The Nature of Iodine-131:

Iodine-131 is a radioactive particle that is produced by the fission of uranium atoms within nuclear reactors. It can also be produced by plutonium and/or uranium in the detonation of nuclear weapons. It takes the form of a purplish-blackish crystal.

"Sublimation" of Iodine-131 means that it can from a solid state to a gas without first becoming a liquid. Iodine-131 dissolves in water.

Occurrence: Iodine-131 can get into the environment through air or water. It is created by fission and it often gets into the environment through leaks. It spreads fairly quickly through water.

Health Effects of Iodine-131, According to Water Technology Magazine

• Long term exposure to radioactive Idodine-131 can cause thyroid cancer.
• Low doses of Iodine-131 can also lower the activity of the thyroid gland by lowering the production of hormones.
• Doctors sometimes use small doses of Iodine-131 to treat an overactive thyroid, however the equilibrium must be perfect or the treatment will cause cancer in the area.
• If large quantities of Iodine-131 are released via some sort of nuclear accident, government agencies can use stable, non radioactive iodine to ensure that people do not absorb too much Iodine-131.

The Japanese Accident.

Following the collapse of the Fukushima nuclear plant in Japan due to the earthquakes that took place in early March 2011, Iodine-131 has been a large concern worldwide. Iodine-131 made water in Japan unsafe for consumption by infants, and in March of 2011 Iodine-131 was discovered in small amounts in rain water wells in Philadelphia. The water, however, was considered safe to drink because of the small amounts of the contaminant

Regulation:

The U.S. Environmental Protection Agency (EPA) has applied both water-based and airborne regulations to Iodine-131, limiting the amount of Iodine-131 that can legally be released by nuclear plants and various industrial facilities. No MCL is established for Iodine-131.

Water treatment: How To Remove Iodine-131 from Water.

The EPA recommends both reverse osmosis and ion exchange for the reduction of Iodine-131.

Sources: Forbes, Los Angeles Times, Philadelphia Inquirer, U.S. Environmental Protection Agency. Water Technology.