The McNett Water Purifier Tablets (Pack of 20) use chlorine dioxide the clean water to meet EPA guidelines. In other words, add one tablet to one liter of water to make it safe to drink. The tablets kill bacteria, cysts including giardia and cryptosporidium as well as viruses without creating a nasty aftertaste. These tablets are great for making safe drinking water if you are in an area were none is available. Use it in third world countries or just out in the wild where there are no shops or running water.