CT values are how microbiologists actually measure disinfection. CT = concentration × contact time. It's the product of chlorine level (ppm) and how long that chlorine touched the pathogen (minutes). Every pathogen has a required CT — the total exposure needed to inactivate 99.9% of it.
CT values at pH 7.5 (free chlorine, no CYA)
| Pathogen | Required CT (ppm·min) | At 1 ppm FC, kills in… |
|---|---|---|
| E. coli | ~0.04 | < 1 minute |
| Hepatitis A | ~16 | 16 minutes |
| Giardia lamblia | ~45 | 45 minutes |
| Cryptosporidium parvum | ~15,300 | 10.6 days |
This is why Cryptosporidium is a different kind of emergency. You can't kill it in a reasonable time at normal chlorine levels. Crypto requires hyperchlorination: 20 ppm FC held for 12.75 hours at pH 7.5 with no CYA. That gives CT = 20 × 765 = 15,300.
Why CYA slows everything down
Cyanuric acid protects free chlorine from UV by loosely binding to it. The bound form still disinfects, but more slowly. A pool with 40 ppm CYA has a CT curve that's roughly 3–5× slower than an unstabilized pool at the same FC reading. This is why CYA must be reduced to zero before a Crypto hyperchlorination— you're trying to maximize CT, and CYA sabotages it.
CT math in the field
For fecal-incident response, you typically need to demonstrate you've achieved a specific CT value before reopening. The math:
Required hold time (minutes) = Required CT ÷ Average FC during hold
Example: Formed stool, Giardia CT required = 45 ppm·min. You hold FC at 2 ppm with pH ≤ 7.5. Required hold: 45 ÷ 2 = 22.5 minutes.
(In practice, CDC guidance is at least 25 minutes for formed-stool incidents — a small margin for field variability.)
The turnover connection
CT only counts when all the water has circulated through the contact zone. For a homogeneous disinfection CT, you need multiple turnoversduring the hold period. This is why the CDC uses 12.75 hours for Crypto (at a 6-hour turnover, that's ~2 turnovers + safety margin).
ORP and CT
ORP (oxidation-reduction potential) measures disinfection activity in millivolts and correlates with HOCl concentration — but it is not a ppm or CT substitute. Two pools at 650 mV can have very different actual kill-rates depending on CYA, pH, and temperature. Use ORP for automated control, not for fecal-incident response decisions.