Do you have some brand names or websites? Clorine was the purifying medium ( and still is for municipal water supplies) until Iodine gained favor ( except for pregnant women and people with thyroid conditions.) Mechanical filtration is neccessary for cryptosporidium. There are some who debate this, but again municipal agencies consider mechanical filtration neccessary. The surest measure is of course adequate boiling. The site review on water filters will give you something to measure performance parameters by.