Japanese survivors of the tsunami and earthquake who live near the four stricken nuclear reactors of the Fukushima Dai-ichi plant are being told to take potassium iodide or iodine tablets as a precaution against thyroid cancer.

It is believed that the radiation leaking from the plants is radioactive iodine, which can cause thyroid cancer.

Potassium iodide (or KI, as it’s known chemically) is a common form of salt and is known to protect the thyroid gland from radiation and cancer caused by radioactive iodine.

The chemical works quickly by saturating the thyroid with non-radioactive iodine. That makes it difficult or impossible for radioactive iodine to be absorbed by the thyroid. If the gland has enough iodine in it, the thyroid doesn’t absorb any radioactive iodine and it is flushed out of the system in urine, experts say.

“The thyroid needs iodine to produce the thyroid hormone,” explained Alvin Powers, an endocrinologist and professor of molecular physiology and biophysics at Vanderbilt University’s School of Medicine in Nashville, Tennessee.

“If there is radioactive iodide in anything we eat or drink it gets concentrated in the thyroid, stays there and emits radiation in the thyroid,” When taken before exposure, potassium iodide can provide protection for 24 hours. It also is beneficial if taken three or four hours after exposure, medical experts advise.

If radioactive iodide is in the thyroid it increases the chances of cancer of the thyroid in future years,” said Powers. “How much the risk is increased is controversial, but it increases the risk of thyroid cancer over all, particularly in young people and children.”

Potassium iodine, however, does not protect against other forms of radiation sickness. “All it protects against is the thyroid,” Powers said. “It’s important people don’t think it protects against all the other things that can happen from radiation exposure.”

“Potassium iodide can provide important protection for one organ from radiation due to one radionuclide,” says the Health Physics Society website.