Total Dissolved Solids (TDS) is the sum of all ion particles that are smaller than 2 microns. They are considered a 'nuisance chemical' by the EPA and aren't harmful on their own. However, high TDS concentrations can indicate a problem with harmful ions that can cause health concerns.
Total Dissolved Solids in Drinking Water
High concentrations of TDS in water supplies can be indicated by a salty taste, deposits, hardness, and colored water.