Biodegradability is the ability of a substance or material to be broken down into its fundamental parts by exposure to the natural action of elements like microorganisms, sunlight, and water. Products that are biodegradable do not accumulate in the environment. Further, their biodegradation means that they have become a nutrient to other biological processes, which is necessary for the planet Earth to be self-sustaining.
When a product does not biodegrade, it remains in the environment indefinitely. As more and more of the substance accumulates, its chemical concentration and toxic effects increase which means that a relatively safe chemical can then become a dangerous one.