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Re: [pct-l] Re: Iodine

Charcholla <Charcholla@aol.com> wrote:

<<Have you seen (or heard of) the kits that provide chlorine crystals for
treating raw water? A measured amount super-chlorinates the water; hydrogen
peroxide added later neutralizes the taste? We used this on the JMT when we
were in camp where larger volumes of water were needed. We treated a 3-gallon
bag at a time for meals and the next day's initial supply of drinking water.
On the trail, we used iodine tablets because it's simple.
<snip>
It eithered worked or we were lucky on our 21-day JMT hike in '96. >>

I honestly think that the Giardia threat is greatly overstated. You not
have ingested any cysts, you may have a resistance to the bug, or it may
have worked. Dunno really, I just repeat what I've read in research. Very
little good stuff is said about using chlorine to kill Giardia

http://www.gorp.com/gorp/health/wtrflfaq.htm says

"Avoid halazone and Clorox, because chlorine is volatile, slow to disinfect,
and works differently against protozoa and viruses at various pH levels.
It also reacts with organic compounds to form carcinogenic chloramines.
Iodine is not highly toxic, and in fact is an essential ingredient of
human nutrition.  However, continuous ingestion of large doses may cause
health problems, particularly for people with thyroid problems."

http://io.datasys.swri.edu/wisdom/Panel_09_Water.html says 

"The effect of chemical disinfection, such as chlorine, on the viability of
Giardia cysts is an even more complex issue. It is clear from the number
of waterborne outbreaks of Giardia that have occurred in communities where
chlorine was employed as a disinfectant that the amount of chlorine used
routinely for municipal water treatment is not effective against Giardia
cysts. These observations have been confirmed in the laboratory under
experimental conditions (25,26,27). This does not mean, however, that 
chlorine does not work at all. It does work under certain favorable
conditions. Without getting too technical, one can gain some appreciation
of the problem by understanding a few of the variables that influence the
efficacy of chlorine as a disinfectant. 

1) Water pH: at pH values above 7.5, the disinfectant capability of
chlorine is greatly reduced.  2) Water temperature: the warmer the water,
the higher the efficacy.  Thus, chlorine does not work well in ice-cold
water from mountain streams. 3) Organic content of the water: mud,
decayed vegetation, or other suspended organic debris in water chemically
combines with chlorine making it unavailable as a disinfectant. 4)
Chlorine contact time: the longer Giardia cysts are exposed to chlorine ,
the more likely it is that the chemical will kill them. 5) Chlorine
concentration: the higher the chlorine concentration, the more likely
chlorine will kill Giardia cysts. Most water treatment facilities try to
add enough chlorine to give a free ( unbound)  chlorine residual at the
customer tap of 0.5 mg per liter of water.

The five variables above are so closely interrelated that an unfavorable
occurrence in one can often be compensated for by improving another. For
example, if chlorine efficacy is expected to be low because water is
obtained from an icy stream, either the chlorine contact time or chlorine
concentration, or both could be increased. In the case of
Giardia-contaminated water, it might be possible to produce safe drinking
water with a chlorine concentration of 1 mg per liter and a contact time
as short as 10 minutes if all the other variables were optimal (i.e., pH
of 7.0, water temperature of 25 deg C, and a total organic content of the
water close to zero). On the other hand, if all of these variables were
unfavorable (i.e., pH of 7.9, water temperature of 5
 deg C, and high organic content), chlorine concentrations in excess of 8
mg per liter with several hours of contact time may not be consistently
effective. Because water conditions and water treatment plant operations
(especially those related to water retention time and, therefore, to
chlorine contact time) vary considerably in different parts of the United
States, neither the U.S. Environmental Protection Agency nor the CDC has
been able to identify a chlorine concentration that would be safe yet
effec tive against Giardia cysts under all water conditions. Therefore,
the use of chlorine as a preventive measure against waterborne giardiasis
generally has been used under outbreak conditions when the amount of
chlorine and contact time have been tailored to fit specific water
conditions and the existing operational design of the water utility. "
--
Brick Robbins
San Diego, CA          
brick@ix.netcom.com              
http://www.netcom.com/~brick 

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