Plastic is actually categorized into seven types for recycling, not four. The plastic types are coded numerically from one to seven; some of the categories are rarely recycled.
Polyethylene terephthalate constitutes the first plastic recycling category; it includes water and soda bottles. High-density polyethylene milk and shampoo bottles make up category two. Category three, PVC plastic, is seldom recycled. The fourth category, low-density polyethylene shopping bags, is also seldom recycled. Polypropylene, the fifth category, includes plastic utensils and medicine bottles. Polystyrene egg cartons and CD cases are the sixth type of plastic. Every other type of plastic, such as polycarbonate and nylon, falls into the seventh plastic recycling category.