Plastic identification numbers exist to tell recycling workers and sorting machines exactly which type of plastic a product is made from. The system was created in 1988 by the Society of the Plastics Industry, assigning a number from 1 to 7 to each major category of plastic resin. The original goal was straightforward: help waste recovery facilities sort plastics quickly and accurately, since different plastic types can’t be melted down and reprocessed together.
The Problem These Numbers Solve
Most plastics look and feel similar, but they’re chemically distinct. A water bottle and a milk jug might both seem like generic “plastic,” but the water bottle is made from one type of polymer (number 1) and the milk jug from another (number 2). These two plastics melt at different temperatures, have different chemical structures, and produce unusable material if they’re mixed during recycling. A single misplaced item can contaminate an entire batch.
Before the numbering system, sorters at recycling facilities had to rely on experience, product shape, or burn tests to figure out what they were dealing with. The stamped number on the bottom of a container gave them an instant answer.
What Each Number Means
Each number corresponds to a specific plastic recipe:
- 1 (PET or PETE): The clear, lightweight plastic used in soda bottles and food containers. Widely accepted in curbside recycling and often turned into polyester fiber for clothing and carpet.
- 2 (HDPE): A dense, sturdy plastic found in milk jugs, detergent bottles, and grocery bags. Also widely recycled through curbside programs.
- 3 (PVC): Used in plumbing pipes, vinyl siding, shower curtains, and some food packaging. Rarely recycled in household programs.
- 4 (LDPE): A flexible, soft plastic used in produce bags, six-pack rings, and squeeze bottles. Not typically collected curbside, though many grocery stores accept it for drop-off recycling.
- 5 (PP): Found in yogurt cups, bottle caps, food storage containers, and auto parts. Increasingly accepted in curbside programs.
- 6 (PS): Polystyrene, including rigid utensils, CD cases, and foam products like Styrofoam. Too lightweight and low-value to recycle economically, so most programs don’t accept it.
- 7 (Other): A catch-all for anything that doesn’t fit numbers 1 through 6, including specialty plastics and multi-layer packaging. Rarely recyclable through standard programs.
How Sorting Facilities Actually Use Them
Modern recycling facilities don’t rely on workers squinting at tiny numbers on the bottom of containers. Instead, they use near-infrared (NIR) optical sorters, machines that scan plastics on high-speed conveyor belts and identify the resin type by reading how infrared light bounces off the material’s surface. Each plastic polymer reflects infrared energy in a unique pattern, essentially a chemical fingerprint. Once the machine identifies a container as, say, PET (number 1), air jets blast it off the belt and into the correct collection stream.
These automated sorters are both faster and more accurate than human workers. They process the massive volume of mixed recyclables that flow through single-stream facilities, where all household recyclables arrive jumbled together. The resin identification number still matters as a standardized language for the industry, even though the physical sorting has moved beyond visual inspection. It ensures that everyone along the recycling chain, from manufacturers to processors to buyers of recycled material, is talking about the same thing.
Why a Number Doesn’t Mean “Recyclable”
This is the single biggest misunderstanding about the system. A plastic item stamped with a number is not necessarily recyclable in your area, or anywhere at all. The number identifies the resin type. That’s it. Whether your local facility can actually process that resin depends on market demand, available equipment, and the economics of selling the sorted material.
Numbers 1 and 2 are recycled almost everywhere. Number 5 has become widely accepted in recent years. But numbers 3, 4, 6, and 7 are difficult or uneconomical to recycle, and most curbside programs reject them. When people see a number inside the familiar triangular symbol and assume it means “please recycle,” they often toss non-recyclable items into the bin. This contaminates otherwise good batches of recyclable material.
The confusion was partly baked into the original design. Until 2013, the symbol surrounding the number was a set of “chasing arrows,” the looping triangle universally associated with recycling. ASTM International, which now manages the standard, changed the symbol to a solid equilateral triangle specifically to break that mental connection. The goal was to refocus the code on its actual purpose: identifying the resin, not making a recycling promise. In practice, both versions still appear on products, and the confusion persists.
Health Concerns Tied to Specific Numbers
The numbering system has taken on a secondary role as a quick reference for potential chemical exposure. The American Academy of Pediatrics has recommended avoiding plastics labeled 3, 6, and 7 for food contact, especially for children.
Number 3 (PVC) often contains phthalates, chemicals added to make plastic flexible. Phthalates can interfere with hormone function. Number 6 (polystyrene) can leach styrene, a suspected carcinogen, particularly when heated. Number 7 is the broadest concern because it includes plastics that may contain bisphenol A (BPA) or bisphenol S (BPS), both of which can disrupt the endocrine system. The exception within number 7 is products labeled “biobased” or “greenware,” which are typically made from plant-based materials and don’t contain these compounds.
Numbers 1, 2, 4, and 5 are generally considered safer options for food storage and everyday use.
State Laws That Require the Labels
The numbering system isn’t just voluntary. Multiple U.S. states have passed laws requiring plastic bottles and containers to display the resin identification code before they can be sold. New Jersey, for example, has mandated since 1991 that all plastic bottles and containers sold in the state carry the symbol, identification number, and an abbreviation of the resin name. These laws were designed to support recycling infrastructure by ensuring that every piece of plastic entering the waste stream carries the information sorters need.
Digital Watermarks as a Next Step
The seven-number system has limits. It groups an enormous variety of plastics into just seven buckets, with number 7 serving as a catch-all that tells sorters very little. It also can’t distinguish between food-grade and non-food-grade versions of the same resin, a distinction that matters for producing high-quality recycled material.
A European initiative called HolyGrail 2.0, backed by over 130 companies, has been testing digital watermarks as a more advanced alternative. These are invisible codes, each about the size of a postage stamp, printed across the entire surface of a package. High-resolution cameras at sorting facilities can read the watermarks and extract detailed information: the specific resin, whether it held food or chemicals, the manufacturer, and the packaging composition. This level of detail could allow sorting into dozens of precise categories instead of just seven, producing cleaner recycled material. The technology has shown promise in pilot testing, though it would need to be adopted industry-wide to replace or supplement the existing numbered system.