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Juneau State Office Mailing Address Update

All mail sent to the DEC office in Juneau through the U.S. Postal Service must be addressed to P.O. Box 111800, Juneau, AK, 99811. Beginning October 1, all mail addressed to our Willoughby Avenue address will not be delivered and will be returned to sender by the Post Office. This change only impacts incoming mail to the Juneau office.

Half Lives Explained

Time is one of the three basic principles of radiation exposure. Hearing that cesium has a half-life of 30 years can be concerning. However, there are different types of half-lives that describe different things. The 30 years is the physical half-life. Here are the different half-life measures and what they mean.

Half Lives

  • Physical Half Life is the time for a quantity of radioisotope to decay by half (Cs-137 = 30 years)
  • Biological Half Life is the time for 1/2 of the amount of a radionuclide to be expelled from the body (CS-137 = 70 to 100 days)
  • Effective Half Life takes into account both physical & biological half lives

1/T effective= 1/T physical + 1/T biological

The below table presents some of the radioactive isotopes associated with a nuclear power plant with their physical and biological half-lives.

Physical and Biological Half Lives of Select Isotopes
Isotope Physical Half-Life Biological Half-Life
Cesium (Cs-137) 30 years 70 days
Cs-134 2 years 70 days
Iodine-131 (I-131) 8.1 days 138 days
Sr-90 28 years 18,000 days