What are A1 and A2 values?

A1 and A2 are nuclide-specific activity limits that set how much radioactive material a Type A package may contain. A1 applies to special-form material — an indispersible solid or sealed capsule — while A2 applies to normal-form material that could disperse. Both are listed for each radionuclide in 10 CFR Part 71 Appendix A and 49 CFR 173.435, in terabecquerels (with approximate curie values).

When is a Type B package required instead of Type A?

A Type A package may contain up to A1 of special-form or A2 of normal-form material. If a shipment exceeds those limits, a Type B package — engineered and tested to survive severe accident conditions — is required. Type A packages are designed to retain their contents under normal transport conditions and minor mishaps, not major accidents, which is why the activity they may carry is capped.

How are A1 and A2 values derived?

They come from the IAEA Q-system, which limits the dose to a person near a damaged package. The system evaluates several exposure pathways — external photon and beta dose, inhalation, skin and ingestion contamination, and immersion — against dose criteria of 50 mSv effective dose, 500 mSv skin equivalent dose, and 150 mSv lens equivalent dose. A1 uses only the external pathways; A2 uses all pathways, so A2 is generally lower.

How do you handle a mixture of radionuclides?

Use the sum-of-fractions rule in 49 CFR 173.433: divide each nuclide's activity by its applicable A1 or A2 value and add the fractions. If the sum is 1.0 or less, the mixture fits within a Type A package; if it exceeds 1.0, a Type B package is required. Special-form and normal-form contributions are combined in the same sum.

What is the transport index?

The transport index (TI) is the maximum radiation dose rate in millisieverts per hour at one meter from the package surface, multiplied by 100 and rounded up to the next tenth. It governs carrier controls and, together with surface dose rate, determines the package label category — Radioactive White-I, Yellow-II, or Yellow-III.

Do these rules apply to a clinic that only receives isotopes?

Yes, indirectly. A receiving facility does not design the package, but it must know the package type and category to perform the required receipt survey under 10 CFR 20.1906, verify labeling, and handle contamination correctly. Understanding A1/A2 and the category system is part of a defensible radiation-safety program even for a facility that never ships.

Type A and Type B Packages: A1/A2 Limits

Whether a shipment of radioactive material can travel in a simple Type A package or requires a robust, accident-resistant Type B package comes down to two numbers: A1 for special-form material and A2 for normal-form material. Those limits are set nuclide by nuclide in 10 CFR Part 71 and 49 CFR 173.435, derived from international dose criteria, and combined for mixtures with the sum-of-fractions rule.¹⁶⁸

Getting the package type right is a core radiation-safety and compliance duty for any facility that ships or receives isotopes. This guide explains what A1 and A2 mean, where the values come from, how to apply them to single nuclides and mixtures, and how the result connects to package labeling and the receipt survey.³⁴⁵

Introduction

The transport regulations are built on a simple risk idea: the more hazardous a package would be if it failed, the tougher the package has to be. Type A packages are designed to retain their contents under normal conditions of transport and minor mishaps. Type B packages are engineered and tested to survive severe accident conditions — fire, impact, crush, and immersion. The dividing line between the two is set by activity, and that line is defined by A1 and A2.¹⁸

For a medical facility, this matters in both directions. A nuclear pharmacy or manufacturer that ships doses must select and certify the correct package. A clinic that only receives doses must still understand the package type and category to perform its receipt survey, verify labels, and respond correctly to any contamination.³¹⁰

This article defines A1 and A2, presents representative values for common medical radionuclides, works through the single-nuclide and mixture calculations, explains the Q-system that produces the values, and connects the result to labeling, the transport index, and the receiving survey.

Topic Explanation

A1, A2, and physical form

A1 is the maximum activity of special-form radioactive material permitted in a Type A package; A2 is the maximum activity of normal-form material.³ The distinction is physical form:

Special form — an indispersible solid or a sealed capsule that will not break up or leach even if the package fails, tested against specified impact, percussion, bending, and heat criteria. Because a special-form source cannot disperse, the only accident hazard is external dose, so its limit (A1) is comparatively high.
Normal form — any material not qualified as special form, including liquids, powders, and most unit doses. Normal-form material can disperse in an accident, adding inhalation, ingestion, and contamination hazards, so its limit (A2) is generally lower than A1.¹³

The relationship $A_{2} \leq A_{1}$ holds for essentially every radionuclide because A2 must account for more exposure pathways than A1.¹

Representative A1 and A2 values

The values are tabulated for each radionuclide in 10 CFR Part 71 Appendix A and, identically, in 49 CFR 173.435; the two are internationally harmonized with the IAEA transport regulations.¹⁶⁸ Representative values for common medical radionuclides are shown below (terabecquerels, with approximate curie equivalents).

Radionuclide	A1 (TBq)	A1 (Ci, approx.)	A2 (TBq)	A2 (Ci, approx.)
Tc-99m	10	270	4	110
I-131	3	81	0.7	19
Mo-99	1	27	0.6	16
In-111	3	81	3	81
Tl-201	10	270	4	110

Curie values are approximate conversions (1 TBq ≈ 27 Ci) shown for orientation; the regulatory values are stated in terabecquerels, and the applicable edition of the table should always be consulted directly for the exact figures and for any nuclide not listed here.¹⁶

Type A versus Type B

The two package types differ in engineering and testing, not just in the activity they may carry.

Feature	Type A package	Type B package
Activity limit	≤ A1 (special form) or ≤ A2 (normal form)	Above A1/A2 limits
Design basis	Normal transport conditions and minor mishaps	Severe accident conditions (fire, impact, crush, immersion)
Approval	Meets DOT performance specifications	Requires competent-authority design approval/certification
Typical medical use	Unit doses, most nuclear-pharmacy shipments	Large Mo-99 generators in bulk, high-activity sources

A Type A package is the workhorse of routine medical radiopharmaceutical transport; a Type B package is required only when the activity exceeds the A1/A2 limits.³⁴

Key Technical Principles

The single-nuclide test

For a single radionuclide, the package-type decision is direct: compare the activity to be shipped against the applicable limit. If the material is normal form, compare to A2; if special form, compare to A1. If the activity is at or below the limit, a Type A package is permissible; above it, a Type B package is required.⁴

For example, a routine therapeutic iodine-131 capsule shipment of a few gigabecquerels is far below the I-131 normal-form limit of 0.7 TBq (700 GBq), so it ships in a Type A package. Only bulk quantities well beyond clinical unit doses would push a single nuclide over its A2 value.⁶

The sum-of-fractions rule for mixtures

When a package contains more than one radionuclide, 49 CFR 173.433 requires the sum-of-fractions (unity) rule. Divide each nuclide's activity by its applicable limit and add the fractions; the package qualifies as Type A only if the total is at most 1:

i \sum \frac{A _{i}}{A _{2, i}} \leq 1 (normal form)

Special-form and normal-form contributions are combined in the same sum, each divided by its own A1 or A2 value. If the sum exceeds 1, a Type B package is required.⁵

Worked mixture example

Consider a normal-form shipment combining molybdenum-99 and technetium-99m — the situation for a bulk generator:

Mo-99: 0.50 TBq, with $A_{2} = 0.6$ TBq
Tc-99m: 2.0 TBq, with $A_{2} = 4$ TBq

Compute each fraction and sum:

\frac{0.50}{0.6} + \frac{2.0}{4} = 0.833 + 0.500 = 1.333

Because the sum of fractions is 1.333, which exceeds 1, this shipment cannot travel as a Type A package — it requires a Type B package.⁵⁶ Had the Mo-99 activity instead been 0.20 TBq, the sum would be $0.333 + 0.500 = 0.833 \leq 1$ , and a Type A package would be permissible.

The transport index and package category

Independent of the A1/A2 decision, the external radiation level determines the package's label category. The transport index (TI) is the maximum dose rate at one meter from the surface, converted to a control number:

TI = \dot{H} (1 m) [mSv/h] \times 100

rounded up to the next tenth.³ The label category is then set by whichever of surface dose rate or TI is more restrictive:

Category	Surface dose rate	Transport index
Radioactive White-I	≤ 0.005 mSv/h	0
Radioactive Yellow-II	> 0.005 to 0.5 mSv/h	> 0 to 1.0
Radioactive Yellow-III	> 0.5 to 2 mSv/h	> 1.0 to 10

For example, a package reading 0.30 mSv/h at the surface (Yellow-II by surface dose) but 0.04 mSv/h at one meter has a TI of $0.04 \times 100 = 4.0$ , which falls in the Yellow-III range; the package is labeled Yellow-III because the category is set by the more restrictive of the two criteria.⁷

Clinical Impact

For most medical facilities the day-to-day impact is on the receiving dock, not the shipping bay. A clinic receiving unit doses handles Type A packages labeled White-I or Yellow-II almost exclusively, but the radiation safety officer still has to recognize the category, perform the receipt survey, and confirm that the labeling and paperwork match the contents.³¹⁰

Type A packaging has a real safety record in the medical setting. A reusable Type A container developed and tested at a medical center passed the required water-spray, free-drop, compression, and penetration tests and carried radioactive material for years without loss of contents — a concrete illustration that Type A performance requirements are meaningful, not nominal.¹¹ Occupational dose during transport itself is generally low when packages are correctly categorized and handled, though it is not zero and is monitored where warranted.¹²

The compliance stakes are also real. Package type and category drive labeling, placarding, the transport index summed across a conveyance, documentation, and carrier controls. An error in the A1/A2 determination or the category assignment can turn a routine shipment into a reportable event.¹⁰¹³

Practical Optimization Tips

Confirm the physical form before choosing the limit

The single most common conceptual error is applying A1 (special form) to material that is actually normal form. Unless the source is a qualified special-form capsule or indispersible solid, use A2. When in doubt, use the more restrictive A2 value.³

Use the current, correct table

A1/A2 values are internationally harmonized but are periodically reviewed. Always read the value from the edition of 10 CFR Part 71 Appendix A or 49 CFR 173.435 in force, and for a nuclide not individually listed, follow the rules in Appendix A for unlisted or unknown nuclides rather than guessing.¹⁶

Apply the sum-of-fractions rule whenever more than one nuclide is present

Generators, mixed-isotope research shipments, and decayed multi-dose packages all require the unity calculation. Divide each activity by its own A1 or A2, add, and compare to 1. Do not simply add activities or compare against a single nuclide's limit.⁵

Separate the A1/A2 decision from the category decision

Package type (A vs B) is set by activity against A1/A2; label category (White-I / Yellow-II / Yellow-III) is set by external dose rate and transport index. A low-activity package can still carry a meaningful surface dose rate, and both determinations must be made.³⁷

Build receipt into the program

Even a receive-only clinic should train staff to read the category, measure the package on receipt, and document the survey under 10 CFR 20.1906. See our guides to radioactive package receipt and wipe testing and shipping radioactive material under DOT 49 CFR.

Regulatory Considerations

Radioactive material transport is jointly governed by the U.S. Department of Transportation and the Nuclear Regulatory Commission, harmonized with the international IAEA regulations. The frameworks fit together as follows:

DOT / PHMSA (49 CFR). 49 CFR 173.403 defines A1, A2, special form, normal form, package types, and the transport index; 173.431 sets the Type A activity limits; 173.433 gives the sum-of-fractions rule and treatment of unlisted nuclides; 173.435 is the master table of A1/A2 values; and 173.441 sets the radiation-level limits and category system.³⁴⁵⁶⁷
NRC (10 CFR 71). 10 CFR Part 71 governs package design, testing, and certification; Appendix A determines A1 and A2; and 71.4 provides the parallel definitions.¹²
International basis. The values and the underlying Q-system come from the IAEA transport regulations, SSR-6 (Rev. 1), 2018 Edition, with advisory material in SSG-26 explaining the Q-system derivation.⁸⁹ The Q-system limits dose to a person near a damaged package using pathway analyses against criteria of 50 mSv effective dose, 500 mSv skin equivalent dose, and 150 mSv lens equivalent dose; A1 reflects only external pathways while A2 reflects all pathways.⁹
State context. In Florida, radioactive-material medical use is administered by the state under Florida Administrative Code Chapter 64E-5; DRPS also serves Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware, where parallel state radiation-control rules apply on top of the federal transport framework. Always confirm requirements with the authority having jurisdiction.

A defensible program documents the physical form determination, the A1/A2 comparison or sum-of-fractions calculation, the package type, and the category assignment for every shipment, and it trains receiving staff to close the loop on the other end.¹⁰¹³

Frequently Asked Questions (FAQs)

Are the A1/A2 values the same in DOT and NRC regulations?

Yes. The A1/A2 table in 10 CFR Part 71 Appendix A and the table in 49 CFR 173.435 are harmonized, and both align with the IAEA transport regulations, so the value for a given nuclide is the same regardless of which framework you consult.¹⁶⁸

Why is A2 usually smaller than A1?

Because A2 (normal form) must account for dispersal hazards — inhalation, ingestion, and contamination — in addition to external dose, while A1 (special form) considers only external dose. More pathways mean a lower limiting activity.⁹

What happens if a nuclide is not in the table?

49 CFR 173.433 and 10 CFR Part 71 Appendix A provide default A1/A2 values and rules for unlisted or unknown nuclides, generally requiring conservative assumptions based on decay mode and atomic number.⁵¹

Does an excepted or limited-quantity package avoid these limits?

Excepted packages carry only very small fractions of A1/A2 and are subject to reduced requirements, but they are still governed by activity limits and low surface-dose criteria. They are a subset of the same framework, not an exemption from it.³

Who determines the package type for a medical shipment?

The shipper is responsible, typically the nuclear pharmacy or manufacturer, with the radiation safety officer or a qualified medical physicist confirming the physical form, the A1/A2 comparison, and the labeling. Receiving facilities verify the category and perform the receipt survey.³¹⁰

Key Takeaways

A1 (special form) and A2 (normal form) are nuclide-specific activity limits that decide whether a shipment fits in a Type A package.¹³
Values are tabulated in 10 CFR Part 71 Appendix A and 49 CFR 173.435, in terabecquerels, and are internationally harmonized.¹⁶⁸
Above A1/A2, a Type B package engineered for severe-accident conditions is required.⁴
For mixtures, apply the sum-of-fractions rule; a total at or below 1 permits Type A.⁵
A1/A2 come from the IAEA Q-system dose criteria (50 mSv effective, 500 mSv skin, 150 mSv lens).⁹
Package category (White-I / Yellow-II / Yellow-III) is set separately by surface dose rate and transport index.³⁷
Even receive-only facilities must recognize the category and perform the receipt survey under 10 CFR 20.1906.¹⁰

How DRPS Can Help

Diagnostic Radiation Physics Services (DRPS) supports nuclear medicine and radiopharmacy facilities across Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware with radioactive material license support, radiation safety officer services, transport and receipt program development, package-category and receipt-survey training, and radiation-safety documentation prepared by board-certified medical physicists.

A strong transport-safety program is not just about paperwork. It is about correctly classifying physical form, applying A1/A2 and the sum-of-fractions rule, assigning the right category, and closing the loop with a documented receipt survey — so every shipment and every delivery is defensible.

Conclusion

A1 and A2 are the two numbers that translate a radionuclide's hazard into a package requirement. Special-form material is limited by A1, normal-form by A2, mixtures by the sum-of-fractions rule, and anything above those limits requires an accident-resistant Type B package. The values trace back to the IAEA Q-system's dose criteria, and the parallel category system based on surface dose rate and transport index governs labeling. For a medical facility, mastering these determinations — and building receipt surveys into the program — is a practical, defensible part of radiation safety.¹⁵⁸⁹

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR Part 71, Appendix A, Determination of A1 and A2. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 71.4, Definitions. ecfr.gov
U.S. Department of Transportation. 49 CFR 173.403, Definitions (Class 7 radioactive materials). ecfr.gov
U.S. Department of Transportation. 49 CFR 173.431, Activity limits for Type A and Type B packages. ecfr.gov
U.S. Department of Transportation. 49 CFR 173.433, Requirements for determining A1 and A2 values for radionuclides and for the listing of radionuclides on shipping papers and labels. ecfr.gov
U.S. Department of Transportation. 49 CFR 173.435, Table of A1 and A2 values for radionuclides. ecfr.gov
U.S. Department of Transportation. 49 CFR 173.441, Radiation level limitations and exclusive use provisions. ecfr.gov
International Atomic Energy Agency. Regulations for the Safe Transport of Radioactive Material. IAEA Safety Standards Series No. SSR-6 (Rev. 1), 2018 Edition. Vienna: IAEA; 2018. iaea.org
International Atomic Energy Agency. Advisory Material for the IAEA Regulations for the Safe Transport of Radioactive Material. IAEA Safety Standards Series No. SSG-26 (Rev. 1). Vienna: IAEA; 2014. iaea.org
Chen MY. Radiation protection and regulations for the nuclear medicine physician. Seminars in Nuclear Medicine. 2014;44(3):215-228. doi:10.1053/j.semnuclmed.2014.03.005. doi.org
Landsworth R, Gross G. Compliance testing of reusable containers for transport of type A packages of radioactive material. Health Physics. 1991;60(5):721-723. doi:10.1097/00004032-199105000-00014. doi.org
Donadille L, Itié C, Lahaye T, Muller H, Trompier F, Bottollier-Depois JF. Workplace characterisation in case of rail transport of radioactive materials. Radiation Protection Dosimetry. 2006;125(1-4):369-375. doi:10.1093/rpd/ncl168. doi.org
González AJ. Radiation safety standards and their application: international policies and current issues. Health Physics. 2004;87(3):258-272. doi:10.1097/01.hp.0000130400.90548.5e. doi.org