Mercury is a toxic heavy metal released into the atmosphere, most significantly, through the burning of coal in coal-fired power plants and from Hg-cell chloralkali plants where Hg acts as a flowing electrode used to reduce Na⁺ to Na⁰ in an amalgam. While levels of mercury in the atmosphere aren't directly toxic, mercury deposition into lakes and rivers leads to elevated levels of mercury in these organisms, which can eventually be transmitted to humans through the eating of fish and other organisms which bioaccumulate Hg and other toxic heavy metals. Long range transport has moved this element---released in combustion processes---to all of the continents on the planet. Mercury is methylated biochemically in the biosphere into a much more toxic form, methyl mercury, CH₃HgX, where X can be variously -CH₃, -OH, or -Cl or other halogens.