Some natural water systems do exist in which little CO₂ is present, but most contain important amounts of dissolved CO₂. Thus, in a pe–pH diagram for a metal, if some CO₂ is present, then we need to consider that there may be predominance region for a metal carbonate solid, possibly more than one carbonate solid variation. For metals in a III oxidation state or higher, generally carbonate and phosphate solids do not need to be considered, because hydroxide, oxide, and oxyhydroxide solids are, in general, more stable. Pb(II) solids are generally more soluble than PbO₂(s) and Pb(s). Like Pb, CO₂ species are subject to redox reactions. since some period of stagnancy will be required to reach equilibrium between water and hydrocerrusite scale within in a service line pipe, flushing clean water through the pipe and plumbing before consumption can be enormously helpful in further reducing Pb levels in drinking water, allowing conformance with the current US EPA action level.