37

The cookbook "Let's Cook It Right", by Adelle Davis contains the following claim (on page 4 of at least one edition - I'm not sure which edition this is but it's likely from the late sixties or early seventies):

Stainless steel was at first welcomed as a metal unlikely to contaminate foods. Investigation, however, has shown that if even the best stainless steel utensils are scoured only once with an abrasive powder, scratch pad, or steel wool, small amounts of chromium, nickel, and other highly toxic metallic compounds dissolve into every food cooked in them thereafter. If stainless-steel utensils are always soaked clean and no abrasives are used, they appear to be completely harmless. Unfortunately, food is frequently burned the first time such a utensil is used, and the pan is quickly scoured. Some authorities believe chromium and nickel are so toxic that none should be ingested; they recommend discarding any utensil which has been scoured even once. Exactly how dangerous these metals are is unknown at present, but they do appear to be considerably more toxic than aluminium. Ingested aluminium combines with phosphorus, a nutrient excessively abundant in the American diet; the resulting aluminium phosphate is excreted from the body.

(My emphasis in quoted text)

Can anyone identify the original research and authorities behind the bolded claims?

Are scoured stainless steel pans still considered to be a significant health risk by any modern authorities?

If an abraded stainless steel surface does continually release metal particles, are the chromium and nickel atoms in a stainless steel particle significantly bio-available?

I'm surprised that any amount of abrasion or scouring would turn a utensil into an ongoing source of significant metal contamination, considering the ductility of steel. I would expect that since the pan is made from formed steel, abraded and unabraded surfaces would not be significantly different in terms of metallurgical composition (e.g. percentage of nickel or chromium vs iron atoms).

7
  • 11
    It's curious that they claim that chromium is "so toxic that none should be ingested". Many popular multivitamins include chromium. Apr 13 at 19:00
  • 22
    Adelle Davis was a food fad writer in the 1960s. Her books were filled with misinformation, and multiple deaths, including those of children, were associated with her advice. She was an early example of the same sort of quackery and scamming that permeate social media today.
    – barbecue
    Apr 13 at 19:37
  • 6
    Even with substances known to be toxic, the right question is never "is the substance present?" but "how much is present is that level a risk?"
    – matt_black
    Apr 14 at 12:39
  • 6
    @EndAnti-SemiticHate Hexavalent chromium and chromate salts, in any appreciable amount, really are highly toxic. Trivalent chromium is possibly an essential nutrient, and at worst non-toxic in the same sense that iron is. Many people think mostly of those chromate salts, especially lead(II) chromate or zinc chromate (both historically used as yellow pigments and both nasty toxic), but know very little about trivalent chromium, which is what’s in those multivitamins. Apr 14 at 20:50
  • 5
    @AustinHemmelgarn makes a very good point about specifying the compound rather than relying on colloquial simplified terms. Sodium will literally ignite in your mouth and chlorine is a Geneva-convention-violating part of this daily breakfast, but combined they're an essential and delicious nutrient.
    – Jay McEh
    Apr 15 at 16:29

1 Answer 1

87

The claim is wildly overblown

TL;DR While chromium and nickel (and iron) do leach into your food from stainless steel, they do so in forms and levels which are safe, even essential. Chromium and nickel are already naturally present in soil and food.

The author has likely confused this with extremely toxic forms of chromium and nickel.

There's no point to the abrasion claims if it isn't toxic, I'll just address the toxicity claims.

Yes, chromium and nickel are in your food...

There's no need to look to abrasion, most food already contains trace amounts of chromium and nickel. Cooking for long periods with an acidic food such as tomato sauce in a new stainless steel pan will leach more chromium and nickel. Repeated use reduces the leaching significantly until it effectively stops. The amounts are in micrograms (1 millionth of a gram).

Stainless Steel Leaches Nickel and Chromium into Foods During Cooking goes for the maximum leaching effect by cooking tomato sauce (one of the most acidic foods you're likely to cook) in brand new pans for hours. Even then the amounts were very low, and they drop with every use. They were unable to reach the Tolerable Upper Intake Level (UL), also known as Tolerable Daily Intake (TDI), used by the paper.

Note: UL/TDI is for chronic lifetime intake, it is not harmful to occasionally exceed the TDI. The paper demonstrates a worst case and unsustainable level of exposure. More on UL/TDI below.

...but safe forms of chromium and nickel.

Some authorities believe chromium and nickel are so toxic that none should be ingested;

Chromium and nickel are elements, but bare elements are rarely found in nature. They are almost always found combined together, often with other elements, into compounds. For example, the "lead" in your pencil is graphite, pure carbon. A diamond is also pure carbon. These are both "carbon", just arranged differently. The food we eat is also mostly carbon.

It's the same for chromium and nickel. There are many forms, some are extremely toxic, some are very safe. The author does not specify. Trace amounts of chromium and nickel are present naturally in the soil, water, and many foods, so I don't know what those "authorities" expect you to eat. I suspect the author mixed up the chromium and nickel present in stainless steel with highly toxic hexavalent chromium and nickel tetracarbonyl.

If someone is trying to make you scared of an element or "chemicals" or even "heavy metals", take it with a grain of sodium and chlorine; ask for details. Let's look at chromium and nickel in more detail.

Chromium

Chromium(III), trivalent chromium, is used by the body. A healthy individual should be getting about 20-30 micrograms a day. It's present in the soil and shows up in trace amounts in the food we eat. This is the form present in stainless-steel. Any scouring would produce chromium(III) oxide.

The chromium achieves corrosion resistance by forming a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible so the metal always appears shiny. It is, however, impervious to water and air, protecting the surface of the metal beneath. Also, when the surface is scratched, this layer quickly reforms

From A Guide to Hexavalent Chromium Cr(VI) for Industry by the North Carolina Department of Labor

EFSA’s experts establish a tolerable daily intake (TDI) of 0.3 milligrams per kilogram of body weight per day for chromium III – which occurs naturally, is an essential nutrient and the main form of chromium present in food.

From the European Food Safety Authority (EFSA)

For a 70 kg person the TDI is 21 mg per day every day. That's 8 times higher than the UL used in the paper.

Chromium(VI), hexavalent chromium is extremely carcinogenic. However, this is of a concern for people who work with chromium, such as welders, or might be drinking contaminated drinking water. Hexavalent chromium is not present in stainless steel but it can be produced in its manufacture.

Hexavalent chromium can also be formed when performing “hot work” such as welding on stainless steel, melting chromium metal or heating refractory bricks in kilns. In these situations, the chromium is not originally in the hexavalent state but at sufficiently high temperatures undergoes oxidation (i.e., loses electrons) to yield the hexavalent form.

Nickel

Nickel is also present in most foods, it's essential for plants, but it has not been identified as an essential nutrient for humans. Most that you eat is urinated out. While people can develop an allergy to nickel, it's not considered toxic at the levels leaching out of a stainless steel pan.

Nickel Tetracarbonyl is extremely toxic. It is made by reacting nickel with carbon monoxide (CO), if you have high concentrations of CO in your kitchen you have more pressing problems, and it decomposes very quickly in air. It also isn't present in stainless steel.

Experts increased the safe level, known as the tolerable daily intake (TDI), from 2.8 micrograms per kilogram of body weight to 13 micrograms per kilogram of body weight.

From EFSA

For a 70 kg person that's about 1 mg of nickel per day, every day. This is the same as the UL used in the leaching paper.

What about abrasion?

Knowing all that, how much steel would you have to scrape off your pan every day to exceed the TDI of chromium and nickel? Is it even plausible? Let's do some quick back of the envelope calculations.

The austenitic stainless steel used in most stainless steel pans is about 18% chromium and 9% nickel.

To reach the TDI of 1 mg of nickel you'd have to scrape over 11 mg of steel off your pan every day.

To reach the TDI of 21 mg of chromium you'd have to scrape 117 mg of steel off your pan every day.

A stainless steel pan weighs about a pound or 453 grams. At 117 mg per day you're losing 10% of the pan per year. Unless you're cleaning with a sandblaster, this isn't plausible.

16
  • 5
    @Hydrargyrum I've added more references. I don't plan on digging into their abrasion claims, the abraded material is not toxic at anything near these concentrations unless you have a nickel allergy. "even an unabraded pan can leach a fair amount of metal" No, it's very, very little metal. I'll update the answer to explain.
    – Schwern
    Apr 15 at 2:51
  • 5
    @Hydrargyrum Updated. The leaching in the worst-case scenario does not even reach TDI.
    – Schwern
    Apr 15 at 3:06
  • 8
    @Hydrargyrum I added some back of the envelope calculations for how much you'd need to scrape off the pan to even reach TDI. It's a lot.
    – Schwern
    Apr 15 at 3:16
  • 6
    @BЈовић this answer states, quite conclusively, that it is fine to continue using scratched pans made of the specific metals listed. I don't know if the advice also covers things like cheap teflon or other anti-stick coatings.
    – terdon
    Apr 15 at 15:27
  • 3
    @BЈовић It's ok to use scratched stainless steel pans (or cast iron or carbon steel). Non-stick pans, ceramic pans, enameled pans... these vary depending on how it's made.
    – Schwern
    Apr 15 at 19:12

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .