Great Question!!

Aunt Jean asked a great question about the Emergency post I did so I thought I'd blog about it!

~~O.K. question???? Are you sure about that last part using the pressure cooker? Cause I thought that if you were to boil water... it has to be an open kettle? Would you please get back to me on this Roseanne.... thanks...
Aunt Jean

Hi there,
About your question. I'm not sure how much you know or don't know so I thought I'd just write a whole ton of stuff and I'll hope that you don't think I'm being condesending or anything as I can use your question to help other readers. :)

So as you probably know "boiling" can happen at any temp. Water in space would boil at a near freezing temp and wouldn't kill so much as a fish. Once water reaches it's "Boil" the temperature can no longer be increased. On earth we are under 14 lbs of pressure and to boil at my elevation the temp of my water needs to be around 208 which is an ok temp to kill off microorganisms under a ten min. rolling boil. BUT a pressure cooker can add an additional 14 lbs of pressure to the already 14 we are under creating a much higher temp for a boil probably somewhere close to 250 degrees. Nothing is as important to killing microorganisms like heat. (So basically pressure and heat are relatively synonimous terms so going to a higher pressure means higher heat.) Because Pressure cookers can reach higher boil temp they shorten the the need for the standard 10 min. boil, thus useing less energy. You take them all the way up and then turn off the stove.
SO now we get to your question:
An open kettle is good if you were cooking to collect clean water out of something like swamp water, salt water, seaweed or even grass but you'd want it still to be covered with a plastic catch to catch the clean steam. HOWEVER water contaminated with something like let's say gasolene which has a higher vapor pressure should most definetely be in an open kettle so as to release that vapor first. But if the contaminate had a lower vapor pressure ...yikes! So if your water has more than a few microorganisms, say like the water in the streets after Katrina... it's safer to not use it at all. (Unless we talk filtration but that's a whole other ball of wax). Water baked out of grass and leafs would be WAY safer.
In the grand scheme of things a glorified pressure cooker is what is used to "Auto clave" surgical instaments and is simply a much better way to clean water that is contaminated with bacteria.
Additionally having an open kettle is less desirable because you loose precious water through steam vapor as well.

So anyhow, Let me know if I over looked anything. Maybe you were thinking about how much longer you have to wait for it to cool down before you can drink it! :)
Water purification is definetly something My dad made sure we understood. I called him to double check.
Hope this is helpful!
I'm sure wikipedia or something out there like it has a much more scientific answer.
Thanks,
~Roseanne S.