Styrofoam does not decompose in the environment under normal circumstances. Much like plastic, Styrofoam is made from a polystyrene-based petroleum product that is not biodegradable. Plastic takes hundreds of years to decompose, and Styrofoam takes much longer because it is a stronger form of plastic.
Styrofoam does not decompose because the polymer beads that comprise it are resistant to photolysis. It is harmful to the environment because it requires hydroflourocarbons that negatively impact the ozone layer and because it uses up limited stores of petroleum. During the manufacturing process, Styrofoam releases chemicals that can cause headache, depression, weakness, fatigue and adverse kidney effects.