Elemental heavy metal. Is a liquid at normal temperature. It is also a systemic poison focused on brain, bowels, kidneys, then other body organs. Its maximum allowable concentration is 0.05 parts per million (PPM). Affects the central nervous system most severely. Children and elderly are the most susceptible to its poisoning. As with other heavy metals, mercury is a bio-accumulant that climbs up the food chain over time and leaves the body very slowly, collecting in body tissue.