming pools.
Establishing a risk-based CYA/FC standard based on the Giardia
curve (Figure 7
) reflects a worst-case risk scenario since the probability of infection is higher for Giardia
than for Cryptosporidium
or E. coli
O157:H7. The Giardia
curve is, thus, the most appropriate for establishing a risk-based CYA/FC standard. In Figure 7
, the untreated recreational water acceptable risk (36/1,000) line crosses the Giardia
curve at a CYA/FC ratio of 22, suggesting a ratio of 22 is a reasonable CYA/FC standard in swimming pools. Although the sensitivity analysis showed the Giardia
curve can shift up or down for different values of each input parameter, resulting in the curve crossing the untreated recreational water acceptable risk line at different CYA/FC ratios, the curve is based on the selected input parameters and represents conservative operating conditions for a public pool. A CYA/FC ratio of 22 could be considered as a standard in swimming pools despite the uncertainty associated with using point-estimates (maximum, minimum, mean, etc.) in the risk model.
If the maximum CYA/FC ratio in the MAHC would be reduced, the arguments above provide several possible values for the recommended CYA/FC ratio, namely, 5–10, 15, 22, and 22.5. The value chosen should be a round number for convenience of use by pool operators and public health officials. A maximum CYA/FC ratio of 20 is recommended as a compromise between the various suggested values and provides a modest reduction in infection risk from the currently allowed value of 45 without imposing major changes in pool operations.
Using a ratio is slightly more complicated than a simple limit on CYA but makes more sense for risk reduction. Using a round value of 20 makes implementation of a ratio easy; thus, if FC is 1 mg/L, CYA should not exceed 20 mg/L, and if FC is 2 mg/L, CYA should not exceed 40 mg/L, etc. (Table 9
The results in Table 7
indicate that lowering the maximum allowed CYA/FC ratio from 45 to 20 would reduce the annual probability of Giardia
infection by about a factor of two and E. coli
infection by about a factor of five. The U.S. EPA criteria for beaches are based on an annual illness rate of 3.6 × 10−2
, which has a history of acceptance by the public. With a ratio of 20:1, the U.S. EPA criteria are met for the three pathogens included in this study.
Regulations with low FC < 1.0 mg/L, such as Deutsches Institut für Normung (DIN)19643 used in Germany and some other countries in Europe, do not use any stabilized chlorine sources or any added cyanuric acid; 19643-1 with a low of 0.3 FC with no CYA and a pH up to 7.5 has the same HOCl concentration (HOCl ~0.15 mg/L) as a pool with a CYA/FC ~3.9 and so is well within our recommendation of CYA/FC ≤ 20 (HOCl ≥ 0.02 mg/L) when CYA is present. If there are countries using 0.3 to 1.0 mg/L with CYA, then they should consider raising their required minimum FC based on the CYA level.