Selenium in drinking water can come from discharge from mines, natural deposits, dischage from refineries, or from agricultural runoff leaching natural selenium compounds from dry, undeveloped land.
Prolonged exposure to selenium above the maximum contaminant level can cause fingernail or hair loss, numbness in extremities, or circulation problems.
EPA has set the selenium standard for drinking water at .05 parts per million (50 parts per billion) to protect consumers served by public water systems from the effects of long-term, chronic exposure to selenium.
NSF/ANSI Standard 53: Drinking Water Treatment Units - Health Effects
Overview: Standard 53 addresses point-of-use (POU) and point-of-entry (POE) systems designed to reduce specific health-related contaminants, such as Cryptosporidium, Giardia, lead, volatile organic chemicals (VOCs), MTBE (methyl tertiary-butyl ether), that may be present in public or private drinking water.
NSF/ANSI Standard 58: Reverse Osmosis Drinking Water Treatment Systems
Overview: This standard was developed for point-of-use (POU) reverse osmosis (RO) treatment systems. These systems typically consist of a pre-filter, RO membrane, and post-filter. Standard 58 includes contaminant reduction claims commonly treated using RO, including fluoride, hexavalent and trivalent chromium, total dissolved solids, nitrates, etc. that may be present in public or private drinking water.
NSF/ANSI Standard 62: Drinking Water Distillation Systems
Overview: Standard 62 covers distillation systems designed to reduce specific contaminants, including total arsenic, chromium, mercury, nitrate/nitrite, and microorganisms from public and private water supplies.
The foregoing information was compiled from the the links listed above.