Cryptosporidium (Crypto) is the single most challenging RWI pathogen for pool operators. It's chlorine-resistant, common, easily transmitted, and responsible for the majority of treated-water outbreaks tracked by the CDC. Every pool operator — public or private — needs to know the protocol.
Why Crypto is different
Crypto forms oocysts— tough, resilient egg-like structures with an outer shell that shrugs off normal chlorine residuals. At 1 ppm free chlorine, Crypto requires over 15,000 ppm·minutes of contact to inactivate. That's 10+ days of exposure at 1 ppm.
No pool can maintain 1 ppm FC for 10 days of Crypto exposure. So the answer isn't “add more time” — it's “add dramatically more chlorine, briefly.”
Transmission and epidemiology
- A diarrheal person sheds up to one billion oocysts in a single bowel movement
- As few as 10 oocysts can cause infection
- Incubation period: 2–10 days
- Diarrhea lasts 1–3 weeks in healthy adults; longer in immunocompromised patients
- Infected people can continue shedding oocysts for up to 2 weeks after symptoms resolve
- Per CDC guidance, infected people should not return to any pool for 2 weeks after diarrhea stops
The CDC hyperchlorination protocol
- Close the pool. No swimmers.
- Remove CYA if present. Partial drain/refill to reduce CYA to 15 ppm or lower. CYA slows the kill.
- Adjust pH to 7.5 or lower.
- Raise free chlorine to 20 ppm using unstabilized chlorine (liquid hypo or cal-hypo).
- Hold for 12.75 hours at 20 ppm FC. Check and redose every 2–4 hours — chlorine demand will be high.
- Verify the math: CT = 20 ppm × 765 min = 15,300 ppm·min. This meets the published 3-log Crypto inactivation target.
- Replace the filter media or chemical-soak cartridges. Oocysts accumulate in filters and can reintroduce infection.
- Lower FC to normal before reopening. Test patron-safe levels.
Filter-enhanced filtration during response
Standard sand and cartridge filters can't catch Crypto oocysts (they're ~4 microns). To remove them from the water column:
- Add a coagulant or polymeric clarifier to clump oocysts into larger, filterable particles
- Run the filter continuously during hyperchlorination
- Vacuum to waste any sediment that settles
- Consider UV or ozone supplementation permanently if Crypto is a recurring risk (splash pads, high-volume public facilities)
Prevention: the “Healthy Swimming” basics
- Signage requiring pre-swim showers (removes perianal fecal material)
- Frequent bathroom breaks for young children
- “Don't swim with diarrhea” posted clearly
- Swim-diaper-only zones for kiddie pools
- For high-risk venues (splash pads, therapy pools): add UV or ozone as secondary disinfection
Residential vs. commercial
For residential pools, the risk is low but not zero. If a family member has had diarrhea, keep them out of the pool for 2 weeks post-recovery. If contamination is suspected, the same hyperchlorination protocol applies — just at smaller volumes. For commercial pools, this is regulatory territory: your county health department may require closure, testing, and documented response before reopening.