Radionuclide Fact Sheet 
IODINE - 125
Symbol: 125I
States/Forms: Inorganic, Free Iodine
Physical Characteristics
- Radioactive Half Life (T½): 60 days
- Decay Type: Electron (e-) capture, Gamma
- Maximum Energy Emission: 0.035 MeV (Gamma)
- Hazard Category
- A Level (High Hazard): >10 mCi
- B Level (Moderate Hazard): >200 μCi to 10 mCi
- C Level (Low Hazard): 0.1 μCi to 200 μCi
Radiological Characteristics
- Critical organ: Thyroid
- Routes of intake: Ingestion, inhalation, skin contamination (absorption).
- Internal exposure and contamination: Primary radiological concerns with 125I
- External exposure: Concern for 125I.
Dosimetry
- Film badges and dosimeter rings are required if 5 millicuries (mCi) or more is to be handled at a given time, or if mCi levels are handled frequently.
Bioassay Requirements
- Thyroid scans.
External Radiation Hazards and Shielding
- The exposure rate at 1 cm from 1 mCi is 1.5 rads per hour (R/hr). The exposure rate varies directly with activity and inversely with the square of the distance. 1/32 of an inch of lead shielding should be used when working with concentrations > 1 microcurie (μCi). ½ inch of lead shielding should be used when working with concentrations > 1 mCi.
Hazards if Internally Deposited
ALI (Annual Limit of Intake):
- 400 μCi – by ingestion
- 600 μCi – by inhalation
DAC (Derived Air Concentration):
- 3 x 10-8 µCi/mL
Survey Instrumentation
- GM survey meters have a poor efficiency of detection for 125I.
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Survey by smear test or use NaI (TI) Scintillation probes.
Problems and Precautions
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Store NaI-125 solutions at room temperature because freezing results in subsequent volatilization of radioiodine.
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Avoid acidic solutions to minimize volatilization.
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Wear two pair of gloves when handling radioiodine compounds.