Though we have all long been told that the problem with plastics is that they don't ever breakdown, recent studies show a far more insidious and pervasive threat. Worse than littered beaches and clogged waterways, worse even than the entanglement, suffocation and ingestion deaths of countless birds and sea creatures.
While it is technically correct that plastics do no not biodegrade (i.e. breakdown into harmless organic compounds), the millions of tons of plastic that finds its way into the world's oceans every year do break down - and fast (http://news.nationalgeographic.com/news/2009/08/090820-plastic-decomposes-oceans-seas.html).
Sunlight, wave and wind action disintegrate plastics into a soup of toxins and carcinogens, and estrogen-like compounds against which no living thing has any defenses. These compounds are having grave effects from the very bottom of the food chain all the way to the very top – effects which may include significantly reduced reproductive activity in key areas of the food chain.
Advancing more quickly and with more serious immediate consequences than any other environmental threat, these same compounds are already having directly measurable effects on humans including feminization of boys, premature maturation in girls, and a variety of serious affects on pregnancy and gestation, cancer risk and aging, and more.
Most of the research to date has focused on the effects of using plastics for eating, drinking, storage and cooking. Reducing these uses is an important step to reduce health problems and plastic waste, but plastics from hundreds of other uses - bags, packaging, industrial materials, decorations, toys - would continue to find a path to waterways and eventually to the oceans where the potential damage is even greater and the threats far more difficult to avoid.
If the existing huge masses of marine plastic in open ocean and tidal zones continue to increase and are left to breakdown at currently documented rates, the result will be a concentration of toxins, carcinogens and endocrine disrupters including estrogen-like compounds that will affect almost every living thing.