Elemental mercury has very little availability in the environment, until biological action turns it into the far more poisonous methyl mercury. The bulbs don’t contain free liquid mercury like they used to, however; they contain solid metal elements that hold the mercury as an amalgam until the heat of the bulb’s operation causes it to evaporate into the bulb’s glass tube (i.e. the bulb “warms up.”) Then when the bulb is shut off and cools, the mercury condenses back onto the elements and gets absorbed again. This cuts down on the amount of mercury that can get loose from these bulbs if they are later broken.