Several types of protozoan parasites infect people through ingestion of contaminated water containing their infectious forms, called “cysts” or “oocysts” (pronounced oh-oh-sists). Cysts and oocysts are tiny packets of two to four immature forms contained in tough, leathery shells, which protect them from drying out. They form in the intestines of infected people or animals and pass out the feces. They are extremely common, being found in virtually all surface water sources used for public water supplies. Unfortunately, their shell also protects them from the action of the disinfectants used by waterworks to purify drinking water, and they are so small that many municipal filtration systems are unable to remove them reliably. Therefore, to be completely safe, contaminated water supplies must either be boiled for one minute or fine-filtered at the point of use. The most notorious protozoan parasites are:
Entamoeba histolytica, 10-20 um cysts, cause of “amoebic dysentery”
Giardia lamblia, 8-16 um cysts, cause of “giardiasis”
Cryptosporidium parvum, 4-7 um oocysts, cause of “cryptosporidiosis”
These protozoan parasites are unusually efficient at causing infections—most bacteria and viruses require hundreds or thousands of them to evade the body’s defenses, but consuming as few as one of two cysts or oocysts is likely to lead to illness. The illnesses they cause are generally just a few days of diarrhea, cramps, nausea, etc. for most healthy people, but individuals with AIDS or HIV, cancer patients on chemotherapy, organ transplant patients on immune system therapy, and many infants and the elderly are more susceptible to long-term, life threatening disease. In some communities it is estimated that as much as 25% of the population may belong to endangered groups. Therefore, the efficiency of the fine-filtration used to remove them is of paramount importance.
There are no official point-of-use filtration standards, but public health officials have approved systems and products which demonstrate 3-log or 99.9% efficiency for filtration of either live cysts/oocysts or test particles with a diameter of 1 um (micro-meter, or micron). Also, NSF International, a third party certifying agency, has produced a voluntary filtration performance standard (Standard 53: Drinking Water Treatment Units—Health Effects) which has been accepted by most public health officials. NSF Std. 53 requires demonstrating at least 99.95% reduction of live cysts/oocysts, or alternately, at least 99.95% of test particles in the 3-4 um size range. That is about half the size of Crytosporidium oocysts, which are the smallest of these parasites.
Everpure precoat carbon filters are efficient fine-filters capable of removing these cysts/oocysts. They have been tested and Certified by NSF International to ANSI/NSF Standards 42 and 53 for more than 99.9% reduction of particles 0.5 to 1.0 um in size in the general filtration test, and for more than 99.99% reduction of 3-4 um particles in the Cyst Reduction test. Therefore, concerned individuals and businesses may use their water after filtration through these filters, even in the face of a “boil water order” or other notification of the presence of cysts/oocysts, but only if it is confirmed that the water is believed to be potable in all other respects.