ALI, DAC & Internal Dose Limits (10 CFR 20)

Q: When is individual monitoring for internal dose required?

Under 10 CFR 20.1502, a licensee must monitor the occupational intake of radioactive material and assess committed effective dose equivalent for adults likely to receive, in one year, an intake exceeding 10 percent of the applicable ALI, and for declared pregnant women and minors above lower thresholds. Many programs monitor more broadly than the minimum to support ALARA.

ALI and DAC are the two numbers that make internal radiation dose limits usable: the annual limit on intake (ALI) converts the dose limit into a single activity per radionuclide, and the derived air concentration (DAC) converts it into an airborne concentration you can compare against air-sampling data. Both are tabulated for occupational exposure in 10 CFR Part 20 Appendix B, and both trace back to the same underlying limit on committed dose.¹²

You cannot measure committed effective dose equivalent with an instrument the way you read an ion chamber. Instead, the regulatory framework gives you surrogates — activity taken in, or concentration in the air breathed — that map onto the dose limit through defined assumptions about the reference worker. Understanding how ALI, DAC, and DAC-hours relate to the 5 rem annual limit is what lets a radiation safety officer set monitoring triggers, interpret bioassay and air-sampling results, and demonstrate compliance.¹³

Introduction

External dose is comparatively easy: a dosimeter on the body integrates the dose as it is delivered. Internal dose is fundamentally different. Once a radionuclide is inhaled or ingested, it delivers dose over time — sometimes years — from inside the body, to different organs at different rates depending on its chemistry and the radiation it emits. The quantity that captures this is the committed effective dose equivalent (CEDE): the effective dose equivalent that will accumulate over the 50 years following an intake.³

You cannot put a badge on a thyroid or a lung. So the rules work backward from the dose limit to quantities you can measure or estimate — how much activity entered the body (intake), and the concentration of activity in the air a worker breathes. The annual limit on intake and the derived air concentration are precisely those backward-derived quantities, tabulated radionuclide by radionuclide in 10 CFR Part 20 Appendix B.¹²

This article explains how ALI and DAC are defined, how DAC-hours let you track airborne intake against the dose limit, how internal dose sums with external dose to form the total effective dose equivalent, when individual monitoring is required, and how a defensible internal-dosimetry program is documented. It is written for radiation safety officers, authorized users, and facility leadership responsible for occupational radiation protection.

Topic Explanation

The dose limit behind the numbers

The occupational dose limit in 10 CFR 20.1201 is an annual total effective dose equivalent (TEDE) of 5 rem (0.05 Sv), with a separate, more restrictive deterministic limit on dose to individual organs and tissues.⁴ ALI and DAC exist to translate that limit into the internal-exposure domain.

The TEDE is itself a sum:

TEDE = DDE_{external} + CEDE_{internal}

where DDE is the deep-dose equivalent from external exposure and CEDE is the committed effective dose equivalent from intakes.³⁴ This summation, required by 10 CFR 20.1202, is why internal dose cannot be treated as a separate budget: a worker with meaningful external exposure has correspondingly less headroom for intake.⁵

What the ALI actually is

The annual limit on intake is the smaller of two intakes: the intake of a given radionuclide that would commit the reference worker to 5 rem (0.05 Sv) CEDE (the stochastic limit), or the intake that would commit 50 rem (0.5 Sv) to any single organ or tissue (the deterministic, or non-stochastic, limit).¹ Whichever is more restrictive becomes the ALI for that radionuclide and intake route.

This dual basis matters in practice. For many radionuclides the stochastic (whole-body) limit controls. But for radionuclides that concentrate in a single organ — radioiodine in the thyroid is the classic example — the organ dose limit can be the controlling factor, so the ALI is set by the committed dose to that organ rather than the whole-body CEDE.¹ That is why our discussion of thyroid bioassay for I-131 workers focuses on the thyroid specifically.

Appendix B lists separate ALIs for ingestion and inhalation, because the body handles the two routes differently.²

What the DAC actually is

The derived air concentration is the airborne concentration that, breathed by the reference worker for a full working year, delivers exactly one ALI.¹ The "reference working year" is defined as 2,000 hours at a light-work breathing rate of 1.2 m³/h. That gives the simple definitional relationship:

DAC = \frac{ALI _{inhalation}}{2 , 000 h \times 1.2 m ^{3} / h} = \frac{ALI _{inhalation}}{2 , 400 m ^{3}}

DAC is the bridge between an air-sampling measurement (a concentration) and the dose framework (an intake). It is what lets you look at a measured airborne concentration and immediately judge it against the dose limit.¹⁶

The structure of Appendix B

10 CFR Part 20 Appendix B is organized into tables that serve different audiences and exposure pathways:

Appendix B table	Population / pathway	Quantities listed	Typical use
Table 1	Occupational	Oral ingestion ALI, inhalation ALI, inhalation DAC	Worker intake limits and air-sampling comparison
Table 2	Public (effluents)	Air and water effluent concentration limits	Release to unrestricted areas
Table 3	Sewerage	Monthly average concentration for sewer release	Liquid waste disposal to sanitary sewer

The occupational ALI and DAC values in Table 1 are based on the ICRP Publication 26/30 methodology in force when the rule was adopted; ICRP has since issued updated dose coefficients (Publications 68 and 119, and the Occupational Intakes of Radionuclides series), but the Appendix B values remain controlling for U.S. compliance unless a license specifies otherwise.²⁷⁸⁹ For sewer disposal specifically, see our guide to sewer disposal of radioactive material.

Key Technical Principles

Deriving an ALI from a dose coefficient

Conceptually, the stochastic ALI is just the dose limit divided by the committed effective dose per unit intake — the dose coefficient $e (50)$ , in Sv/Bq:

ALI_{stochastic} = \frac{0.05 Sv}{e ( 50 )}

For illustration only, a radionuclide with an inhalation dose coefficient of $e (50) = 1.0 \times 1 0^{- 8}$ Sv/Bq would give a stochastic ALI of:

ALI = \frac{0.05}{1.0 \times 1 0 ^{- 8}} = 5 \times 1 0^{6} Bq \approx 135 μ Ci

The deterministic ALI is derived the same way against the 0.5 Sv organ limit using the organ-specific committed dose equivalent per unit intake, and the smaller of the two is the tabulated ALI.¹ (These numbers are illustrative of the method; always use the actual Appendix B value for a specific radionuclide.)

DAC-hours: the working currency of internal exposure

The most useful practical quantity is the DAC-hour — exposure to a concentration of one DAC for one hour. Because one ALI corresponds to 2,000 DAC-hours, DAC-hours let you accumulate airborne exposure over time and convert it to dose:

Intake (fraction of ALI) = \frac{DAC-hours}{2 , 000}

CEDE \approx \frac{DAC-hours}{2 , 000} \times 5 rem

This is how air-sampling data become a dose estimate: sum the product of measured concentration (in DAC) and time over all exposure periods, then apply the relationships above.⁶

Worked example: an air-sampling intake estimate

Suppose air sampling in a work area shows an average concentration of 0.3 DAC for a particular radionuclide, and a worker spends 50 hours in that area during the project. The accumulated exposure is:

DAC-hours = 0.3 DAC \times 50 h = 15 DAC-hours

The intake as a fraction of an ALI is:

\frac{15}{2 , 000} = 0.0075 ALI

and the estimated committed effective dose equivalent from inhalation is:

CEDE \approx 0.0075 \times 5 rem = 0.0375 rem = 0.375 mSv

Two compliance conclusions follow. First, the intake (0.75% of an ALI) is well below the 10% of ALI threshold that triggers mandatory individual intake monitoring under 10 CFR 20.1502, though documenting the estimate is still good practice.¹⁰ Second, that 0.375 mSv of CEDE must be added to the worker's external deep-dose equivalent for the year to form the TEDE that is compared against the 5 rem (50 mSv) limit.⁴⁵ If the same worker also received, say, 8 mSv externally, the TEDE would be about 8.4 mSv — still compliant, but the internal contribution is not optional to track.

Clinical Impact

In a medical facility, internal exposure is usually small — but "usually small" is a conclusion you have to earn with monitoring, not an assumption. The settings where intake matters most are radiopharmacy and hot-lab operations, radioiodine therapy preparation and administration, generator elution, and any work with volatile or aerosolizable material. Radioiodine is the recurring concern because it is volatile and thyroid-seeking, which is exactly why thyroid bioassay programs exist.¹

The practical impact of the ALI/DAC framework is that it gives a radiation safety officer defensible decision points. Air-sampling results expressed in DAC tell you immediately whether engineering controls (fume hoods, ventilation, containment) are adequate. Intake estimates expressed as a fraction of ALI tell you whether a worker needs formal bioassay. And the requirement to sum internal CEDE with external DDE means the RSO has to maintain a complete dose picture, not just a stack of dosimeter reports.³⁵

Getting this wrong has real consequences: an unmonitored intake can mean an undetected dose, an incomplete TEDE, and a compliance gap that surfaces during inspection. The framework also feeds directly into ALARA — tracking intakes and DAC-hours is how a program demonstrates that internal exposure is being kept as low as reasonably achievable, not merely below the limit. This connects closely to our guides on occupational exposure monitoring and the NRC occupational dose limits in 10 CFR Part 20.

Practical Optimization Tips

Use DAC to evaluate engineering controls, not just compliance

A measured airborne concentration in DAC is an immediate readout of how well your containment is working. If routine operations approach a meaningful fraction of a DAC, the fix is usually better engineering controls — a hood, improved ventilation, containment — rather than relying on the dose limit as a backstop.⁶

Set monitoring triggers at a fraction of the ALI

The regulatory floor for individual intake monitoring is a likely annual intake exceeding 10% of the ALI.¹⁰ Many well-run programs set internal investigation levels lower for ALARA, so that a trend toward intake is caught and corrected well before it approaches the regulatory threshold.

Always close the loop with the TEDE

Internal CEDE is meaningless in isolation — it must be summed with external DDE for compliance. Build your recordkeeping so that bioassay and air-sampling results flow into the same annual dose record as dosimeter results, consistent with NRC Regulatory Guide 8.34 methodology.³⁵

Match the intake route to the work

Appendix B lists separate ingestion and inhalation ALIs. Inhalation is the dominant concern for volatile or aerosolizable material; ingestion controls hinge on contamination control and hygiene. Choose the relevant pathway when assessing a task, and remember that an organ-limited radionuclide (like radioiodine) may be governed by organ dose, not whole-body CEDE.¹²

Document the basis, not just the result

A bioassay or air-sampling program is only defensible if the assumptions are recorded: the dose coefficients or Appendix B values used, the breathing-rate and occupancy assumptions, the biokinetic model, and the calculation method; the reference biokinetic models that underlie the dose coefficients are themselves periodically reviewed and revised as new human data emerge.¹² NRC Regulatory Guide 8.9 provides accepted concepts, models, and assumptions for a bioassay program.¹¹

Common pitfalls to avoid

Treating internal dose as negligible without monitoring. "Small" must be demonstrated.
Forgetting the summation rule. TEDE is internal CEDE plus external DDE.
Using the wrong intake route. Ingestion and inhalation ALIs differ.
Ignoring organ-limited radionuclides. For radioiodine, the thyroid may control.
Confusing Appendix B tables. Occupational (Table 1) is not the same as effluent (Table 2) or sewerage (Table 3).
Failing to document assumptions. An undocumented intake estimate is hard to defend at inspection.

Regulatory Considerations

The ALI/DAC framework is built directly into 10 CFR Part 20, and a defensible internal-dosimetry program references each relevant section explicitly.

10 CFR 20.1003 defines ALI, DAC, CEDE, committed dose equivalent, and TEDE — the vocabulary the rest of the rule depends on.³
10 CFR 20.1201 sets the occupational limit at 5 rem TEDE per year, with separate organ and skin/extremity limits.⁴
10 CFR 20.1202 requires summing external and internal dose to demonstrate compliance with the TEDE limit.⁵
10 CFR 20.1204 specifies how to determine internal exposure from bioassay or air-sampling data, including the use of DAC-hours.⁶
10 CFR 20.1502 establishes when individual monitoring of intakes is required (likely annual intake exceeding 10% of the ALI for adults, with lower thresholds for declared pregnant women and minors).¹⁰
10 CFR Part 20 Appendix B tabulates the ALI and DAC values themselves.²
NRC Regulatory Guides 8.34 and 8.9 provide accepted monitoring criteria, dose-calculation methods, and bioassay-program models.³¹¹

Jurisdiction follows the usual pattern for byproduct material: the NRC (10 CFR Parts 20 and 35) governs radioactive material directly in non-Agreement jurisdictions, while Agreement States administer equivalent rules. Among the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States, while Washington DC and Delaware are regulated directly by the NRC for radioactive material. Confirm the requirements of the authority having jurisdiction, since Agreement State limits are equivalent but the citations and forms differ.²⁴ For the broader program context, see our guide to building an internal program around the NRC occupational dose limits.

Frequently Asked Questions (FAQs)

What is an annual limit on intake (ALI)?

The annual limit on intake is the activity of a given radionuclide that, if taken into the body of a reference adult worker by inhalation or ingestion in a year, would result in a committed effective dose equivalent of 5 rem (0.05 Sv) or a committed dose equivalent of 50 rem (0.5 Sv) to any single organ or tissue, whichever is smaller. It converts the internal dose limit into a single activity value per radionuclide.¹

What is a derived air concentration (DAC)?

The derived air concentration is the airborne concentration of a radionuclide that, if breathed by a reference worker for 2,000 working hours a year at a breathing rate of 1.2 cubic meters per hour (light work), would result in an intake of one ALI. Numerically, DAC equals the inhalation ALI divided by 2,400 cubic meters. DAC makes air-sampling results directly comparable to the dose limit.¹

What is a DAC-hour and how is it used?

A DAC-hour is the exposure from breathing air at one DAC for one hour. Because one ALI corresponds to 2,000 DAC-hours, you can sum DAC-hours from air-sampling data over a year, divide by 2,000 to get the fraction of an ALI taken in, and multiply by 5 rem to estimate the committed effective dose equivalent from inhalation.⁶

When is individual monitoring for internal dose required?

Under 10 CFR 20.1502, a licensee must monitor occupational intake and assess committed effective dose equivalent for adults likely to receive, in one year, an intake exceeding 10 percent of the applicable ALI, and for declared pregnant women and minors above lower thresholds. Many programs monitor more broadly than the minimum to support ALARA.¹⁰

How do internal and external doses combine?

The total effective dose equivalent (TEDE) is the sum of the deep-dose equivalent from external exposure and the committed effective dose equivalent from intakes. The 10 CFR 20.1201 occupational limit of 5 rem per year applies to the TEDE, so a worker with both external exposure and an intake must have the two summed for compliance.⁴⁵

Are the ALI and DAC values the same as the latest ICRP dose coefficients?

Not exactly. The ALI and DAC values in 10 CFR Part 20 Appendix B are based on the ICRP Publication 26/30 methodology used when the rule was written. Internationally, ICRP has since published updated dose coefficients (Publications 68 and 119, and more recently the Occupational Intakes of Radionuclides series). For U.S. compliance, the Appendix B values remain controlling unless your license specifies otherwise.⁷⁸⁹

Key Takeaways

ALI converts the internal dose limit into an activity per radionuclide; DAC converts it into an airborne concentration; both are tabulated in 10 CFR Part 20 Appendix B.¹²
The ALI is the smaller of the stochastic (5 rem CEDE) and deterministic (50 rem organ) limits; for organ-seeking radionuclides like radioiodine, the organ limit can control.¹
DAC equals the inhalation ALI divided by 2,400 m³, and one ALI corresponds to 2,000 DAC-hours — the working currency for air-sampling intake estimates.¹⁶
Internal CEDE must be summed with external DDE to form the TEDE compared against the 5 rem limit.⁴⁵
Individual intake monitoring is required above 10% of the ALI, but ALARA often warrants lower internal investigation levels.¹⁰
Appendix B values rest on ICRP 26/30 methodology and remain controlling for U.S. compliance even though ICRP has since updated its dose coefficients.²⁷⁸

Conclusion

ALI and DAC are not abstractions — they are the practical machinery that makes an unmeasurable quantity, committed internal dose, into something a radiation safety program can monitor, evaluate, and document. Once you see ALI as "the dose limit expressed as activity" and DAC as "the dose limit expressed as air concentration," the rest follows: DAC-hours track airborne intake, the 10% ALI rule sets monitoring triggers, and the summation rule folds internal dose back into the total effective dose equivalent. A program that understands and documents these relationships can defend its internal-dose assessments to any inspector — and, more importantly, can keep its workers' actual exposures low.¹³⁵

How DRPS Can Help

Diagnostic Radiation Physics Services (DRPS) helps medical and research facilities build defensible internal-dosimetry and radiation safety programs across Florida, Maryland, Virginia, Washington DC, California, Nevada, Pennsylvania, New York, New Jersey, and Delaware. Our board-certified medical physicists and radiation safety professionals provide radiation safety officer support, intake and bioassay program design, air-sampling and DAC evaluation, radioactive material license support, and radiation safety training aligned with 10 CFR Part 20 and applicable Agreement State rules.

A strong internal-dose program is not about memorizing Appendix B. It is about knowing which numbers control, monitoring the right people, and being able to show the math when it matters.

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR 20.1003: Definitions (annual limit on intake, derived air concentration, committed effective dose equivalent, total effective dose equivalent). ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR Part 20, Appendix B: Annual Limits on Intake (ALIs) and Derived Air Concentrations (DACs) of Radionuclides for Occupational Exposure; Effluent Concentrations; Concentrations for Release to Sewerage. ecfr.gov
U.S. Nuclear Regulatory Commission. Regulatory Guide 8.34: Monitoring Criteria and Methods to Calculate Occupational Radiation Doses. Washington, DC: NRC. nrc.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.1201: Occupational dose limits for adults. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.1202: Compliance with requirements for summation of external and internal doses. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.1204: Determination of internal exposure. ecfr.gov
International Commission on Radiological Protection. Limits for Intakes of Radionuclides by Workers. ICRP Publication 30. Ann ICRP. 1979. icrp.org
International Commission on Radiological Protection. Dose Coefficients for Intakes of Radionuclides by Workers. ICRP Publication 68. Ann ICRP. 1994;24(4). icrp.org
International Commission on Radiological Protection. Compendium of Dose Coefficients based on ICRP Publication 60. ICRP Publication 119. Ann ICRP. 2012;41(Suppl). icrp.org
U.S. Nuclear Regulatory Commission. 10 CFR 20.1502: Conditions requiring individual monitoring of external and internal occupational dose. ecfr.gov
U.S. Nuclear Regulatory Commission. Regulatory Guide 8.9: Acceptable Concepts, Models, Equations, and Assumptions for a Bioassay Program. Washington, DC: NRC. nrc.gov
Blanchardon E, Leggett RW, Eckerman KF. Some elements for a revision of the americium reference biokinetic model. Radiat Prot Dosimetry. 2007;127(1-4):131-135. doi:10.1093/rpd/ncm262. PubMed