We might never know the exact health effects of cast iron pans; too much iron lined to Alzheimer's and colorectal cancer

Very good article - https://examine.com/nutrition/are-cast-iron-pans-unsafe/

Summary:

Aside from stone, iron is the oldest cooking surface still in use. This provides evidence for its lack of obvious harm, but doesn’t necessarily mean it’s totally safe.

(You can’t just dig up pure iron from the ground. Pure iron is rare and mainly comes from fallen meteorites. And it's actually pretty soft, so not great for making pans without adding in some carbon for hardening. But still, around 97-98% of a cast iron pan is plain ol’ iron, which is why we’re so interested in its health effects. Our discussion also applies to carbon steel cookware (such as woks), which is made up of 99% iron.)

Cast iron and carbon steel pans are very similar in their makeup and kitchen use, so potential health concerns from cast iron pans also apply to carbon steel pans, which are also known as “blue steel” or “black steel” pans. 

Too much iron has been linked to a wide variety of conditions, such as Alzheimer’s, heart disease, and colorectal cancer to name just a few. There’s a couple groups of people who don’t have to worry quite as much about iron overload though: menstruating women and vegetarians/vegans. But for others, especially those who regularly eat red meat, it doesn’t take much to push yourself into excess iron territory.

Iron is almost universally labeled as “good” among laypeople and even some health professionals. But too much iron is uniquely harmful, as the body cannot get rid of it, and iron has a tendency to produce free radicals. Thus, excess iron is linked to many diseases.

How much iron are we talking?

We’re talking a pretty decent amount of iron, depending on the condition of your cast iron pan, and what specifically you’re cooking.

To put it into context: men need 8 mg of iron a day, and a serving of tomato sauce cooked in a cast iron skillet can provide 5 mg of iron!

Iron content of foods cooked in a cast iron pan

Cast iron pans can leach a sizeable amount of iron into your food, exceeding dietary intake in some cases. Acidic foods will contribute to much more leaching while an old, heavily-seasoned pan will leach much less iron than a newer one.

Stainless steel doesn’t leach much iron, due to its protective shield of chromium oxide. But it may still leach small amounts of other metals such as nickel, which some people have allergic reactions to.

Seasoning isn’t just recommended for cast iron pans, it’s a requirement. The seasoning layer is comprised of broken down then polymerized unsaturated fatty acids. Multiple thin layers of seasoning built up over time are a sign of a well-used and largely non-stick cast iron pan.

The pros of the seasoning process are numerous: you can eventually cook eggs without them sticking, you don’t have to re-season as often, the pan won’t rust, and you can get much cooking cred from your foodie friends.

The cons are harder to quantify. Bits of the seasoning will come off over time (and be replaced by more seasoning). Nobody knows exactly how much comes off over time, nor do they know what the health effects are of eating tiny bits of this type of broken down fat. If you heat the pan up fairly high over long periods, might carcinogenic fumes or free radicals develop from the oxidized oil? Would small amounts of these hypothetical byproducts even be of concern, given the natural antioxidant defenses our bodies employ?

Despite this uncertainty, you shouldn’t be overly alarmed. The flaxseed oil seasoning on your cast iron pan may be oxidized, but it’s not rancid. Meaning, it doesn’t impart undesirable odors or flavors (for the most part). This may seem confusing at first, since all the double bonds in a bottle of flaxseed oil mean that it can go rancid easily, when not refrigerated. This is because the double bonds are easily attacked by air and light, among other factors.

Seasoning is basically oxidized then polymerized polyunsaturated oil. The risks, if any, of eating tiny amounts of seasoning every day for years is unknown. But they’re probably not very large.

Even with all the options available, there is no perfect pan, given the wide variety of factors people look at. These include non-stickiness, searing ability, ease of use, even heating, and so on and so on. Cast iron does well with some of these and poorly with others.

The decision to choose cast iron or a different cooking material depends on a variety of personal preferences, including risk aversion, what you enjoy cooking with, and what you already own. Trying out more than one type of pan may be wise, or even using more than one type of pan on a regular basis, depending on what’s best suited for the job at hand.

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Question:
I have used iron skillets for years.  Some belonged to my Mother.  A friend asked me if it was safe to use because of the iron in the skillet.  I told her I have been eating from them and using them for years & never heard anything about the iron in the skillet not being safe.  Please let me know.  Thank-you for your informative care of the skillets. – Brenda Weldon (12/16/05)

Answer: 
Yes, cooking in a cast iron skillet can add significant amounts of iron to your food and into your body… if you eat it. This was proven by researchers who tested 20 foods, the results of which were published in the July 1986 issue of the Journal of the American Dietetic Association.  They measured the iron and moisture content of these items when raw, and after cooking in an iron skillet and a non-iron (Corning ware) dish, separately.  A new, seasoned iron skillet was used, in the event prior use might have affected iron absorption.  The researchers also compared iron absorption when using a new iron skillet versus an older one.

Foods tested (100 g./3 oz.)	Iron content - raw	Iron content - cooked in Cast iron
Applesauce, unsweetened	.35 mg.	7.38 mg.
Spaghetti sauce	0.61	5.77
Chili with meat and beans	.96	6.27
Medium white sauce	.22	3.30
Scrambled egg	1.49	4.76
Spaghetti sauce with meat	.71	3.58
Beef vegetable stew	.66	3.4
Fried egg	1.92	3.48
Spanish rice	.87	2.25
Rice, white	.67	1.97
Pan broiled bacon	.77	1.92
Poached egg	1.87	2.32
Fried chicken	.88	1.89
Pancakes	.63	1.31
Pan fried green beans	.64	1.18
Pan broiled hamburger	1.49	2.29
Fried potatoes	.42	.8
Fried corn tortillas	.86	1.23
Pan-fried beef liver with onions	3.1	3.87
Baked cornbread	.67	.86

 https://whatscookingamerica.net/Information/IronCastIron.htm