Plastics are considered a threat to the environment because they are non-biodegradable and take hundreds of years to decompose. When plastic waste is burnt, it releases toxic gases that pollute the air. Animals sometimes ingest plastic debris which can block their respiratory systems and cause death. Plastic bags thrown away can clog sewage systems and allow contaminated water to flow into streets, spreading disease. Plastics are made from fossil fuels and plastic production releases harmful emissions, while plastic waste persists indefinitely in landfills where it contaminates soil and water.