cast iron pans contain two materials, the seasoning (oxidized and polymerized food oils, occasionally oxidized iron) and the pan itself (an iron-carbon-silicon alloy, potentially with impurities).
some minor outputs of food oil oxidation/polymerization are believed to be probable carcinogens. these compounds will be present in all food cooked on any surface. this varies more significantly based on your food choice and preparation method than your cookware selection. if you're eating, you're consuming oxidized and polymerized food oils.
the pan itself could potentially contribute iron to your food, or molecular variants like rust or magnetite (oxidized iron). this iron isn't harmful. you're more likely to be deficient than have too much iron. in fact, cooking with iron is occasionally advocated as a way to supplement iron nutrition, to treat iron-deficient anemia.
there are potential impurities in the iron alloy of the pan. most impurities are removed by the foundry process as 'slag'. when iron is heated to molten temperature, everything reactive will burn off or evaporate. other metals will float or precipitate. slag is removed before casting, but some may remain mixed - these will be oxides of foundry inputs used to regulate the melt such as calcium, magnesium, aluminum, and barium. these are controlled to low fractions, but even so are nontoxic or nutritious when ingested. if you're using metal cookware, there is some slag in your cookware.
i just now tried to find a study indicating some harmful property of cast iron but i couldn't find one.
just stop using plastic to cook. it's not hard, and it's not expensive. it's even easier than being miserable on the internet