Chlorine inactivates most pathogens that cause diarrheal disease in humans. The tables below detail the effectiveness of chlorine against disease-causing bacteria, viruses, and protozoa. The Ct factor can be used to compare the effectivenessof chlorine against different pathogens, and is calculated by multiplying the concentration of chlorine (in mg/L or ppm/parts per million) needed to inactivate a certain percentage of the pathogen by the time (in minutes) the pathogen was exposed to that concentration of chlorine. Higher Ct factors indicate relatively higher tolerance to chlorine, while lower Ct factors indicate relatively low tolerance to chlorine. The Ct factors shown in the tables below were calculated from data in peer-reviewed research articles. The efficacy of disinfection using chlorine is dependent not only on the pathogen itself, but also on the pH and temperature of the water. In general, disinfection is more effective at higher temperatures and lower pH. Attachment to particulate matter, aggregation, encapsulation of the pathogen, ingestion by protozoa, and water turbidity may also affect chlorine efficacy. The results below reflect conditions of low water turbidity (<1 NTU), chlorine demand-free water systems. The Safe Water System accounts for variations in water quality by doubling the chlorine used for turbid drinking water. The maximum Ct factor created by adding 1.9 mg/L sodium hypochlorite to water for 30 minutes (the minimum chlorine dosage recommended by the Safe Water System for clear, non-turbid, demand-free water) is 56 mg·min/L 1. For turbid water, the dose is doubled to 3.8mg/L, with a resulting maximum Ct factor of 112 mg·min/L.



Inactivation of Bacteria Pathogen From WHO guidelines for drinking

water quality Concentration of chlorine (mg/L) Time of chlorine exposure (min) Ct

factor % Inactivation Variables affecting Ct factor Health significance Persistence in water supplies Tolerance to chlorine Relative infectivity Temp (C) pH Burkholderia pseudomallei 2 Low May multiply Low Low 1.0 60 60 99% 22.0-

25.0 6.25-

7.0 Campylobacter jejuni 3 High Moderate Low Moderate 0.1 5 0.5 99-99.9% 25.0 8.0 Escherichia coli 4 High Moderate Low Low 0.5 <0.5 <0.25 99.99% 23.0 7.0 E. coli

(entero-

hemhorrhagic) 4 High Moderate Low High 0.5 <0.5 <0.25 99.98-99.99% 23.0 7.0 Salmonella typhi 5 High Moderate Low Low 0.05 20 1 99.2% 20-25 7.0 Shigella

dysenteriae 5 High Short Low Moderate 0.05 <1 <0.05 99.9% 20-25 7.0 Shigella sonnei 6 – – – – 0.5 1 0.5 99% 25.0 7.0 Vibrio cholerae (smooth strain) 7 High Short Low Low 0.5 <1 <0.5 100% 20.0 7.0 Vibrio cholerae (rugose strain) 7 High Short Low Low 2.0 20 40 99.99% 20.0 7.0 Yersinia enterocolitica 8 High Long Low Low 1.0 >30 >30 82-92% 20.0 7.0

Inactivation of Viruses Pathogen From WHO guidelines for drinking

water quality Concentration of chlorine (mg/L) Time of chlorine exposure (min) Ct

factor %

Inactivation Variables affecting Ct factor Health significance Persistence in water supplies Tolerance to chlorine Relative infectivity Temp (C) pH Enteroviruses Coxsackie A 9 High Long Moderate High 0.46-0.49 0.3 0.14-0.15 99% 5.0 6.0 Coxsackie B 9 High Long Moderate High 0.48-0.50 4.5 2.16-2.25 99% 5.0 7.81-

7.82 Echovirus 9 High Long Moderate High 0.48-

0.52 1.8 0.86-

0.94 99% 5.0 7.79-

7.83 Hepatitis A 10 High Long Moderate High 0.41 <1 <0.41 99.99% 25.0 8.0 Poliovirus 11 High Long Moderate High 0.5 12.72 6.36 99.99% 5.0 6.0 Adenoviruses 11 High Long Moderate High 0.17 4.41 0.75 99.99% 5.0 7.0 Noroviruses 11 High Long Moderate High 1.0 0.07 0.07 99.99% 5.0 7.0 Rotavirus 12 High Long Moderate High 0.20 0.25 0.05 99.99% 4.0 7.0

Inactivation of Protozoa Pathogen From WHO guidelines for drinking

water quality Concentration of chlorine (mg/L) Time of chlorine exposure (min) Ct

factor %

Inactivation Variables affecting Ct factor Health significance Persistence in water supplies Tolerance to chlorine Relative infectivity Temp (C) pH Entamoeba histolytica 13 High Moderate High High 2.0 10 20 99% 27-30 7 Giardia intestinalis 14 High Moderate High High 1.5 10 15 99.9% 25.0 7.0 Toxoplasma gondii 15 High Moderate High High 100 1440 >144,000* – 22.0 7.2 Cryptosporidium parvum 16 High Long High High 80 90 15,300* 99.9% 25.0 7.5 * Toxoplasma and Cryptosporidium oocysts are highly resistant to chlorine disinfection. Chlorine alone should not be expected to inactivate these pathogens in drinking water. Filtering water supplies with a 1 micron absolute filter is recommended to physically remove the oocysts before chlorination if these pathogens are of concern. Cyclospora cayetanensis oocysts are also thought to be highly chlorine tolerant but there are no Ct values derived at this time.

References

Lantagne D. Sodium hypochlorite dosage for household and emergency water treatment. JAWWA. 2008;Aug 100(8):106-19. Howard K, Inglis TJ. The effect of free chlorine on Burkholderia pseudomallei in potable water. Water Res. 2003;37(18):4425-32. Blaser MJ, Smith PF, et al. Inactivation of Campylobacter jejuni by chlorine and monochloramine. Appl Environ Microbiol. 1986;51(2):307-11. Zhao T, Doyle MP, et al. Chlorine inactivation of Escherichia coli O157:H7 in water. J Food Prot. 2001;64(10):1607-9. Butterfield CT, Wattie W, et al. Influence of pH and temperature on the survival of coliforms and enteric pathogens when exposed to free chlorine. Public Health Rep. 1943;58(51):1837-1880. King CH, Shotts EB, et al. Survival of coliforms and bacterial pathogens within protozoa during chlorination. Appl Environ Microbiol. 1988;54(12):3023-33. Morris JG, Sztein MB, et al. Vibrio cholerae O1 can assume a chlorine-resistant rugose survival form that is virulent for humans. J Infect Dis. 1996;174(6):1364-8. Paz ML, Duaigues MV, et al. Antimicrobial effect of chlorine on Yersinia enterocolitica. J Appl Bacteriol. 1993;75(3):220-5. Engelbrecht RS, Weber MJ, et al. Comparative inactivation of viruses by chlorine. Appl Environ Microbiol. 1980;40(2):249-56. Grabow WO, Gauss-Muller V, et al. Inactivation of hepatitis A virus and indicator organisms in water by free chlorine residuals. Appl Environ Microbiol. 1983;46(3):619- 24. Thurston-Enriquez JA, Haas CN, et al. Chlorine inactivation of adenovirus type 40 and feline calicivirus. Appl Environ Microbiol. 2003;69(7):3979-85. Vaughn JM, Chen YS, et al. Inactivation of human and simian rotaviruses by chlorine. Appl Environ Microbiol. 1986;51(2):391-4. Stringer RP, Cramer WN, et al. Comparison of bromine, chlorine, and iodine as disinfectants for amoebic cysts, p. 193-209. In J. D. Johnson (ed.), Disinfection: water and wastewater. Ann Arbor Science Publishers, Inc. Ann Arbor, Mich. Jarroll EL, Bingham AK, et al. Effect of chlorine on Giardia lamblia cyst viability. Appl Environ Microbiol. 1981;41(2):483-7. Wainwright KE, Miller MA, et al. Chemical inactivation of Toxoplasma gondii oocysts in water. J Parasitol. 2007;93(4):925-31. Shields JM, Hill VR, Arrowood MJ, Beach MJ. Inactivation of Cryptosporidium parvum under chlorinated recreational water conditions. J Water Health. 2008;6(4):513–20.

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