Calculate pool closure time after a fecal incident using CDC and Model Aquatic Health Code (MAHC) guidelines. Accounts for CYA impact on chlorine effectiveness and secondary disinfection systems.
Enter your current water parameters and incident details below to get a closure time estimate with a step-by-step action plan.
Diarrheal incidents require much longer closure due to Cryptosporidium risk
Current FC reading
0 for indoor/unstabilized pools
Must be 7.5 or lower during remediation
UV or ozone systems rated for Cryptosporidium inactivation
Used for chemical dosing estimates
25 min
Formed stool protocol
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Your closure time is manageable.
The Centers for Disease Control and Prevention (CDC) and the Model Aquatic Health Code (MAHC) provide specific protocols for responding to fecal incidents in commercial and public swimming pools. These guidelines are designed to protect swimmers from recreational water illnesses (RWIs), particularly Cryptosporidium, which is highly resistant to chlorine.
Every commercial pool operator should have a written fecal incident response plan. This calculator helps you determine the required closure time based on your specific water chemistry and disinfection systems, so you can respond quickly and confidently.
The CDC distinguishes between two types of fecal incidents, each requiring a different response:
Formed stool is considered lower risk because Cryptosporidium is far less likely to be present. The CDC protocol calls for:
Diarrheal incidents are treated as high-risk because loose stool is more likely to contain Cryptosporidium. The response is significantly more aggressive:
CT value stands for "Concentration x Time" and is measured in mg·min/L (milligrams per liter multiplied by minutes of contact time). It represents the total disinfection dose the water receives.
For Cryptosporidium inactivation, the CDC recommends a CT value of 15,300 mg·min/L at 77°F (25°C), pH 7.5, with 1 ppm CYA or less. This is the standard used in this calculator.
The formula is straightforward:
At 20 ppm FC with no CYA: Time = 15,300 / 20 = 765 minutes (12.75 hours). This is why diarrheal incidents often result in overnight pool closures.
Cyanuric acid (CYA, also called stabilizer or conditioner) protects chlorine from UV degradation in outdoor pools, but it comes with a significant trade-off: CYA dramatically reduces the amount of free chlorine available to kill pathogens. This is especially critical during fecal incident remediation.
The relationship between CYA and effective chlorine is approximated by:
Effective FC = Free Chlorine / (1 + CYA / 25)
This means:
As you can see, even moderate CYA levels can make closure times impractical for commercial operations. This is why many health departments recommend keeping CYA below 15 ppm in commercial pools, and some jurisdictions ban CYA entirely in commercial aquatic facilities.
Secondary disinfection systems that are rated for Cryptosporidium inactivation can significantly change the response protocol:
Medium-pressure UV systems rated at 40 mJ/cm² or higher can inactivate Cryptosporidium as water passes through the UV chamber. When a UV system is present and operational:
Ozone is a powerful oxidizer that can inactivate Cryptosporidium. Similar to UV, facilities with properly rated ozone systems may follow reduced protocols for diarrheal incidents. However, ozone must be properly dissolved and have adequate contact time to be effective.
Many states and local jurisdictions require commercial pool operators to notify the health department after certain fecal incidents. General requirements include:
Check with your local health department for specific notification requirements in your jurisdiction. Requirements vary significantly by state and county.
Thorough documentation protects both pool operators and swimmers. For every fecal incident, record:
Keep these records on file for at least three years, or as required by your local health code.
No. The pool must remain closed to all swimmers during the entire remediation period. At elevated chlorine levels (especially 20 ppm for diarrheal incidents), the water is unsafe for swimming and can cause skin and eye irritation. Do not allow anyone to enter the water until the CT value has been achieved and free chlorine has returned to safe swimming levels (typically below 5 ppm, or per your local code).
When in doubt, treat it as a diarrheal incident. The CDC recommends erring on the side of caution. If the stool is not clearly solid and formed, follow the diarrheal incident protocol. It is always better to over-respond than to risk exposing swimmers to Cryptosporidium or other pathogens.
Cryptosporidium oocysts have a thick outer shell that makes them highly resistant to chlorine disinfection. While most bacteria and viruses are killed within seconds to minutes at normal pool chlorine levels, Cryptosporidium can survive for over 10 days at 1 ppm free chlorine. This is why the CDC requires such high CT values (15,300 mg·min/L) for crypto inactivation, resulting in extended closure times.
In some cases, partial or full draining and refilling may be faster and more practical than hyperchlorination, especially if your pool has high CYA levels. If your CYA is above 15 ppm and you are dealing with a diarrheal incident, the closure time with hyperchlorination alone may be impractical (multiple days). Draining and refilling with fresh water, then treating at target chemistry, can be faster. Consider water costs, refill time, and rebalancing time when making this decision.
Yes, keep the filtration system running continuously during remediation. The filter helps distribute chlorine evenly throughout the pool and can physically remove some Cryptosporidium oocysts, especially with DE (diatomaceous earth) filters. However, filtration alone is not sufficient, and the full CT value must still be achieved. After remediation, backwash or clean the filter thoroughly.
After achieving the required CT value, you need to bring free chlorine back to safe swimming levels (typically 1-5 ppm per local code). Options include: allowing chlorine to naturally dissipate (can take hours to days), using sodium thiosulfate (chlorine neutralizer) to chemically reduce FC, or diluting with fresh water. Always test and confirm FC is at safe levels before reopening. UV systems can also help reduce chlorine levels faster.
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