I am seasoning a carbon steel wok and from what I understand it basically involves heating cooking oil on it until it gets to the smoke point and then forms a polymerized layer that makes it like a non stick pan. I have also heard that as oil gets heated close to and past the smoke point it breaks down releasing free radicals and other unhealthy compounds. My question is that when the heated oil polymerizes do these compounds get permanently locked up in the polymer or will they eventually get into the food cooked in the pan?
I read several articles in the topic in the past 5 hours, and I have a different conclusion than the other answerer.
My experience that the TG polymer layer on the surface of your wok is far from being stable. Every time you use it, smaller or bigger particles of the polymer can end up in your food. Some of them are even visible, that's why I stopped using a wok a long time ago. If you want to use a seasoned wok no matter what, then do the seasoning with an oil high in polyunsaturated fatty acids (e.g. flax seed oil), so the polymer will be somewhat more stable, because of the additional bonds, but there is no guarantee that it won't break down in time. Don't use these heat sensitive oils for cooking, they are good only for seasoning or for cold salads.
High polymers can be absorbed depending on whether the enzymes of the digestive tract can handle them. For example starch is a high polymer of glucose and amylases can easily hydrolize it, while cellulose is a different high polymer of glucose, which we don't have the enzymes to digest. As of TG polymers, they contain mostly fatty acid monomers, dimers and trimers. So the polimerization is far from being complete, you won't get long fatty acid polymers by cutting down the glycerols with a lipase. According to the articles I read, lipases can hydrolize around 10-50% of the polymers (they are inhibited by them at some degree), so the resulting dimers, trimers, oxidized monomers, cyclic monomers, etc. created in the seasoning process, can be absorbed. It is hard to find studies about their health effect, but they are not considered healthy.
If you use fresh cooking oil containing unsaturated fats (like olive or canola oil) to season your pan, it will thermally polymerize to form a stable layer that gives the cooking surface its nonstick properties. Note that a saturated fat like coconut oil, lard, or ghee will not work very well or at all for this, since it does not have any double bonds to undergo polymerization. The radical formation you are concerned about results over time from air oxidation of unsaturated fats to form peroxides--these in turn break down to give aldehydes and acids that cause the bad flavor of rancid fat, and have adverse health effects. (Not a problem for coconut oil.) Heating fresh oil in a pan to create a nonstick coating may result in some air oxidation in addition to polymer formation, but I think it is not a big cause for concern. High polymers are generally nontoxic because the body does not absorb them.
Based on inf3rno's answer, this is from "Deep Frying Chemistry, Nutrition, and Practical Applications" by Michael Erickson
Digestibility of oxidized fatty acid monomers was considerably high, averaging 76.6%. Among polymeric fatty acids, nonpolar dimers had the lowest digestibility (10.9% on average),whereas oxidized dimers and polymers possessed higher apparent absorbability than expected, ranging from 22.7-49.6% (96).
Basically, the question around all safe cookware questions is, "is a coating of X on the surface" considered safe. In stainless steel it would be chromium oxide. Nonstick coatings are manmade thermoplastic polymers. Glass is silicon dioxide. There seems to be a blind consideration of "cast iron" as safe yet it is quite difficult to figure out what the coating actually is, so this question of asking if it releases free radicals makes a lot of sense.