Another popular purification method is filtering. Filters come in two varieties: ceramic and charcoal. The latter is more popular with backpackers since it’s lighter and cheaper, but ceramic ones are reusable. A filter size of 0.2 microns effectively removes parasites, protozoa such as cryptosporidium cysts, and bacteria. However, filtering won’t clear out viruses. Even filters than contain iodine resins aren’t entirely reliable because of how briefly water is in contact with them.