It wasn't until recently that I gave much thought to the containers that I carried my water in when I'm out and about in the outdoors. Over the years I have used British and US military water bottles, bottles made from aluminum, plastic and now stainless steel. Does it make a difference? You bet it does! My first awareness that there might be a problem occurred a couple of years ago when I walked into REI (www.rei.com) to replace a water bottle that I had lost. (Hate it when that happens!) As I wandered through the isles I paused in front of a rack of Nalgene water bottles and noticed a sign that stated "BPA Free!" What I wondered was "BPA?" Come to find out BPA is a health endangering chemical that interferes with the body's endocrine system. BPA leaches from your plastic water bottle into the water you consume - especially if you put hot water into your water bottle. This problem has been largely resolved with the production of BPA free plastic but what is not clear is how many other harmful chemicals contained in the plastic remain to be identified! If you are going to use a plastic water bottle at least make sure it is certified BPA free.
Aluminum water bottles, both lined and unlined, are also available but once again you run into potential health problems. Aluminum, in minute amounts, is released into the water you drink and has been linked to the early onset of Alzheimer's disease. The water bottle manufactures have attempted to fix this problem by lining the interiors of the bottles with plastic or resin but once again you have plastics of unknown quality in contact with the water you are drinking. Resins have been known to crack and are damaged if heat is applied to the bottle.
Which brings me to water bottles made from stainless steel. Such bottles are widely available on the internet and from some of the better outdoor retailers. I recommend them. I particularly like the Klean Kanteen brand. (www.KleanKanteen.com) Bottles made from 18/8 food grade stainless steel should be apart of your gear. 18/8 grade stainless steel is completely inert and is easy to clean. Unlike plastic, a stainless steel water bottle can be used to melt snow and ice or heat water if the need arises. Granted they are a bit heavier but in my opinion this disadvantage is far outweighed by the peace of mind I get knowing that I am not slowly poisoning myself.