We've been sprinkling salt over our food and measuring it into batches of cookies for years, and never once has it really occurred to us to wonder why table salt is iodized. Does it change the salt? Improve its nutrition? Prevent it from caking inside the canister? The answer is much more straightforward than you might think.
Iodized salt is quite simply salt to which iodide (a form of iodine) has been added. Iodine is one of those essential elements we need to live, especially for proper functioning of the thyroid gland. It's found naturally in seawater, seaweeds like kelp, ocean fish, and other sea creatures. In other words: seafood!
Iodine deficiency can lead to health problems like goiters and cause mental development issues. It shows up most frequently in populations that don't have access to saltwater - or more specifically, to the iodine-rich seafood living in the saltwater.
In the United States, table salt and iodine became intertwined around the time of World War I when recruits from the Midwestern states showed up for service with iodine deficiency. This prompted a national health movement to add iodide to table salt, a readily available seasoning that nearly every household kept in their kitchens and used daily. (Since most table salt is mined from the land, it didn't contain iodine naturally.) Since then, iodine deficiency has practically disappeared in the US.
As a side note, this is one of the earliest instances of the US government and the food industry working together to provide a "nutritionally-enriched" product for mass-consumption. Interesting, don't you think?
Related: Food Science: Salting to Taste
(Image: Emma Christensen)