The sources of potable water in the backcountry can be from lakes, streams, glacial melt water, or snow. Several methods exist for treating drinking water in the backcountry including boiling, chemical treatment, and filtration. There are advantages and disadvantages to each method. Surface water and snow may contain organisms that cause intestinal ailments in humans. Boiling water for 5 minutes or adding chemicals such as bleach, iodine, or halozone will kill these organisms. Chemical treatment, such as iodine, requires 30 minutes to be effective. Water can also be filtered to remove these organisms if the filter is effective to 5 microns. Glacial melt water sources contain fine particulates that may cause diarrhea and requires mechanical filtration before the water can be safely consumed. Treatment at each location varies depending on the type of contaminants that need to be neutralized or removed.