[at-l] Safe Water and Statistics

[rickboudrie at hotmail.com (rick boudrie)]

Jim writes:

"Not quite - that was 1 in 5000 PER LITER of untreated water - not 1 in 5000 
overall.  So for a thruhiker who's on the Trail for 180 days and drinking 4 
liters per day, the odds are more like 720/5000 or 14.4% (about 1 in 7).  
And that's an overall average - for the whole country.  The rate of 
contamination for the Appalachians is 50% (more than twice the average) 
which makes the odds for an AT thruhiker more like 31.3% or almost 1 in 3.  
Ooops ---"

Not exactly, Jim.  With the information given, it could be far greater than 
1 in 3.

Determining the odds as 1 in 3 are not so easy from the information 
provided.  Lets say we accept that everyone (a gross simplification) gets 
sick if they drink a 500 cooties a day, and no one gets sick if they drink 
less than that amount (also a gross simplification).  Now lets assume that 
4999 out of 5000 liters of water has 300 cooties in it.   Pretty easy to 
figure out that almost 100% of the people who who drink one liter won't get 
sick, and almost 100% of those who drink who drinks two liters a day will 
get sick.

The important thing I get out of the article (and Jim's observation that 
hikers drink alot)  is that there need be some daily critical mass of 
cooties before any individual gets sick, and yet that your body does not 
accumulate this critical mass over days.

I am not sure what I will do with this knowledge, other than to be a bit 
less concerned about stuff like reusing a water bottle that once held stream 
water, or by accidental contamination of a filter hose, or after fishing a 
cap out of a stream that has been dried off.   I am also going to be 
increasingly wary of those situation where I am likely to be dealing with a 
critical mass of bugs, like when taking my filter apart.  No way am I ever 
going to make the mistake of soaking the entire filter element in a 
bleach/iodine solution again.

The nature of a daily critical mass of bugs also makes me wonder whether or 
not those who don't filter are better off getting there water from many 
different sources thorughout the day, or from just one.  With realistic 
assumptions, I'll bet that is something that someone with a good 
undertanding of probabilities could figure out.

Anyway, this is interesting stuff to think about.  I enjoy reading of 
everone's perspectives.   It's one of those topics that will no doubt be 
discussed for years.  In the absence of any concensus (if you think there is 
little here, go check out www.whiteblaze.net) I think its great that 
Backpacker has finally published an article that tried to do more than 
simply parrot the same old maxims, and personal anectdotes.

Rick B

(who was never sick along the trail or in a number of hell-holes but still 
filters most of the time)

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