What's the meaning of the numbers from 1 to 5 under milk cartons?

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Different types of consumer products and packaging are made from different types of plastics.  The Society of the Plastics Industry (SPI) came up with a standardized coding system for the different types of plastics used numbered 1 through 7 that are stamped onto the plastic itself.  These numbers help with identifying and separating different plastics to help make recycling faster and more efficient.  The different codes are identified below.

1:  Polyethylene tetraphthalate.  This is commonly used for beverage bottles.

2:  High Density Polyethylene.  This is commonly used for milk cartons and shampoo bottles.

3:  Polyvinyl Chloride (PVC).  This is commonly used for plumbing pipes.

4:  Low Density Polyethylene.  This is commonly used for trash bags and sandwich type bags.

5:  Polypropylene.  This is used for more temperature durable plastics like drinking cups.

6:  Polystyrene.  This is Styrofoam and is used for cups and shipping packaging.

7:  This is the catch all category for everything else, like polycarbonate for CD's.

Of all of the above materials, type 1 and 2 (like the milk cartons you referenced) are by far the most commonly recycled in standard household consumer materials.

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