I-131 Nuclear Data: Half-Life, Decay, Radiation
Iodine-131 (131I) is a radioactive isotope of iodine, atomic number 53, mass number 131. It is produced as a fission product in nuclear reactors and is the primary radionuclide used in nuclear medicine for thyroid cancer treatment and hyperthyroidism therapy. Its 8-day half-life makes it the dominant short-term air safety concern in nuclear medicine facilities, and a significant intermediate-term concern in nuclear power plant decommissioning and severe accident scenarios.
| Parameter | Value | Units / Notes |
|---|---|---|
| Element | Iodine (I) | Atomic number 53; halogen group |
| Mass number | 131 | 53 protons + 78 neutrons |
| Physical half-life (t½) | 8.0197 days | 8 days, 4 hours, 44 minutes (NNDC) |
| Decay constant (λ) | 0.08664 day⁻¹ | = ln(2) / 8.0197 = 8.64×10⁻² day⁻¹ |
| Specific activity | 4.599 × 10³ Ci/g | 1.702 × 10¹⁴ Bq/g |
| Decay mode | β⁻ (100%) | → Xe-131m (1.3%) + Xe-131 stable (98.7%) |
| Principal β⁻ energy | 606.3 keV (endpoint) | 89.9% intensity; mean energy 191.6 keV |
| Principal γ energy | 364.5 keV | 81.2% intensity — primary dosimetry/imaging energy |
| Secondary γ energies | 636.9 keV (7.2%), 284.3 keV (6.1%) | Additional dose pathway contributions |
| Daughter nuclides | Xe-131m (t½=11.9d) + Xe-131 (stable) | Xenon — noble gas, chemically inert |
| Biological half-life (thyroid) | ~80 days | Varies with thyroid function and iodine status |
| Effective half-life | ~7.28 days | = (8.02 × 80) / (8.02 + 80) |
Physical vs Biological vs Effective Half-Life
Three distinct half-life values are relevant for I-131 dosimetry and air safety decision-making. Confusing them leads to errors in clearance time calculations and radiation protection program design.
| Half-Life Type | Value for ¹³¹I | Definition | Air Safety Relevance |
|---|---|---|---|
| Physical (t½phys) | 8.0197 days | Time for half the ¹³¹I atoms to undergo radioactive decay; determined by nuclear physics, fixed, identical for all I-131 regardless of chemical form or biological location | Primary parameter for room clearance calculations |
| Biological (t½bio) | ~80 days (thyroid) | Time for the body to eliminate half the ¹³¹I via metabolic processes (thyroid secretion, urinary/fecal excretion); varies by individual thyroid function and dietary iodine status | Used for patient release decisions, thyroid dose calculation |
| Effective (t½eff) | ~7.28 days | Combined effect of physical decay AND biological elimination: 1/t½eff = 1/t½phys + 1/t½bio → t½eff = (8.02 × 80) / (8.02 + 80) = 7.28 days | Used for internal dose calculations; shorter than physical t½ |
Decay Formula & Calculation
The radioactive decay of I-131 follows the standard first-order exponential decay law. This is the fundamental equation used for all I-131 decay calculations — from room clearance time to patient release criteria.
Interactive I-131 Decay Calculator
Calculate remaining activity, percent remaining, or time to reach a target activity level for any I-131 quantity.
Decay Table: I-131 Remaining Activity (0–80 Days)
The following table shows the percentage of initial I-131 activity remaining at key time points. Use this for rapid clearance estimates without a calculator. All values calculated using A(t)/A₀ = e−0.08664t × 100%.
| Day (t) | Half-Lives Elapsed | Fraction Remaining | % Remaining | Reduction Factor | Activity Level |
|---|---|---|---|---|---|
| 0 | 0 | A₀/A₀ = 1.000 | 100.000% | 1× | |
| 8.0 | 1.00 | 1/2 = 0.5000 | 50.000% | 2× | |
| 16.0 | 2.00 | 1/4 = 0.2500 | 25.000% | 4× | |
| 24.0 | 3.00 | 1/8 = 0.1250 | 12.500% | 8× | |
| 32.0 | 3.99 | 1/16 = 0.0625 | 6.250% | 16× | |
| 40.1 | 5.00 | 1/32 = 0.03125 | 3.125% | 32× | |
| 48.1 | 5.99 | 1/64 = 0.01563 | 1.563% | 64× | |
| 56.1 | 6.99 | 1/128 = 0.00781 | 0.781% | 128× | |
| 64.2 | 8.00 | 1/256 = 0.00391 | 0.391% | 256× | |
| 80.2 | 10.00 | 1/1024 = 0.000977 | 0.0977% | 1,024× | |
| 160.4 | 20.00 | 1/1,048,576 | ~0.0001% | ~10⁶× |
After 10 half-lives (80 days), only 0.1% of the original I-131 activity remains — a factor of 1,024 reduction. This is the standard health physics rule for radioactive decay "to negligibility" for most practical purposes. For nuclear medicine I-131 therapy rooms, if a spill introduces 100 mCi into room air, after 80 days only ~100 µCi remains — which may still exceed the DAC in a small room without ventilation.
Airborne Chemical Forms of I-131
The airborne chemical form of I-131 significantly affects both inhalation dose and filtration efficiency. Different chemical forms have different lung deposition characteristics and different adsorption affinities for activated carbon — this is why the NRC specifically requires methyl iodide (the most challenging form) as the carbon adsorber qualification test.
Radiation Protection: DAC, ALI, and Thyroid Dose
The primary radiation protection concern from I-131 inhalation is thyroid dose. Iodine — including radioactive I-131 — is selectively concentrated in the thyroid gland following inhalation or ingestion. A single inhaled breath in a high-concentration environment can deliver significant thyroid dose. The following values govern radiation protection decisions for I-131 airborne exposure.
| Parameter | Value | Regulatory Source | Application |
|---|---|---|---|
| Derived Air Concentration (DAC) | 2×10⁻⁸ µCi/mL | NRC 10 CFR 20 Appendix B | Air monitoring action level; respirator requirement threshold |
| Annual Limit on Intake (ALI) | 50 µCi (soluble) | NRC 10 CFR 20 Appendix B | Intake that produces 0.5 Sv thyroid dose or 5 rem whole body |
| Thyroid dose coefficient (inhalation) | 5.1×10⁻⁸ Sv/Bq | ICRP 68, adult, Type F | Dose per becquerel inhaled; convert: 50 µCi = 1.85 MBq → thyroid dose = 94.4 mSv |
| Thyroid dose coefficient (ingestion) | 7.4×10⁻⁹ Sv/Bq | ICRP 68, adult | Ingestion pathway (nuclear medicine spill, contaminated food) |
| Occupational thyroid limit | 0.5 Sv/year (500 mrem) | NRC 10 CFR 20.1201(a)(2)(ii) | Annual dose limit for radiation workers |
| General public thyroid limit | 0.05 Sv/year (50 mrem) | NRC 10 CFR 20.1301 | Dose to members of the public at facility boundary |
| Potassium Iodide (KI) blocking dose | 130 mg (adult) | FDA Guidance 2001 | Saturates thyroid before emergency I-131 exposure; reduces uptake >95% |
A radiation worker breathing air at the DAC (2×10⁻⁸ µCi/mL) for 40 hours without respiratory protection inhales: 40 hrs × 20 L/min × 60 min/hr × 2×10⁻⁸ µCi/mL = 0.96 µCi intake. Thyroid dose: 0.96 µCi × 3.7×10⁴ dps/µCi = 35,520 Bq × 5.1×10⁻⁸ Sv/Bq = 1.8 mSv thyroid dose. One 40-hour workweek at DAC = 1.8 mSv thyroid. The DAC is designed so a full year (2000 hrs) at DAC = ALI = 50 µCi intake = 94 mSv thyroid — approaching but within the 500 mSv annual limit with significant margin.
NRC 10 CFR 35.75 Ventilation Requirements for I-131 Therapy Rooms
NRC 10 CFR 35.75 and its supporting guidance documents specify engineering controls for hospital rooms containing patients receiving I-131 therapy. The ventilation requirements are driven by the need to protect staff, adjacent patients, and the public from I-131 exhaled by therapy patients, and to manage any accidental spills during administration.
Clearance Time Calculation
After an I-131 spill or accidental release in a facility, the primary clearance mechanism (with adequate ventilation) is radioactive decay — not physical removal. The following formula calculates how long ventilation + natural decay takes to reduce the room air concentration to acceptable levels.
Engineering FAQ
What is the half-life of Iodine-131?
The physical half-life of Iodine-131 (¹³¹I) is 8.0197 days (approximately 8 days and 4.5 hours). This is the time for half of any given I-131 quantity to decay to Xe-131m/Xe-131 via beta emission. The biological half-life (thyroid) is approximately 80 days. The effective half-life — combining physical decay and biological elimination — is approximately 7.28 days: calculated as (8.02 × 80) / (8.02 + 80). For air clearance calculations, always use the physical half-life (8.02 days) and decay constant λ = 0.08664 day⁻¹.
What is the DAC (Derived Air Concentration) for I-131?
The NRC DAC for Iodine-131 (soluble form, inhalation) per 10 CFR 20 Appendix B is 2×10⁻⁸ µCi/mL (2×10⁻⁸ µCi/cm³). This concentration, if breathed continuously for a working year (2,000 hours), would result in an intake equal to the Annual Limit on Intake (ALI = 50 µCi). The DAC is used as the air monitoring action level — when room air concentrations exceed the DAC, respiratory protection (typically a full-face respirator with HEPA and activated carbon) is required. Contact IAS for room I-131 monitoring and filtration solutions: (650) 646-5199.
What ventilation does NRC require for I-131 therapy rooms?
NRC 10 CFR 35.75 and supporting guidance require: (1) negative pressure ≥0.01 in.WG (25 Pa) relative to adjacent corridors; (2) HEPA filtration ≥99.97% + TEDA-impregnated carbon adsorber ≤0.175% CH₃I penetration (ANSI N510) on all room exhaust before recirculation or discharge; (3) minimum 6-12 ACH supply air; (4) continuous negative pressure monitoring; (5) annual in-place HEPA and carbon test documentation. The IAS-NC700-HI (700 CFM, HEPA ≥99.97% + TEDA carbon ≥99.95%) provides all filtration requirements in one portable cart.
Why is methyl iodide (CH₃I) the hardest form of I-131 to filter?
Methyl iodide (CH₃I) is more challenging to capture on activated carbon than elemental I₂ because of its lower polarity and reduced interaction with the carbon surface at normal conditions. While elemental iodine I₂ is easily chemisorbed on standard activated carbon via reaction with carbon surface, CH₃I requires triethylenediamine (TEDA) impregnation of the carbon to achieve nucleophilic displacement reactions that chemically bind the CH₃ group to the TEDA nitrogen. Unimpregnated carbon may remove 60-90% of CH₃I at favorable conditions, but TEDA-impregnated carbon achieves ≥99.95% removal at the ANSI N510 test conditions (70% RH, 30°C). This is why NRC RG 1.52 specifically requires TEDA-impregnated carbon qualified via CH₃I penetration testing.
How long until an I-131 contaminated room is safe to occupy without respiratory protection?
Clearance time depends on initial concentration, room ventilation rate, and target clearance level (DAC = 2×10⁻⁸ µCi/mL). For a room with 12 ACH ventilation and a filtration-equipped exhaust, the effective removal rate is λ_total = 0.08664 (decay) + 12 (ventilation/day) ≈ 12.09/day. Time to reach DAC from 1,000× DAC: t = ln(1000) / 12.09 = 6.908 / 12.09 = 0.57 days (13.7 hours). Same calculation for a sealed room (no ventilation): t = ln(1000) / 0.08664 = 79.7 days — far longer. This demonstrates why adequate ventilation dramatically shortens clearance times. Contact IAS for site-specific clearance calculations: (650) 646-5199.
What is the decay constant (λ) of I-131?
The decay constant of I-131 is λ = 0.08664 per day, calculated as ln(2) / t½ = 0.6931 / 8.0197 = 0.08664 day⁻¹. In other units: λ = 3.609×10⁻³ per hour = 6.015×10⁻⁵ per minute = 1.003×10⁻⁶ per second. The decay rate (disintegrations per second) of any I-131 quantity is: dN/dt = −λN, where N is the number of I-131 atoms. For practical calculations, use the simpler formula: A(t) = A₀ × e−0.08664t (t in days).
NRC 10 CFR 35.75 Compliant I-131 Filtration
IAS-NC700-HI provides HEPA ≥99.97% + TEDA carbon ≥99.95% CH₃I removal in a single 700 CFM portable cart — with factory PAO test certificates and full NRC documentation package.