The petroleum-based plastic bags take decades to break down, so if they are not recycled they litter. It creates visual pollution: in the streets, on the beaches etc. Also, they can clog roadside drains, which could cause street flooding during heavy rainfall.
Plastic bags can be recycled but it rarely happens; according to the U.S. Environmental Protection Agency, only 1 percent of plastic bags were recycled in 2000, against 20 percent for paper bags.
They endanger wildlife and particularly sea life such as sea turtles and dolphins, which can die of entanglement, suffocation, and ingestion because they assume that these bags are jellyfish.