I interpreted the report differently. The sentence you are referring to:

“Although some Cryptosporidium oocysts remained viable, one group of researchers found that a 30-minute contact time with 0.22 mg/L chlorine dioxide could significantly reduce oocyst infectivity (Peeters et al., 1989).”

… is immediately followed by:

“In contrast, other researchers have found that CT values in the range of 60 to 80 mg·min/L were necessary to provide 1- to 1.5-log inactivation (Korich et al., 1990; Ransome et al., 1993). Finch et al. (1995) reported that the CT values for 1-log inactivation was in the range of 27 to 30 mg·min/L. For 2-log inactivation, the CT value was approximately 40 mg·min/L, and 70 mg·min/L for 3-log inactivation. Finch et al. (1997) found 3-log inactivation of Cryptosporidium oocysts with initial chlorine dioxide residual concentrations of 2.7 and 3.3 mg/L for contact times of 120 minutes, at pH of 8.0 and a temperature of 22°C.”

The “in contrast” the second part seems to refer to the contract between reducing the Cryptosporidium infectivity and the actual complete, measureable inactivation of the Cryptosporidium oocysts.


You are correct. Chlorine dioxide is not 100% effective. This paper simply lays out the dosages and contact times as well as their effectiveness for both Giardia and Cryptosporidium. It shows the difference in difference in resistance between Giardia and Cryptosporidium, but it lists methods of obtaining the same level of disinfection (3-log, i.e., 99.9% I believe) of both. The report lays out the reality pretty well in one sentence:

“Both studies concluded that chlorine dioxide is an excellent disinfectant against cysts and that chlorine dioxide is better than or equal to chlorine in terms of inactivation. Chlorine dioxide was found to be superior to chlorine at higher pHs.”

Now, for credibility, that sentence applies directly to the disinfection of Naegleria gruberi, however it seems to be true for Cryptosporidium as well when looking at the EPA results on regular chlorine (Alternative Disinfectants and Oxidants Guidance Manual (EPA 815-R-99-014, April 1999): Chapter 2 — Page 37 (2-37), Section 2.7.4.3):

“Chlorine has little impact on the viability of Cryptosporidium oocysts when used at the relatively low doses encountered in water treatment (e.g., 5 mg/L). Approximately 40 percent removals (0.2 log) of Cryptosporidium were achieved at CT values of both 30 and 3,600 mg·min/L (Finch et al., 1994). Another study determined that “no practical inactivation was observed” when oocysts were exposed to free chlorine concentrations ranging from 5 to 80 mg/L at pH 8, a temperature of 22°C, and contact times of 48 to 245 minutes (Gyürék et al., 1996). CT values ranging from 3,000 to 4,000 mg·min/L were required to achieve 1-log of Cryptosporidium inactivation at pH 6.0 and temperature of 22°C. During this study, one trial in which oocysts were exposed to 80 mg/L of free chlorine for 120 minutes was found to produce greater than 3-logs of inactivation.”

And it is easily verified that chlorine dioxide is significantly more effective against Cryptosporidium than iodine.


This is presumed true, and the statement deserves qualification, despite its being presumed technically true.

http://www.foodsafety.gov/~mow/chap24.html
“Infectious dose--Less than 10 organisms and, presumably, one organism can initiate an infection.”

One can be infected due to only one viable Cryptosporidium oocyst. That does not mean that you will be infected if there is even one viable Cryptosporidium oocyst in the water you drink.

The basic point is this: Does chlorine dioxide eliminate any chance of being infected by wild ground water? No, it does not. Neither does any other treatment. Even the effective time for boiling is debated, and even distillation does not remove 100% of everything, and I have not even mentioned how difficult it is to carry your firewood in a pocket. There really is no such thing as the elimination of the chance.

So, take chlorine dioxide for what it is. It is a self-contained, portable solution for significantly reducing your chances of infection as long as it is used correctly. It is meant to replace regular chlorine and iodine, which have shown to be ineffective at inactivating Cryptosporidium at dosages safe for human consumption. It is not meant to replace filters or boiling or even UV treatment; it is meant only to be the next generation of self-contained chemical treatments.

You cannot view this issue with a black and white monitor, and you will be severely handicapped even with 256 shades of gray. You have to switch into the 16 million color range for this, because there are multiple different categories of water treatment, and each item in each category has a different effectiveness. You cannot just measure one item against another when the items are a different beast altogether.

And on a side note, for anyone who is interested, one of the better no-fluff breakdowns of all the different methods of water treatment can be found at http://zenbackpacking.net/WaterFilterPurifierTreatment.htm. Can anyone find an updated link to the Army’s chart on the different treatments and their ratings? It looks like the Army moved or removed the page and did not supply a redirect.