Turn over the yogurt tub in your refrigerator and you will find a small triangle with a 5 inside it. Turn over the bottle of window cleaner under the sink and you will find a 3. A lot of people read that pair of numbers as a ranking, with the 5 somewhere near the safe end and the 3 somewhere near the worrying end.
It is not a ranking. The number tells a sorting facility which polymer the container is made from so it can be separated from the other polymers. That is the whole job. It carries no information about whether the container is safe for food, how it behaves in heat, or whether you should refill it.
Here is the number that reframes the subject. A 2021 review in Environmental Science & Technology worked through 63 industrial, scientific and regulatory data sources and identified more than 10,000 substances used in making plastics, counting the building blocks, the additives and the processing aids. Seven codes. Ten thousand substances. The code was never designed to summarize that, and it does not.
This article covers what the codes are, what each resin is, why the additives matter more than the polymer for most of what people worry about, and what heat, fat and acid do.
The code is a sorting label, not a grade
The numbering scheme was introduced in 1988 by the Society of the Plastics Industry, an industry trade group, as a way to mark containers so they could be identified after they entered the waste stream. Michigan’s environment department describes it in exactly those terms in its public recycling guidance: the code identifies which plastic resin a container is made from.
Think about who the audience is. A sorting line reads the code, or more often a machine reads the material directly and the code is a backstop for human sorters. The consumer standing at a bin was never the intended reader. The code ended up on the bottom of the container because that is where moulds carry it, and it ended up inside a triangle made of chasing arrows, which is where the confusion starts. The arrows look like a recycling promise. They are a shape, not a claim.
Food safety runs on a completely separate system, and this is the clearest proof that the codes are not a safety scale. In federal regulation, plastics used with food are authorized by polymer and by use. Title 21 of the Code of Federal Regulations, Part 177, lists them by name: Section 177.1630 covers polyethylene phthalate polymers, which is PET, Section 177.1520 covers olefin polymers, which is the polyethylene and polypropylene family, and Section 177.1640 covers polystyrene and rubber-modified polystyrene. Not one of those sections mentions a number in a triangle. Article 712 covers how food contact substances get authorized and what the words on food packaging mean.
So the reader correction is simple. The number is a sorting code. A 5 is not safer than a 3 because 5 is bigger.
What the seven resins are
Grouping these by number teaches you nothing, because the numbers were assigned by sorting convenience. Grouping them by what the material is like teaches you quite a lot.
The two clear, stiff ones
PET, code 1, is the clear, glassy, slightly springy plastic of soda and water bottles. It takes a high gloss, it holds carbonation, and it is the single most valuable plastic in a recycling stream.
Polystyrene, code 6, is also clear in some forms and brittle rather than springy. It cracks instead of flexing, which is why a clear deli lid shatters at the hinge. Expanded into foam it becomes the familiar white cups and protective packaging. Egg cartons are a common example of the moulded form.
The two polyethylenes, same chemistry, different packing
HDPE, code 2, and LDPE, code 4, are both polyethylene. The difference is how the molecular chains pack together. The high-density version packs tightly, which makes it stiff, cloudy and good at holding liquids under their own weight. Milk jugs and detergent bottles are the household examples. The low-density version packs loosely, which makes it floppy, translucent and good at being thin. Dry shopping bags, squeeze bottles and the bread bag are all code 4.
That one distinction explains something useful. When people say a plastic feels cheap and flimsy, they usually mean it is low-density polyethylene, and flimsy here is a design choice, not a defect.
Polypropylene, the one that takes heat
Polypropylene, code 5, is the workhorse of reusable food containers, yogurt tubs, ketchup bottles and most screw caps. It is the resin manufacturers reach for when a container has to survive being filled hot or run through a dishwasher.
Notice what that sentence does and does not say. It is a manufacturing choice based on the material’s heat tolerance. It is not a certification, and it does not mean a tub is indifferent to being microwaved with oily food in it. Article 712 owns microwaving and food storage.
Vinyl, the one that changes character
PVC, code 3, sometimes marked V, is the strange one, and understanding why is the key to the rest of this article. On its own, PVC is rigid. It is drainpipe, window frames, credit cards and the stiff bottles used for window cleaner and some shampoos. Add plasticizers and the same polymer becomes a shower curtain, a garden hose, a vinyl floor or medical tubing.
Same code. Same polymer. Entirely different material, because of what was mixed into it.
Seven, which is a shrug
Code 7 means other. It is a bucket for everything that is not one of the six named resins, which makes it the least informative mark on any container. Five-gallon buckets and rigid plastic toys often land here. So does polycarbonate, the hard, clear plastic once used for water bottles and food storage, which is where bisphenols enter the picture. Article 714 covers that chemical family in detail. So do several plant-based polymers designed for industrial composting, which is a different collection system again.
A 7 tells you only that nobody put it in a box.
Plasticizers are additives, not the plastic
This is the idea that makes the rest of the subject tractable. When people talk about chemicals leaching out of plastic, they are almost never talking about the polymer. Polymers are long, heavy molecules that mostly stay put. The small molecules move.
Phthalates are the clearest example. The Centers for Disease Control and Prevention describes them as plasticizers, chemicals used to make plastics more durable, and lists where they show up: vinyl flooring, lubricating oils, personal care products such as soaps and shampoos and hair sprays, and the PVC used in packaging, garden hoses and medical tubing. They are not bonded into the polymer backbone. They sit among the chains, which is how they do their job, and it is also why they can migrate out.
That has two consequences people rarely connect.
First, two containers with the same number can behave completely differently. A rigid PVC bottle and a soft PVC hose share a code and share almost nothing else.
Second, regulators who act on this act on the additive, not on the resin. Federal law is a good illustration of the four kinds of claim you meet in this subject, and this one sits squarely in the regulated category. Under Section 108 of the Consumer Product Safety Improvement Act, the Consumer Product Safety Commission prohibits eight named phthalates above 0.1 percent, which is 1,000 parts per million, in accessible plasticized components of children’s toys intended for children 12 and under and child care articles for children 3 and under. The rule names chemicals. It does not name a resin code.
On what that exposure means for health, the honest position is the one CDC states about its own biomonitoring work. The agency measured 13 phthalate metabolites in the urine of at least 2,636 people aged six and over in the 2003 to 2004 national survey, found them widely, and added the caveat that matters: “Finding a detectable amount of phthalate metabolites in urine does not mean the levels will cause harmful health effects.” Detection and harm are different findings. Articles 718 and 719 cover why, and they are the right place to settle it.
Heat, fat and acid are what move things out of a container
Three conditions drive migration, and they are consistent across the research.
A 2023 systematic review in Comprehensive Reviews in Food Science and Food Safety examined phthalates across food categories and reported that temperature, how long food sits against the material, fat content and acidity all correlate positively with how much migrates. A 2025 systematic review in Reviews on Environmental Health, covering 67 studies and 5,378 samples, reached the same conclusion from the packaging side: migration rates rose with temperature and exposure time, and high-fat or acidic foods pulled more out than neutral ones.
The mechanism is unglamorous. Heat gives small molecules the energy to move through the polymer. Fat dissolves things that water does not, so an oily food acts as a solvent against the container wall. Acid attacks some materials and surface coatings. Time multiplies all three.
That is why the practical question is never which number is on the bottom. It is what is in the container, how hot it got, and how long it stayed there. Tomato sauce in a takeout container, left in a hot car, has hit all three conditions at once. Dry rice in the same container has hit none of them.
Both reviews build quantitative exposure estimates from their data, and both are candid that the numbers depend heavily on assumptions about diet. Treat the direction of the finding as established and the size of any single intake estimate as uncertain.
Reuse, scratches and the dishwasher
Reuse is good for the material and it is where the honest trade-off lives.
The temperature point has been measured directly for one resin. A 2012 study in the Journal of Environmental Monitoring examined 32 PET bottle materials from Britain and Nigeria, where antimony is used as a catalyst during manufacture, and found antimony in the plastic itself at 177 to 310 milligrams per kilogram. When the researchers held water in those bottles at 40 and 60 degrees Celsius, antimony stayed below the European acceptable limit even after 48 hours. At 80 degrees, most exposures exceeded it. Some bottled contents also exceeded the limit after 11 months of storage at room temperature.
Read that carefully, because it says something more useful than a headline would. Ordinary warmth did not do much, sustained heat did, and so did very long storage. Those are European limits rather than US ones, it is one study on one resin, and article 701 covers bottled water specifically. The pattern is the lesson: temperature does most of the work, and time can substitute for temperature if you give it long enough.
Scratches and the dishwasher belong to the same physical story. Abrasion increases surface area and creates channels for liquid to sit in. A worn container also holds residue that no longer rinses out, which is a hygiene point before it is a chemistry point. Repeated hot wash cycles are repeated heat exposures.
So the sensible position is neither to throw out your containers nor to pretend wear does not matter. Keep reusing them. Retire the ones that have gone cloudy, crazed or visibly scored, especially the ones you use for oily or acidic food. If you want numbers on particles shed from worn plastic, article 713 covers microplastics, and the measurement problems there are real enough that nobody should be quoting a figure at you with confidence.
What happens after you put it in the bin
This is the part to state plainly, without lecturing anyone.
Globally, the accounting is not encouraging. A 2017 paper in Science Advances estimated that 8,300 million metric tons of virgin plastic had been produced in total, that about 6,300 million tons of plastic waste had been generated as of 2015, and that roughly 9 percent of that waste had been recycled, with 12 percent incinerated and 79 percent accumulating in landfills or the environment.
At the level of your own bin, the practical fact is sorting economics. Michigan’s recycling guidance says most facilities collect codes 1 and 2, that codes 3 through 6 are difficult to recycle and some centers do not process them, and that code 7 is commonly excluded altogether. It tells readers to check with their local provider first, which is the correct advice and also an admission that the code on the container cannot answer the question.
Federal law anticipates this gap. The Federal Trade Commission’s Green Guides, at 16 CFR 260.12, allow an unqualified claim that something is recyclable only where recycling programs are available to a substantial majority of consumers or communities, and the Guides define substantial majority as at least 60 percent. Below that threshold, marketers are expected to qualify the claim, and the Commission offers suggested wording that tells you everything about the state of plastic recycling: “This product [package] may not be recyclable in your area.”
That is a federal regulator, writing the sentence the triangle cannot.
Three questions that beat reading the number
When you want to know whether a plastic item is a reasonable choice for what you are about to do with it, the code is the last thing to look at. Ask these instead.
What is going in it, and is it hot, oily or acidic? That question does more work than every other consideration combined, because those are the three conditions the migration research keeps identifying.
Is this container still in good condition, or is it cloudy, scratched and softened at the rim? Condition is a better guide than age.
What does the manufacturer say, and in what words? A specific instruction about temperature is a real statement. A vague reassurance on a label, or the absence of one, is not.
One safety note worth carrying away. If a plastic container melts, chars or smokes, get air moving, get people out of the room and do not eat the food. Fumes from burning plastic are irritating and can be worse than irritating in a closed space, and anyone with trouble breathing afterward needs 911 rather than a window. For a swallowed fragment or a chemical taste after eating from a damaged container, call Poison Help at 1-800-222-1222, which answers 24 hours a day, before symptoms rather than after.
And the takeaway that does the most good: the number in the triangle is a note to a sorting facility. What you put in the container, how hot it gets and how long it stays there are the variables that matter, and none of them are printed anywhere on the package.
Sources
- Michigan Department of Environment, Great Lakes, and Energy. “Recycling 101.” michigan.gov
- Centers for Disease Control and Prevention. “Phthalates Factsheet, National Biomonitoring Program.”
- U.S. Consumer Product Safety Commission. “Phthalates Information (Business Guidance).” cpsc.gov
- U.S. Government Publishing Office (eCFR). “21 CFR Part 177, Subpart B, Substances for Use as Basic Components of Single and Repeated Use Food Contact Surfaces.” ecfr.gov
- U.S. Government Publishing Office (eCFR). “16 CFR 260.12, Recyclable claims (FTC Green Guides).” ecfr.gov
- PubMed. “Wiesinger H, Wang Z, Hellweg S. "Deep Dive into Plastic Monomers, Additives, and Processing Aids." Environmental Science & Technology, 2021. PubMed. DOI: 10.1021/acs.est.1c00976.” doi.org
- PubMed. “da Costa JM et al. "Occurrence of phthalates in different food matrices: A systematic review of the main sources of contamination and potential risks." Comprehensive Reviews in Food Science and Food Safety, 2023. PubMed. DOI: 10.1111/1541-4337.13140.” doi.org
- PubMed. “Tanzer M et al. "Phthalates, bisphenols and per- and polyfluoroalkyl substances migration from food packaging into food: a systematic review." Reviews on Environmental Health, 2025. PubMed. DOI: 10.1515/reveh-2025-0027.” doi.org
- PubMed. “Geyer R, Jambeck JR, Law KL. "Production, use, and fate of all plastics ever made." Science Advances, 2017. PubMed. DOI: 10.1126/sciadv.1700782.” doi.org
- PubMed. “Tukur A et al. "PET bottle use patterns and antimony migration into bottled water and soft drinks." Journal of Environmental Monitoring, 2012. PubMed. DOI: 10.1039/c2em10917d.” doi.org