... dipping your head into a cold mountain stream and taking a long refreshing drink is an experience that has basically vanished from the wilderness areas of America. With the increased use of the wilderness there has also been an increase in the amount of bacteriological contamination of backcountry water supplies. The U.S. Environmental Protection Agency reports that 90 percent of the world's water is contaminated in some way. Without getting too much into basic chemistry, there are two methods for water purification tablets: Iodine and chlorine. Iodine is the arguably more effective method, but it leaves an iodine taste. It is suggested the taste may determine how much or how little one drinks. So, how do you deal with it? In this case, Potable Aqua includes a second bottle with tablets you can add to remove the taste, OR, simply adding vitamin C (i.e. Tang), or adding a pinch of salt per quart, adding flavorings (e.g., lemonade mix, etc.) or you can also pour it back and forth between containers (again, AFTER it has been adequately treated). The other option is chlorine, Chlorine should be used for persons with iodine allergies or restrictions like persons with thyroid problems, are on lithium, are allergic to shellfish (they may also be allergic to iodine), women over fifty and pregnant women should consult their physician prior to using iodine for purification. Be aware that iodine is light sensitive and must always be stored in a dark bottle, once opened, it may need to be used within a certain time. The effectiveness of all chemical treatment of water is related to the temperature, pH level, and clarity of the water. Cloudy water often requires higher concentrations of chemical to disinfect. If the water is cloudy, strain it, add the tablets and swish it around. Splash some water with the chemical onto the lid and the threads of the water bottle so all water areas are treated. The water should sit for at least 30 minutes. The colder the water, the less effective the chemical, it works best if the water is over 68F/21C, if the water temperature is below 40F/4C, double the treatment time before drinking. Finally, iodine and chlorine will NOT work on toxic water (water that has dissolved chemicals or solids like salt). In that case, it MAY be safe/safer to add the use of absorbent filters (i.e. with charcoal), but the BEST advice is to simply NOT use toxic water! I always say you should have at least TWO options for water, fire and shelter. Here, tablets are one, the other is a filter like Sawyer Products SP128 Mini Water Filter (It has the distinction of being able to be used almost perpetually by simply backwashing it!). But the best method is boiling, some say you don't actually have to "boil" water to make it safe, that by the time water actually comes to a boil it is already safe, but I like to see bubbles for my own peace of mind!