From January to July 1951, over 20,000curies of radioactive Iodine-131 were released over Eastern Washington. This information was uncovered by HEDR (Hanford Education Dose Reconstruction). Iodine-131 has a half-life of 8 days, which is not so long compared to other elements, but for Hanford workers who were in direct contact with the contaminated air for at least 6 of those 8 days, the radiation exposure would have caused health problems for the thyroid that wouldn't have been immediately discernible. The HEDR project dedicated much of its effort to testing people who had been exposed to Hanford's radiation, Hanford workers, people living on the Columbia River, Richland inhabitants. The Iodine filters were replaced in July; more attention was paid to safety after that.

References

Iodine Half-Life