Why do people say that cooking in a cast-iron pan will build the seasoning, while also saying that you should never cook to the smoking point?

Yes, the two pieces of advice are contradictory. You have to choose which one you prefer to follow. If you decide to only use your pan below the smoke temperature of oil, the seasoning will not build up during cooking.

In typical use, people do heat their pans above the smoking point, and the seasoning does build up. This is how cooking has been done for centuries, with tasty results, while cooking without heating the oil up produces not-very-exciting results for certain foods. People can choose to use low-heat cooking methods and avoid the "smoke point" part too, but that is not what usually happens. There are many factors why more people use the "high heat that seasons" approach:

• They have never heard the "smoking point" advice and use their cast iron pans the usual way
• They are not aware that they are heating it above the smoke point
• They know the "smoking point" advice and choose to disregard it and cook the usual way
• They would like to follow the "smoking point" advice but it is physically very difficult to do it with a cast iron pan
• (Kinda combination of the last two) They are aware of the difficulties of remaining below the smoking point in a cast iron pan, and choose to use a different pan when they want to stay below the smoking point.

Because of the last one, I would suggest that, if you prefer to follow the "smoking point" advice, you switch to a different type of cookware.

I heat my cast iron pans to the smoking point while cooking in them. Indeed, being able to heat them to 250C/450F (or higher) is a big part of the reason to own a cast iron pan in the first place.

However, you don't generally build up your layers of seasoning while cooking in the pan. Instead, where you build it up is through the brief reseasoning you do after every cleaning, by heating and oiling the pan until it smokes.

Rather than doing a large dedicated seasoning "top-up" or trying to season while cooking (by reaching the smoke point), I opt to cook as I normally would and then perform a quick seasoning after washing my pan.

Usual steps I take when cooking with my cast iron (inspired by Kenji Lopez Alt. ):

1. Cook food (e.g. scrambled eggs on a very low temp)
2. Wash with hot soapy water
3. Wipe mostly dry with towel and place pan back on medium high heat
4. Wipe pan all over with dedicated oily rag and then use "clean" side to wipe most of the oil off (like you accidentally put the oil on and are trying to remove it)
5. Let pan reach smoking point and remove from heat
6. One final light wipe with the oily rag to protect the pan in between uses

Several strands to this question...

• Carcinogens: I'm not a specialist but basically burnt carbon (charcoal) and free radicals (broken down oil) should be avoided in large quantities. You'll get both if you overheat oil in cast iron and then burn the protein you're cooking. Is it dangerous? I would read up more but it's not of my everyday worries...

• Seasoning: when I got my first set of cast iron cookware I obsessed about seasoning, burning lots of oil in the process and spending hours trying to get to 'blackness'. It was all a bit of a waste of time: keep cooking in the same pan at normal (high) temperature and minimise washing. If you can wipe clean with a paper towel, that's your preferred approach. If it has some burnt elements, then soak in warm water without soap and scrub with scotch brite or an equivalent scourer. Then dry with a paper towel. You can use soap on the outside and the handle. The only thing to avoid at all is acidic food like tomatoes that will rapidly remove the seasoning build-up.