Although the time it takes for the radiation to decrease by half (the half-life) of I-131 is only 8 days, the damage it causes can increase the risk of thyroid cancer for many years after the initial exposure.
A study led by NCI researchers followed more than 12,500 people who were younger than age 18 at the time they were exposed to a range of doses of I-131 (0.65 Gy on average) from the Chernobyl accident (7). A total of 65 new cases of thyroid cancer were found in this population between 1998 and 2007. The researchers found that the higher a person’s dose of I-131, the more likely they were to get thyroid cancer (with each Gy of exposure associated with a doubling of risk). They also found that this risk remained high for at least 30 years (9).