The introduction of iodine to table salt is arguably one of the biggest public health policy wins of the 20th century, only overshadowed by the near total bans on tetraethyl lead and CFCs.
Few people (in global terms) had flush toilets in the early 20th century. While iodine was huge for the west, toilets are still probably the most effective public health measure you can apply today as billions of people still lack them.