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Appendix E: I-125 Information and Safe Handling Guide


Radioactive half-life 59.6 days
Decay mechanism Electron capture (gamma and x-ray emission)
Energy 27 35 KeV
Contamination monitoring Thin crystal NaI detector, liquid scintillation counter for wipe surveys.
Dosimetry Film badge, TLD ring, thyroid scan.
Shielding thin lead

  • A thyroid bioassay must be scheduled after using 1 mCi or more of NaI, or in cases of suspected contamination.
  • The dose rate at 1 cm from a 1-mCi point source is about 1400 millirem/hr. The quarterly NRC limit of 18,250 millirem for the hands can be reached in 13 hours. The limit for the whole body, 1250 millirem, assuming the source is 3 feet from the body, can be reached in 7440 hours.
  • The dose rate is inversely related to the square of the distance from the source. Thus, while a small amount of I-125 held for a short time can result in a significant dose to the hands, a relatively short separation distance reduces the dose rate to an acceptable level.

Safety Rules

  • The volatility of iodine requires special handling techniques to minimize radiation doses. Solutions containing iodide ions (such as NaI) should not be made acidic or frozen. Both lead to the formation of volatile elemental iodine. Once bound to a protein, the volatility of the radioiodine is tremendously reduced.
  • Always work in a fume hood with a minimum face velocity of at least 125 linear feet per minute when working with NaI. The sash should be lowered as practical as possible.
  • Do not work directly over an open container of I-125.
  • Use lead to shield quantities of 1 mCi or more. Every .7 mm of lead will reduce the gamma radiation emitted from I-125 by 50%. Place the shield near a wall or in a hood away from the main flow of traffic.
  • Avoid opening the septum on delivery vials. It is preferable to remove radioiodine using a syringe.
  • Designate a specific area of the laboratory for all I-125 experiments.
  • All personnel who handle I-125 must wear full-length laboratory coats.
  • Film badges must be worn for all radioiodine work, and finger badges are required when handling 1 mCi or more of I-125.
  • All personnel handling I-125 may wear a ring badge on the hand which is most likely to handle vials, samples, pipettes, etc., containing P-32.
  • All persons in the laboratory must wear whole body film badges, even those who are not handling I-125.
  • A Geiger counter must be in operation during the experiment and preferably at all times. Place Saran Wrap around the counter to avoid contaminating the detector.
  • Use shoulder length gloves with short vinyl gloves on top to minimize skin absorption.
  • Check your gloves frequently with the counter to detect contamination. If contaminated, immediately dispose of the gloves in a radiation waste container.
  • Wearing two pairs of gloves and changing the outer pair every 15 or 20 minutes will reduce the chance of contamination and absorption through the skin.
  • Pipettes dedicated for the use of I-125 should be used. These pipettes should not be used for other purposes as they are easily contaminated by I-125.

Laboratory Cleanup after Use

  • Use the Geiger counter to check your hands, shoes, clothing, all work areas, and equipment used. Check the floor at the area where the isotope was used. If your clothing is contaminated, it will have to be removed and stored until the radiation level decays to background.
  • If any contamination is found on your hands, wash thoroughly with soap and water. This will usually be sufficient to remove surface contamination. If it is not, contact the University Radiation Safety Officer for assistance.
  • If any contamination is found on the work area or equipment, use a commercial radiation contamination remover such as Count Off, with paper towels, to clean the contaminated area.
  • Place the contaminated paper towels in a receptacle labeled as radiation waste.
  • If the contamination cannot be removed, label the area or equipment as radioactive, noting the isotope, the date of contamination, and the maximum dpm found.
  • If any un-removable radiation is found, contact the University Radiation Safety Officer.
  • Check the normal trash container to ensure that no radioactive waste was placed there.
  • Store all radioactive waste in specially marked containers.
  • Send a Radiation Survey report to the University Radiation Safety Officer.

I-125 Decay Rate
Elapsed Time (days) % Remaining Activity Decay Factor
0 100.0 1.0
10 89.1 0.891
20 79.4 0.794
30 70.7 0.707
40 63.0 0.630
50 56.1 0.561
60 50.0 0.500
70 44.5 0.445
80 39.7 0.397
90 35.4 0.354
100 31.5 0.315
110 28.1 0.281
120 25.0 0.250
600 (10 half-lives) 0.1 0.001