Cotton generally takes five to six months to break down if left in a compost pile or in the environment. Cotton is so biodegradable, it can start decomposing in a week.
Cotton biodegrades faster than a plastic bag because cotton is made of natural fibers that microorganisms can break down. Linen biodegrades more quickly than cotton, but wool breaks down more slowly than cotton.
To determine the biodegradability of natural and manmade items, scientists conduct respirometry tests. They place an item into a sealed container along with soil and microorganisms to mimic a compost pile. After a few days, they measure the amount of carbon dioxide that the microorganisms give off to determine how much the item has composted.