Stainless steel explained: grades, nickel leaching and what the numbers mean

Stainless steel is the default answer to “what do I use instead of plastic?” For pans, bottles, lunch boxes and straws it’s often the right one. It’s tough, it’s recyclable, and nobody has found microplastics flaking off a saucepan.

But “stainless” is a family of alloys, not one material, and the label on the box tells you less than you’d think. It also isn’t perfectly inert: new stainless puts measurable nickel and chromium into acidic food. For most people that’s trivia. For people with nickel allergy it’s worth knowing. Here’s what the numbers mean, what the leaching studies found, and how to buy steel without being sold something else.

What makes steel “stainless”

Stainless steel is iron alloyed with chromium. The chromium reacts with oxygen to form an invisible film of chromium oxide on the surface, and that film is what stops rust. It also repairs itself when scratched, which the industry calls “self-passivation”. Keep that film in mind: it’s the reason stainless leaches less as it ages, and the reason scouring and harsh acids matter.

Other elements change how the steel behaves. Nickel gives the common kitchen grades their non-magnetic “austenitic” structure, molybdenum improves resistance to salt, and manganese can stand in for some of the nickel, more cheaply.

The grades, decoded

304, 18/8 and 18/10

The workhorse of kitchen stainless is grade 304, an “austenitic” steel. A typical composition is 18% chromium and 8% nickel; the specification range is 18–20% chromium and 8–10.5% nickel.

“18/8” and “18/10” are marketing shorthand for chromium and nickel percentages, not official grade names. Both numbers fall inside 304’s specification range, so most 18/8 and 18/10 kitchenware is 304-type steel; the stamp alone doesn’t prove it. The labels get muddled even in the research literature: one study refers to “18/10 (grade 316)” pots, though 316 is defined by its molybdenum, not by an 18/10 ratio.

316, “surgical” or “marine” grade

Grade 316 contains 16–18% chromium, 10–14% nickel and 2–3% molybdenum. Molybdenum is what lets it resist salt. One steelmaker’s data sheet puts 304’s limit at waters containing up to about 100 ppm chloride, versus about 2000 ppm for 316. That matters for brines, seawater and long contact with salty food. For a water bottle or a frying pan, 304 is fine. Note that 316 contains more nickel, not less, so it is no help for nickel allergy.

430, the magnetic one

Grade 430 is a “ferritic” stainless steel: roughly 16–18% chromium and no more than 0.75% nickel. Ferritic grades with about 16–17% chromium are used mostly in cooking utensils, and they’re magnetic, which is why you’ll often find 430 on the outside of induction-ready pans. Nickel is barely present.

Knives are a fourth family again: “martensitic” steels such as 420, which can be hardened to hold an edge. In one test of 18 sets of knives, only three exceeded the Italian release limit, and only at 100 °C.

200-series: the budget substitute

Grades 201 and 202 replace much of the nickel with manganese. Grade 201, for example, contains 5.5–7.5% manganese and 3.5–5.5% nickel. Standard 200-series steels have a long track record in North America and India. The trouble is the cheap “new 200-series” variants with less chromium. The stainless industry’s own trade body warned that some of these grades are not comparable to established stainless steels in corrosion resistance, and that some had been fraudulently sold as type 304.

A 200-series pot isn’t automatically dangerous. A test of grade 201 found releases well below the Council of Europe limits under the standard acid test. But if you paid for 304 and got 201, you got a steel that’s less resistant to corrosion, and nobody told you.

How much nickel and chromium get into food?

The study everyone cites

In 2013, researchers at Oregon State University simmered tomato sauce with stainless steel and measured what came out. The headline: after six hours, nickel in the sauce rose up to 26-fold depending on the grade, with chromium rising too.

That figure needs context, because most of the experiments didn’t use pans. They used small chips of reference steel, which put about ten times more steel surface against the sauce than a saucepan does. The researchers also tested one real 316 saucepan. Plain sauce started at 0.130 mg of nickel per kg. After 20 hours in the new saucepan it held 3.84 mg/kg nickel, and chromium rose from 0.2 to about 0.6 mg/kg. The authors calculated that a 126 g serving of sauce cooked that way in a new pan would carry 483 µg of nickel.

Twenty hours is a very long simmer, and that is the worst-case number. Three other findings matter more for everyday cooking:

• New steel leaches most. Nickel release from the chips fell by about 65% by the third cooking cycle and levelled off after the sixth. It never reached zero: by the tenth cycle, a serving still picked up an average of 88 µg nickel and 86 µg chromium (chips, not a pan).
• Grade alone doesn’t predict leaching. Two steels that both count as 304 behaved differently: one leached about 56% more nickel than the other. The nickel content of the steel didn’t line up with the amount released.
• Time and acidity drive it. Longer cooking meant more metal.

What other studies found

The Oregon study is the scariest-sounding, and it isn’t the only one.

• An Italian study cooked tomato sauce and lemon marmalade for one hour in pots from several makers. Release was higher in new pots, in acidic food and with longer cooking, and varied a lot between manufacturers. Every amount measured was below known allergy-triggering levels.
• A 1995 study by the industry-backed Nickel Development Institute, using 19% chromium/9% nickel pans for ordinary household recipes, found pans added 0 to 8 µg of nickel per portion, apart from new pans on first use. The worst case was a new pan used first for acidic fruit, which added about a fifth of normal daily nickel intake. That didn’t recur, even after the pan was scoured.
• Croatian public health testing found many imported cooking utensils failed release limits on the first test, but most passed on repeated extraction.
• Lab tests of grade 201 found most metal came out in the first half-hour, before the surface re-passivated. Abrasion, heat, stronger acid and more steel per volume of food all increased release. A study of grade 430 found the same: more release with more surface, higher temperature and rougher finishes.

The pattern across all of these is consistent. Release is highest from new or freshly abraded steel, with acidic food, at high heat and over long cooking. It drops sharply as the surface oxide settles in. How a surface is finished at the factory matters too, which is why pots of the same nominal grade can behave differently.

Cheap utensils fail more often

Grade isn’t the only variable; quality control is another. When Irish researchers tested 120 stainless serving utensils bought online, 35% of sample types failed the Nordic migration limits, and the metal released tracked what the steel was actually made of. Even so, their exposure estimates found no significant health effect from the iron, aluminium and nickel exceedances. A 2026 review pooling 69 studies found stainless steel exceeded release limits in 11.0% of measurements, compared with 36.1% for other metal cookware, 12.5% for plastics and 5.6% for glass. Those studies used different methods and limits, so read that as rough context, not a league table.

Should you worry? It depends on your skin

For most people: no

Europe has a non-binding benchmark for this. The Council of Europe’s 2013 technical guide on metals in food contact set release limits of 0.14 mg/kg of food for nickel and 0.250 mg/kg for chromium, and said makers should provide cleaning instructions before first use. The 2024 second edition raised the chromium limit to 1 mg/kg.

For chromium, EFSA treats the chromium in food as the trivalent form, not the hexavalent chromium of industrial pollution fame. It set a tolerable daily intake for trivalent chromium of 0.3 mg per kg of body weight, assumed all chromium in food is trivalent, and concluded dietary exposure isn’t a concern.

Nickel is more nuanced. EFSA’s 2020 assessment set a tolerable daily intake of 13 µg per kg of body weight and found that, for most adults, normal diets fall below it. The high end of toddler and child diets generally exceeds it, but that assessment is about nickel already in food and water, not specifically from pans.

For people with nickel allergy: maybe

Nickel is the most common contact allergen. In a five-country European study that patch-tested random adults, 14.5% were allergic to nickel and 0.8% to chromium. A clinical review puts nickel allergy at 8–19% of European adults, mostly women, mostly sensitised by jewellery and piercings.

Most people with a nickel allergy react only to skin contact. A minority also get eczema flare-ups after eating nickel, called systemic contact dermatitis. EFSA chose this reaction as the critical effect for a single meal, with a lowest-observed-adverse-effect level of 4.3 µg of nickel per kg of body weight. For a 70 kg adult that’s about 300 µg. EFSA concluded that ordinary dietary exposure already raises a health concern for sensitised people. A meta-analysis estimated that 1% of nickel-sensitive people may react to normal daily intake of 0.22–0.35 mg.

Put those next to the cookware data. A long-simmered acidic dish in a brand-new pan can add a few hundred micrograms of nickel, enough to matter for the most sensitive people. A one-hour cook in a used pan adds far less. The Italian researchers concluded that 18/10 pots are safe for the majority of nickel-allergic people under common conditions, but that the nickel in food plus pot could still exceed a highly sensitive person’s threshold.

Bottles count too. A 2026 case report in Dermatitis is titled “Systemic allergic dermatitis induced by nickel leaching from a stainless steel bottle containing a citric acid solution”. It is one case, and we could read only the title, but the mechanism is the one the lab studies predict: acid, steel and hours of contact.

Touching stainless is a different matter. Researchers at the French steelmaker Ugine reported that low-sulfur grades including 304, 316L and 430 released very little nickel into artificial sweat and caused no reactions in nickel-sensitised patients. Cutlery handles and pan handles aren’t the problem for allergic skin. Nickel-plated metal is.

Kettles are a known exception. When Danish authorities tested 26 electric kettles, 10 released more than 50 µg/L of nickel into water. Makers redesigned them, and two that still failed on follow-up were withdrawn.

Passivation and the first few uses

“Passivation” is the build-up of that chromium-oxide film. Lab work shows it happens fast: in tests on grade 201, most metal release happened in the first half-hour as the surface passivated. In your kitchen the same settling-in is the likeliest reason leaching falls over the first handful of cooks. No study we found tested a home “conditioning” routine, so treat this as precaution, not proven fix:

• Wash new stainless before first use, as the 2013 Council of Europe guide expects makers to tell you.
• For the first several uses, cook something that isn’t acidic. Don’t christen a new pot with an all-day tomato sauce or a batch of lemon curd.
• Don’t store acidic food (tomato sauce, citrus juice, vinegar pickles) in stainless for days. Cook in it, then transfer leftovers to glass.
• Abrasive scouring removes the surface layer, and lab tests show abraded steel releases more metal until it re-passivates. One older study found no repeat of the first-use spike after abrasive cleaning. If you’re nickel-sensitive, use a non-scratch scrubber and give a freshly scoured pan a non-acidic cook.

“Stainless” bottles that aren’t all steel

Insulated bottles and tumblers are where stainless marketing gets slippery, because the steel shell is often the least interesting part.

• Lids, spouts and straws. Many “stainless” bottles have a plastic lid, a plastic sip spout or a plastic straw. Under our product standard, plastic that touches your drink or your mouth disqualifies a product; a plastic cap that never touches the drink can only make a product “plastic-minimal”.
• Silicone seals. Almost every leak-proof lid has a silicone gasket. Silicone isn’t a conventional plastic, but it isn’t inert either. We allow a small, named silicone seal only in plastic-minimal products.
• Interior coatings. Some bottles are sold as “ceramic-lined” or “coated” inside. That means a coating applied over the steel, and a coating is a different material with its own chemistry. If the maker won’t say exactly what it is, our standard treats that as a disqualifier.
• The base seal. Vacuum-insulated bottles are sealed under the base. In 2024 Stanley said its sealing material “includes some lead”, covered by a stainless steel layer that is “inaccessible to consumers”. An Environmental Working Group scientist told CNN that lead solder has been used to seal insulated cups and that many makers now say they use lead-free alternatives. We found no test showing lead reaching the drink; the concern is a damaged bottle whose base cap has come off, and disposal. Ask whether the vacuum seal is lead-free, and replace a bottle if the base cap comes off.

How to verify a grade (and why the magnet test fails)

The internet’s favourite test is a fridge magnet: if it sticks, it’s “fake”. That’s wrong in both directions.

• A magnet sticks firmly: the steel is probably ferritic (such as 430) or martensitic (most knives). That’s not fake stainless. For nickel-allergic cooks it may even be an advantage.
• A magnet doesn’t stick: the steel is austenitic. That includes 304 and 316, and also the nickel-light 200-series. The industry’s own trade body warned that because new 200-series grades are non-magnetic, buyers may wrongly assume they equal 304.
• A magnet sticks weakly at bends and rims: austenitic steel is non-magnetic when annealed, but cold-forming makes it more magnetic. A deep-drawn 304 bottle can grab a magnet at the base.

What actually works:

1. Ask the maker for the grade of every part that touches food, not just “18/8 stainless”. A reputable maker can tell you whether the body is 304, 316 or 430, and what the lid, gasket and base seal are made of.
2. Ask for a compliance document, such as an EU declaration of compliance for food contact or a test report against the Council of Europe release limits or a national standard. If nothing exists, that’s an answer.
3. Alloy analysis is the only real test. Handheld X-ray fluorescence (XRF) analysers read composition in seconds, and in the utensil study above, XRF composition tracked how much metal migrated. Scrap dealers and metal labs have them; you probably don’t.
4. Be suspicious of a “304” price that’s far below 304’s. Nickel is the expensive ingredient.

Where stainless isn’t the answer

• You have a strong nickel allergy and react to food. Avoid long, acidic cooks in nickel-bearing stainless, especially new pans. A ferritic 430 pan, carbon steel, cast iron, or glass for storage are options. For glazed or enamelled cookware, our standard requires a published lead and cadmium compliance statement.
• Long-term acidic or salty storage. Steel is for cooking and serving. Pickles, ferments, citrus juice and brine are better kept in glass. If you must use steel for salt, 316 handles chloride far better than 304.
• Cheap, unlabelled utensils. Bargain online utensils are where the release-limit failures cluster. Buy fewer, better-documented pieces.
• When the steel is a wrapper for plastic. A steel tumbler with a plastic lid you drink through, or a steel container with an undisclosed coating, isn’t a plastic-free product.
• When you already have something that works. A glass jar you own beats a new steel one. Steel’s environmental case rests on using it for decades.

For which kitchen swaps reduce the most plastic exposure, see our ranked kitchen swaps. For pans specifically, including how stainless compares with nonstick, cast iron and carbon steel, see our nonstick and PFAS guide.

What to do, ranked by evidence

1. Don’t break in new stainless with long, acidic cooking. Strongest evidence: every study found new steel leaches most and acid drives release.
2. If you’re nickel-allergic and react to food, limit long acidic cooks in nickel-bearing stainless. Good evidence for the mechanism and thresholds. Whether a typical pan triggers your reaction is individual.
3. Store acidic leftovers in glass, not steel. Supported by the time-and-acid data. A cheap precaution.
4. Ask for grades and documents, not magnets. Industry sources agree the magnet test can’t separate 304 from 200-series steel.
5. Check every part of a bottle: lid, seal, coating, base. Based on our standard and the makers’ own disclosures, not on leaching studies.
6. Treat first-use routines as a precaution. No study we found tested a home conditioning method. Washing first and a few non-acidic cooks follow the leaching data, but nobody has measured the effect.

The bottom line: stainless steel is one of the best plastic replacements there is, and for most people the metal it sheds is a rounding error next to what’s already in food. Just know which steel you’re buying, and don’t take the label’s word for it.