Two numbers from two studies, both about ordinary free chlorine in clean water. Killing 99.99% of E. coli O157:H7 takes a CT of 0.13. Killing 99.9% of Cryptosporidium oocysts takes a CT between 10,400 and 15,300. Same oxidant, same water, a difference of tens of thousands of times.
That gap is why the picture looks so strange from the shopper’s side: chlorine in a processing flume does something real to bacteria, while a parasite passes through it as if it were not there.
CT: one quantity that explains most of it
CT is concentration multiplied by time: milligrams per liter times minutes of contact. Water treatment measures disinfection this way because ten milligrams for one minute and one milligram for ten minutes deliver the same dose of oxidant.
The arithmetic settles the kitchen question immediately. Put chlorine in a bowl at 10 mg/L and soak the salad for five minutes and the CT comes to 50. For E. coli in water that is ample. For an oocyst it is two hundred times short.
Public pools apply the same arithmetic literally. After a diarrheal incident the water is hyperchlorinated, and the CDC figure for that is a CT of 9,600. The authors of the study behind the 10,400 and 15,300 values note that the recommended figure sits below what they measured on young oocysts. Either way the order of magnitude holds: three logs takes tens of thousands, and a pool on that regime stays closed for hours.
Why a bacterium gives up in seconds
Chlorine in water works as an oxidant: it destroys whatever it reaches. A bacterial cell is separated from the water by a thin envelope the oxidant crosses, and inside there is plenty to wreck.
How much the chemical form matters shows up in the same study that produced the 0.13 figure. Monochloramine took the same E. coli down by the same 99.99%, but needed a CT of about 9.2, roughly seventy times slower. The difference sits in the oxidant, not in the bacterium.
Why an oocyst holds out
An oocyst is a capsule with its contents packed and ready rather than a cell in the usual sense. A Cornell group took the Cryptosporidium wall apart layer by layer in 2010: an outer electron-dense layer, a translucent middle one, two inner dense layers and a suture. The composition includes carbohydrates, medium- and long-chain fatty acids and aliphatic hydrocarbons. Their conclusion was a wall that is variably impermeable and resistant to most environmental pressures, and they associate the way its permeability shifts with temperature with waxy hydrocarbons in the translucent layer.
An impermeable wall closes the question. The oxidant has no way in, and from outside there is little to attack: the capsule is built to survive months in soil and water.
Cyclospora is built differently. The FDA description calls its wall colorless, thinner than a micrometer and bilayered. The four-layer wall belongs to Cryptosporidium, and it is that shell ultrasound cracks in the studies covered in our piece on ultrasonic cleaners. The thin Cyclospora wall shrugs off chlorine all the same.
This is a property of the whole group rather than of one parasite. The FDA and USDA review states it plainly: Cyclospora, and coccidia in general, are highly resistant to sanitizers and disinfectants. That is why the guidance for these parasites carries no line about chlorine concentration and carries one about controlling sources of contamination in the field and the packing house.
The experiment with all three on one leaf
In 2008 a team in Georgia took basil and lettuce, inoculated them with Cryptosporidium, Cyclospora and E. coli O157:H7, and treated everything for twenty minutes with gaseous chlorine dioxide at 4.1 mg/L. One product, one treatment.
| Organism | Reduction, basil | Reduction, lettuce |
|---|---|---|
| E. coli O157:H7 | 2.45 to 3.97 log | |
| Cryptosporidium | 2.6 log | 3.31 log |
| Cyclospora | sporulation unaffected | |
E. coli and Cryptosporidium both gave ground. Cyclospora did not move at all. It is the cleanest illustration of what the CDC states in flat language: routine chemical disinfection does not kill Cyclospora oocysts.
There is no CT for Cyclospora
Cryptosporidium has numbers because you can put it on a cell culture and see whether it infected anything. Nobody has those numbers for Cyclospora, and the reason lies in the parasite: there is no animal model and no culture system, and the only research material is oocysts from clinical stool samples.
So viability is measured the long way round: let the oocysts mature and count how many sporulated. That is why the line in the table above reads “sporulation unaffected” rather than a number of logs, and why the literature contains no CT for this parasite at all. There is nothing to measure it with.
What does work
Heat. No sporulation occurred at 158°F (70°C) or at 212°F (100°C), and pasteurized or thoroughly cooked foods have not been linked to cyclosporiasis outbreaks. That is the one household operation that carries a guarantee.
High pressure. Treatment at 550 MPa and 104°F (40°C) for two minutes on raspberries and basil worked: chickens fed the treated produce shed no oocysts. That is industrial equipment, with no home version.
One chlorine exception deserves a mention: dipping raw vegetables in sodium dichloroisocyanurate at 1 g/L reduced parasite numbers in a study the FDA review cites. No depth of inactivation is given there, so treat the result with care.
Water. Oocysts pass through physical barriers and are unaffected by chlorine and other water treatment disinfectants, and cyclosporiasis has been associated with unchlorinated and inadequately filtered drinking water. In endemic areas the advice is to boil or filter the water used for drinking, cooking and washing raw produce, not to chlorinate it.
The freezer does not work. Oocysts sporulated after a day at minus 59°F (15°C) in dairy products and after two days at minus 68°F (20°C) in water and basil. Only minus 158°F (70°C) and boiling temperatures stopped sporulation.
What follows from all this
Chlorine sits in a processing flume for a different job than sanitizing the leaf. It keeps the water clean so that one contaminated head does not hand the pathogen to the whole lot. The reasoning and the numbers a wash gives on the leaf itself are in our guide to wash water sanitizers.
For Cyclospora that means the work happens before the kitchen, in the field and the packing house. What that looked like in practice is in the breakdown of the Michigan outbreak, and whether a countertop device closes the gap is covered in the piece on the Pure Pod.
Key points
- CT is concentration times time. E. coli needs a CT of 0.13; a Cryptosporidium oocyst needs tens of thousands.
- An oxidant reaches a bacterium through a thin envelope. An oocyst is a capsule with a variably impermeable wall and no running processes inside to stop.
- In one experiment on leaves, chlorine dioxide took 2.45 to 3.97 log off E. coli and left Cyclospora sporulation untouched.
- No CT has ever been measured for Cyclospora: there is no animal model and no cell culture.
- Heat at 158°F (70°C) and high pressure work. The freezer does not: oocysts sporulated after two days at minus 68°F (20°C).
- Chlorine in a flume stands against cross-contamination of the water, not against a parasite on a leaf.
Why some cells are beyond the reach of water altogether is covered in our piece on internalization. The wider picture is in our food safety hub.
