The two main types of biodegradable plastics: oxo-biodegradable and hydro-biodegradable. In both cases degradation begins with a chemical process (oxidation or hydrolysis), followed by a biological process. Both types emit CO2 as they degrade, but hydro-biodegradables (usually starch-based) can also emit methane. Both types are compostable, but only oxo-biodegradable can be economically recycled. **When the material has reached the fragmentation stage it is no longer a plastic, and is "biodegradable" in the same way as nature's wastes such as straw and twigs. The process continues until the material has biodegraded to nothing more than CO2, water, and humus, and it does not leave fragments of petro-polymers in the soil.