Contaminant Level (MCL)
Thallium is a highly toxic metal found in trace amounts in copper, lead, zinc and other ores. It's used in specialized electronic equipment, and can end up in drinking water through leeching from ore-processing sites and discharge from factories.
Thallium is toxic to the nervous, gastrointestinal, and respiratory systems. It can cause diarrhea and vomiting, changes in blood pressure, liver and kidney damage. It has been shown to affect reproductive and developmental health.
According to the EPA:
Some people who drink water containing thallium well in excess of the maximum contaminant level [0.002 milligrams per Liter (mg/L) or 2 parts per billion (ppb)] for many years could experience hair loss, changes in their blood, or problems with their kidneys, intestines, or liver problems.
The EPA recommends activated alumina and ion exchange as water treatment methods.Sources: EPA, EPA Toxicological Review, Wikipedia. Image: images-of-elements.com
(whole house & well units)