Radionuclide Safety Data Sheets

Iodine - 131(131I)


Gamma Energies: 364 keV (82% abundance)
637 keV ( 7% abundance)
284 keV ( 6% abundance)
723 keV (2% abundance)
80 keV (3% abundance)
29-34 keV (4.5% / x-rays)
Beta Energies: 192 keV (89% abundance / average)
606 keV (89% abundance / maximum)

Beta particles with energies of 70 keV and 795 keV can penetrate the dead layer of skin and lens of the eye, respectively.

Fraction of I-131 beta particles (606 keV) transmitted through the dead layer of skin (0.007 cm) is approximately 80%.

Physical Half-Life: 8.05 days
Biological Half-Life: 138 days
Effective Half-Life: 7.60 days
Specific Gamma Constant: 0.22 mR/h at 1.0 meter per millicurie
Specific Activity: 124,068 curies/gram
Maximum Beta Range in Water: 2 mm = 0.20 cm = 0.08 in
Maximum Beta Range in Air: 165 cm = 65.0 in = 5.40 ft



  • Critical Organ (Biological Destination): Thyroid
  • Routes of Intake: Inhalation, Ingestion, Puncture, Wound, Skin Contamination (Absorption)
  • External & internal exposure and contamination are primary radiological concerns

Committed Dose Equivalent (CDE):
(Organ Doses)
1080 mrem/uCi (inhalation / thyroid)
1761 mrem/uCi (ingested / thyroid)
1776 mrem/uCi (puncture / thyroid)
0.45 mrem/uCi (ingested / breast)>

Annual Limit on Intake (ALI):


  • 30 uCi (all compounds / CDE / 50 rems to Thyroid)
  • 90 uCi (all compounds / CEDE / 5 rems to Whole Body)

[1.0 ALI = 90 uCi ingested = 5,000 mrem CEDE (Whole Body)]


  • 50 uCi (all compounds / Class D / CDE / 50 rems to Thyroid)
  • 200 uCi (all compounds / Class D / CEDE / 5 rems to Whole Body)

[1.0 ALI = 200 uCi inhaled = 5,000 mrem CEDE (Whole Body) = 2,000 DAC-hrs]


Skin Contamination Beta Dose Rate: 4,769 mrem/hour per 1.0 uCi/cm2
* [Localized dose to basal cells at 7 mg/cm2 or 0.007 cm depth in tissue without air reflection]

Skin Contamination Beta Dose Rate (Extremity): 1,182 mrem/hour per 1.0 uCi/cm2
* [Localized dose to extremity skin at 30-50 mg/cm2 or 0.03-0.05 cm depth without air refletion]

Skin Contamination Gamma Dose Rate: 61 millirem/hour per uCi/cm2

NOTE: Skin dose assessments must account for gamma dose to the skin as well as beta dose even though the gamma contribution will be small compared to beta dose.

  • Thyroid accumulates 30% of soluble radioiodine in the body. The % uptake for adults and children are similar.
  • Inhaled radioiodine reaches equilibrium with body fluids in about 30-minutes.


Half-Value Layer (HVL / Lead): 0.09 inch = 0.23 cm
Tenth-Value Layer (TVL / Lead): 0.28 inch = 0.70 cm
Half-Value Layer (HVL / Water or Tissue) 2.50 inch = 6.30 cm

* Half-Value Layer (HVL) is the thickness of any given absorber or shield that will reduce the intensity of a radiation beam to 1/2 (50%) of its initial value.

* Tenth-Value Layer (TVL) is the thickness that will reduce the intensity of a radiation beam to 1/10 (10%) of its initial value.

NOTE - Plexiglass, acrylic, plastic, wood, or other low-density material will NOT shield I-131 gamma; use lead bricks.

EXPOSURE RATES: From an unshielded 1.0 millicurie isotropic point source I-131.

Distance mR/hour
1.00 cm 2200.00
10.00 cm 22.00
6.00 in 9.50
100.00 cm 0.22


  • Use a survey meter and, preferably, a G-M pancake/frisker (15.5 cm2 surface area) probe to detect I-131 contamination. G-M pancake/frisker probe efficiency for I-131 is ~ 8%.
  • Use a survey meter and a NaI scintillation probe to obtain highest sensitivity and counting efficiency; however, a G-M survey meter is adequate and most cost-effective for I-131 laboratory work.
  • Liquid scintillation counter (indirect counting) should be used to detect removable I-131 contamination on smears or swabs.

PERSONAL RADIATION MONITORING DOSIMETERS (Whole Body and Finger Tabs): REQUIRED when handling > 5 mCi of I-131 at any time.

THYROID BIOASSAY: REQUIRED after working with > 1.0 mCi of I-131 on an open bench top or > 10.0 mCi in an exhaust hood. Contact RSS (764-4420) for thyroid count.

  • For a continuous inhalation exposure rate of 1/365 of an ALI per day, the following equilibrium are attained:

* Thyroid Activity: 0.262 uCi (Class "D" / Inhalation)
* Daily Urinary Excretion Rate: 0.054 uCi / day


  • Derived Air Concentration (DAC): 2.0E-8 uCi/mL (all compounds)
  • Airborne Effluent Release Limit (Annual Average):* 2.0E-10 uCi/mL (all compounds)
    • * Applicable to the assessment & control of dose to the public (10 CFR 20.1302). If this concentration was inhaled or ingested continuously over one year it would produce a TEDE of 50 millirem.
  • Urinalysis: Not Required; however, may be requested by RSS/OSEH after an I-131 spill or suspected intake.
  • Unrestricted Area Removable Contamination Limit: 200 dpm / 100 cm2
  • Container Labeling Requirement [10 CFR 20.1905]: ≥ 1 uCi


  • Inherent Volatility (STP): SIGNIFICANT [volatilization is a very significant concern with I-131 especially in a disassociated (free) form or acidic solutions]
  • Acidic and frozen solutions enhance radioiodine volatility.
  • Store at room temperature: DO NOT FREEZE (whenever possible)
  • Radioiodine labeled compounds should be assumed to be potentially volatile because decomposition can give rise to free iodine in solution. Maintaining radioiodine solutions at low (dilute) concentration minimizes radiolytic decomposition.
  • Soluble iodide ion is oxidized to elemental (free) iodine that has low solubility in water and a high vapor pressure. Acidic solutions enhance the oxidation of sodium iodide to elemental (free) iodine; thereby, increasing volatility.
  • Regulatory limits on personal intakes and environmental releases of I-131 are quite restricted because of the relatively high radiotoxicity relative to other common university-related radionuclides.
  • Urine bioassays should be conducted approximately 24-hours after a suspected intake of I-131.
  • Thyroid bioassays conducted by Radiation Safety Service (RSS) personnel must be conducted after handling > 1.0 mCi of free or unbound (NaI) form of I-131 on a bench top or > 10.0 mCi in an exhaust hood. Contact RSS/OSEH (764-4420) for a thyroid count.
  • Addition of antioxidants (sodium thiosulfate) to either labeled or sodium iodine solutions of I-131 will help reduce both decomposition and volatilization. Alkaline sodium thiosulfate should be used to chemically stabilize I-131 prior to initiating decontamination of an I-131 spill (0.1 M NaI, 0.1 M NaOH, and 0.1 Na2S2O3).
  • Drying can form airborne I-131 contamination.
  • Radioiodine in the body is eliminated quite rapidly via the urine.
  • Most radioiodine accidents are in a soluble form and will be rapidly absorbed via inhalation, ingestion, absorption through the skin, or any combination of these routes.
  • Due to its volatile character and ease of absorption, potentially exposed individuals should be monitored after any accident or spill either by in-vivo (thyroid count) or in-vitro (urine) analysis.
  • Thyroid counts made within 12-hours after a suspected intake of I-131 often may be unreliable due to skin contamination.
  • Of the iodine entering the transfer compartment of the body, approximately 30% is taken up by the thyroid and the remainder (70%) is assumed to be excreted in the urine (ICRP 54).
  • Iodine is lost from the thyroid in the form of organic iodine. This organic iodine uniformly distributes among all organs & tissues of the body, other than the thyroid, and is retained with a biological half-life of 12 days. 90% of the organic iodine lost from the thyroid is returned to the transfer compartment and the rest is excreted via the feces.
  • The administration of stable iodine (KI or Lugals Solution) blocks the transfer of radioiodine to the thyroid. The onset of inhibition (thyroid blocking) occurs rapidly after administration of stable iodine.
    • NOTE: The use of stable iodine blocking agents is a personal choice. RSS / OSEH will NOT recommend the use of such blocking agents due to any potential personal side effect from such agents.
  • The urinary excretion rate decreases by more than two orders of magnitude within 5 days after intake. Thus, uncertainties in interpretation of urinary excretion that arise because of the unknown time of intake in routine monitoring may be large unless exposure is avoided for 5 days before sampling.
  • Expelling I-131 solutions through syringe needles and pipette tips can generate airborne aerosols.
  • Always wear a lab coat and disposable gloves (preferably, two pairs) when handling I-131.
  • Monitor hands, lab coat, shoes, work areas, and floors using a G-M survey meter equipped with a pancake/frisker probe for gross contamination.
  • Monitor for removable surface contamination by smearing, swiping, swabbing, or wipe testing where I-131 is used. Count smears or swabs in a liquid scintillation counter (LSC), gamma counter, or gas proportional counter (GPC).


  • Iodinations must be conducted in an RSS/OSEH-approved exhaust hood.
  • Iodinations must only be conducted using an RSS/OSEH-approved "closed" system (no pipetting & no open containers during iodination process). Only use rubber-septum sealed vials or containers and syringes.
  • A RSS/OSEH health physicist must observe initial cold and hot iodination runs.
  • Thyroid bioassays are required after using > 1.0 mCi of I-131 on an open bench or iodinating with > 10 mCi in an exhaust hood (Byproduct Material License / Regulatory Guide 8.20).
  • Whenever possible, perform iodination reactions in the original sealed shipping vial when handling potentially volatile radioiodine.
  • Vent the airspace of stock and reaction vials through an activated charcoal-filled syringe trap during iodination procedures.
  • Remove potentially contaminated syringe needles from stock an reaction vials through absorbent material (tissue paper, cotton, etc.).
  • Store I-131 contaminated objects (syringes, stock vials, waste, etc.) in sealed containers (zip-lock bags, plastic containers, etc.).
  • A solution of sodium thiosulfate should be on-hand during iodination procedures.
  • Obtain iodination safety protocols from RSS/OSEH.