ALARA Investigational Levels for Occupational Dose

Investigational Levels I and II are the ALARA action thresholds a radiation safety program sets below the regulatory occupational dose limits, so that a rising dose triggers review and investigation long before anyone approaches a legal limit. They are the practical machinery that turns "as low as reasonably achievable" from a slogan into a monitored, documented process. ¹⁴⁶

A dose limit tells you where the legal ceiling is. An investigational level tells you when to start asking questions. The gap between the two is where a good radiation safety program actually lives: catching a dosimetry trend, an unusual reading, or a changed workflow while there is still ample margin to act. ⁴⁶⁷

Introduction

Every occupational radiation safety program is built around two different kinds of numbers. The first are the regulatory dose limits — hard ceilings in 10 CFR Part 20 that must not be exceeded. The second are investigational levels — internal action thresholds, set well below the limits, that a licensee uses to manage exposures downward and to detect problems early. ³⁴

Investigational levels come in two tiers by long-standing convention. Level I is the lower threshold that triggers a review and a note to the worker. Level II is the higher threshold that triggers a formal, documented investigation. Both sit far below the regulatory limit, and reaching one is not a violation — it is the system working as designed, flagging a dose for attention while the margin to any limit is still large. ⁴⁶

This guide explains where the dose limits come from, how licensees typically derive investigational levels as fractions of those limits, the two-tier action structure, a worked quarterly calculation, an example table, and how to document the whole program under NRC Regulatory Guide 8.10 and the model ALARA program in NUREG-1556. DRPS builds and audits these programs through its radiation safety officer and radioactive material license support services.

Topic Explanation

What an investigational level is

An investigational level is a licensee-chosen dose value, below the regulatory limit, at which the program takes a defined action. It is not a limit and not a regulatory number. Its entire purpose is to create an early-warning band between routine, expected doses and the legal ceiling, so that an unusual dose is investigated while there is still room to intervene. ⁴⁶

The concept flows directly from the ALARA requirement itself. 10 CFR 20.1101(b) requires each licensee to use, to the extent practicable, procedures and engineering controls to achieve occupational doses that are as low as is reasonably achievable. ⁴ Investigational levels are the standard operational tool for meeting that requirement: they define, in advance, the doses that will prompt the program to look more closely. NRC guidance — Regulatory Guide 8.10 and the model ALARA program in NUREG-1556, Volume 9 — describes this two-level approach as good practice for materials licensees. ⁶⁷

For the broader program context in which investigational levels sit, see our guides on building an ALARA program and the annual radiation protection program audit.

Why two levels

A single threshold forces a single response to very different situations. A dose slightly above routine deserves a quick look; a dose well above routine deserves a formal investigation. The two-tier structure matches the response to the signal: ⁴⁶

Level I — the lower trigger. It says: this dose is higher than we expect; the radiation safety officer (RSO) should review it, confirm it makes sense for the work done, notify the worker, and consider whether practices can be improved. No formal committee action is required.
Level II — the higher trigger. It says: this dose is high enough to warrant a documented investigation. The RSO, usually together with the radiation safety committee (RSC), performs a root-cause analysis and defines corrective actions.

Neither event is, by itself, a regulatory report. They are internal management steps that keep the program tight and produce the records that demonstrate ALARA is being actively managed. ⁴⁶

Key Technical Principles

The regulatory limits that anchor everything

Investigational levels are derived from the occupational dose limits in 10 CFR 20.1201. The current annual limits for adult workers are: ³

Dose quantity	Annual limit (conventional)	SI
Total effective dose equivalent (TEDE)	5 rem	0.05 Sv (50 mSv)
Deep-dose + committed dose to any organ/tissue (other than lens)	50 rem	0.5 Sv
Lens dose equivalent (lens of the eye)	15 rem	0.15 Sv (150 mSv)
Shallow-dose equivalent to skin or any extremity	50 rem	0.5 Sv (500 mSv)

One nuance worth stating plainly: the NRC lens limit remains 15 rem (0.15 Sv) per year. International bodies have recommended a substantially lower lens limit, and some jurisdictions have adopted it, but that lower value has not been incorporated into 10 CFR 20.1201. A US program's investigational levels for the lens are therefore anchored to the 15 rem regulatory value, even as many facilities choose to manage lens dose more conservatively. ³⁹

Deriving investigational levels

Most licensees express investigational levels as a fraction of the applicable annual limit, evaluated per calendar quarter. The quarterly regulatory value is simply the annual limit divided by four:

L_{quarter} = \frac{L _{annual}}{4}

The investigational level for a quarter is then a chosen fraction of the annual limit, prorated to that quarter:

IL_{quarter} = f \cdot \frac{L _{annual}}{4}

where $f$ is the fraction the program selects. A widely used convention sets Level I at about 10% and Level II at about 30% of the annual limit. Applying this to the 5 rem/year whole-body (TEDE) limit:

IL_{I} = 0.10 \cdot \frac{5 rem}{4} = 0.10 \cdot 1.25 rem = 0.125 rem = 125 mrem/quarter

IL_{II} = 0.30 \cdot \frac{5 rem}{4} = 0.30 \cdot 1.25 rem = 0.375 rem = 375 mrem/quarter

As an annualized sanity check, 125 mrem/quarter is 500 mrem/year, exactly 10% of the 5,000 mrem/year TEDE limit, and 375 mrem/quarter is 1,500 mrem/year, 30% of the limit. Both remain far below the 5 rem/year regulatory ceiling — which is the entire point of an action level. ³⁶

An example investigational-level table

Applying the same 10% / 30% quarterly convention across the dose quantities gives a table like the following. These are example values only — the specific fractions and the resulting numbers are chosen by the licensee, and real programs vary (some use 25% for Level II, some manage the lens like the whole body rather than against the 15 rem limit, and some select different extremity values). Only the regulatory limits themselves are fixed. ³⁶

Dose quantity	10 CFR 20.1201 annual limit	Quarterly limit (÷4)	Investigational Level I (10%/qtr) → RSO review + notify worker	Investigational Level II (30%/qtr) → RSO/RSC documented investigation
Whole-body deep dose / TEDE	5 rem	1,250 mrem	125 mrem/qtr	375 mrem/qtr
Lens of eye	15 rem	3,750 mrem	375 mrem/qtr	1,125 mrem/qtr
Skin / extremities (shallow-dose)	50 rem	12,500 mrem	1,250 mrem/qtr	3,750 mrem/qtr

Example investigational levels only, computed from a 10% (Level I) and 30% (Level II) quarterly convention. Regulatory limits are from 10 CFR 20.1201; the investigational-level methodology follows NRC Regulatory Guide 8.10 and the model ALARA program in NUREG-1556. Licensees set their own fractions and values below the regulatory limits. ³⁶⁷

Reviewing the data

Monitored dose data — from dosimeters processed by an accredited service — should be reviewed at least once each calendar quarter against the investigational levels. Recording and reporting of occupational dose data follows NRC Regulatory Guide 8.7, which supports the dose-of-record and the documentation the program relies on. ⁸ For the underlying personnel-monitoring methods, see our guide to OSL and TLD personnel dosimetry.

Clinical Impact

Investigational levels convert a pile of dosimetry reports into a management signal. In a busy interventional, nuclear-medicine, or cardiac program, some workers will legitimately accumulate more dose than others. Without action levels, an RSO is left comparing raw numbers to a distant regulatory limit and hoping to notice a trend. With them, a specific, pre-defined value says exactly when to look closer — before a trend becomes a problem. ⁴¹⁰

The early-warning function is where the real protective value lies. A Level I exceedance often surfaces something fixable: a dosimeter worn incorrectly, a shielding aid not used, a new procedure that changed a worker's exposure, or simply a busy quarter. Catching it early, with a friendly review and a practice tweak, is exactly the kind of optimization ALARA is meant to produce. ⁴¹⁰¹¹ The published occupational-dose literature consistently shows that education, workflow attention, and simple protective measures reduce staff dose — precisely the interventions an investigational-level review prompts. ¹⁰¹¹¹²

Investigational levels also protect the program itself. When an auditor or inspector asks how the licensee implements ALARA, a two-tier investigational-level system with quarterly reviews and documented Level II investigations is concrete, defensible evidence that exposures are actively managed, not merely kept under the legal ceiling. ⁶⁷

Practical Optimization Tips

1. Write the levels into the ALARA program document

State the investigational levels, the fractions used to derive them, the monitoring period, and the actions for each tier in the written radiation protection program. A number that lives only in the RSO's head is not a program. ⁶⁷

2. Match the action to the tier

Keep Level I lightweight — a review, a conversation, a note — so it can happen promptly and often. Reserve the formal, committee-level investigation for Level II. Overloading Level I with paperwork discourages timely review. ⁴⁶

3. Investigate the work, not just the number

A dose above an investigational level is a question, not an answer. Confirm the dosimeter was worn correctly and at the right location, reconcile the dose against the actual work performed, and check for a static or contaminated badge before concluding anything about the worker's real exposure. ⁸

4. Document Level II investigations fully

For a Level II exceedance, record the dose, the investigation, the root cause, the corrective actions, and the follow-up. This is the record that demonstrates the program works and that closes the loop on the event. ⁶⁷

5. Revisit the levels periodically

If most workers routinely trip Level I, the level may be set too low for the practice — or a real exposure problem exists. Review the distribution of doses during the annual program audit and adjust the levels or the practices accordingly, keeping optimization, not just compliance, in view. ⁶⁷

Common pitfalls to avoid

Treating an investigational level as a limit. Reaching Level I or II is an internal trigger, not a regulatory violation or report. ⁴
Setting levels and never reviewing the data. Levels without a quarterly review process accomplish nothing. ⁸
Anchoring the lens level to the wrong number. The NRC lens limit is 15 rem/year; derive the lens investigational level from that, while managing lens dose conservatively where warranted. ³⁹
Skipping documentation. An undocumented Level II investigation cannot demonstrate ALARA to an auditor. ⁶⁷
Copying another facility's table blindly. Investigational levels should reflect your own practice, dose distribution, and workload. ⁶

Regulatory Considerations

Investigational levels implement a regulatory requirement even though the specific numbers are not themselves regulatory. The distinction matters for how a program frames and defends them.

10 CFR 20.1101(b) — establishes the ALARA requirement that investigational levels serve; licensees must use procedures and controls to keep doses as low as reasonably achievable. ⁴
10 CFR 20.1201 — sets the occupational dose limits that anchor the investigational-level calculations. ³
10 CFR 20.2203 — governs the separate matter of regulatory reports of exposures exceeding the limits or constraints; investigational-level exceedances are internal actions and are not, by themselves, 20.2203 reports. ⁵
NRC Regulatory Guide 8.10, Revision 2 — the operating-philosophy guidance for maintaining occupational exposures ALARA, which frames the investigational-level approach. ⁶
NRC NUREG-1556, Volume 9 — program-specific guidance for medical-use licenses, including the model ALARA program that materials licensees commonly adopt. ⁷

Because most of the states DRPS serves are NRC Agreement States, the operative rules are typically the state's equivalents of 10 CFR Part 20, administered by the state radiation-control program; Washington, DC and Delaware are regulated directly by the NRC. The ALARA obligation and the investigational-level approach carry across both frameworks, but a program should always confirm the specific requirements of its authority having jurisdiction. For the compliance backdrop, see common radiation safety violations and how to avoid them and preparing for an NRC inspection.

Frequently Asked Questions (FAQs)

What is an ALARA investigational level?

An investigational level is a dose action threshold a radiation safety program sets below the regulatory limit, so that when a worker's monitored dose reaches it, the program investigates before the dose ever approaches a legal limit. Investigational levels are a core tool for keeping occupational exposures as low as is reasonably achievable, and they are set by the licensee, not fixed in the regulations. ⁴⁶

What is the difference between Investigational Level I and Level II?

Level I is the lower threshold that triggers a review: the radiation safety officer looks at the dose, notifies the worker, and asks whether work practices can be improved. Level II is the higher threshold that triggers a formal, documented investigation, typically by the radiation safety officer and the radiation safety committee, including root-cause analysis and corrective action. Level I says pay attention; Level II says investigate and document. ⁴⁶

How are investigational levels calculated?

Most programs express them as a fraction of the applicable annual regulatory limit, evaluated per calendar quarter. A common approach sets Level I at about 10 percent and Level II at about 30 percent of the annual limit, prorated to the quarter. For a 5 rem per year whole-body limit, that gives an example Level I near 125 mrem per quarter and Level II near 375 mrem per quarter. The exact fractions and values are chosen by the licensee, not mandated. ³⁶

Are investigational levels required by NRC regulations?

The specific numeric investigational levels are not fixed in the regulations, but the underlying ALARA requirement is. 10 CFR 20.1101(b) requires licensees to use procedures and engineering controls to keep exposures as low as is reasonably achievable, and NRC guidance such as Regulatory Guide 8.10 and the model ALARA program in NUREG-1556 describe investigational levels as the standard way to implement that requirement. Reaching an investigational level is an internal action trigger, not a regulatory violation. ⁴⁶⁷

What happens when a worker exceeds an investigational level?

Exceeding Level I prompts the radiation safety officer to review the individual's dose, confirm it against work performed, notify the worker, and consider practice improvements — usually without formal committee action. Exceeding Level II prompts a documented investigation with root-cause analysis and corrective actions, generally involving the radiation safety committee. Neither is a regulatory report by itself; both are ALARA management steps, and the records support the program. ⁴⁶

What are the current NRC occupational dose limits?

Under 10 CFR 20.1201, the annual occupational limits are 5 rem for total effective dose equivalent, 50 rem for the sum of deep-dose plus committed dose to any individual organ or tissue other than the lens, 15 rem for the lens of the eye, and 50 rem shallow-dose equivalent to the skin or any extremity. Investigational levels are set below these; note the NRC lens limit remains 15 rem per year and has not been lowered to the value some other bodies recommend. ³⁹

Key Takeaways

Investigational levels are action thresholds, not limits. They sit below the 10 CFR 20.1201 limits and trigger review before a limit is ever in play. ³⁴
Two tiers, two responses. Level I prompts an RSO review and worker notification; Level II prompts a documented RSO/RSC investigation. ⁴⁶
Derived as fractions of the limit. A common convention is ~10% (Level I) and ~30% (Level II) of the annual limit, prorated per quarter — example values, chosen by the licensee. ⁶⁷
The ALARA requirement is regulatory; the numbers are not. 10 CFR 20.1101(b) mandates ALARA; Regulatory Guide 8.10 and NUREG-1556 describe investigational levels as the way to implement it. ⁴⁶⁷
The NRC lens limit is still 15 rem/year. Anchor lens investigational levels to that value, while managing lens dose conservatively. ³⁹
Document and review. Quarterly review of dosimetry against the levels, and full documentation of Level II investigations, is what makes the program defensible. ⁷⁸

Conclusion

Investigational levels are the difference between a program that merely stays under the legal ceiling and one that actively drives dose down. By setting two thresholds well below the regulatory limits, defining exactly what happens at each, reviewing dosimetry every quarter, and documenting the investigations, a licensee turns ALARA from an aspiration into a running process. The numbers are the licensee's to choose, but the discipline they enforce — notice early, investigate thoroughly, record honestly — is what protects workers and what an inspector expects to see. ⁴⁶⁷

How DRPS Can Help

Diagnostic Radiation Physics Services (DRPS) helps medical facilities build and audit the radiation safety programs that investigational levels live within: written ALARA programs, dose-review procedures, radiation safety committee support, personnel-dosimetry program design, and inspection readiness aligned with 10 CFR Part 20, NRC Regulatory Guide 8.10, and NUREG-1556 guidance.

DRPS provides radiation safety officer, radioactive material license support, and radiation safety training across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A strong ALARA program does not wait for a dose to approach a limit. It notices the trend a year early — and acts.

Related Resources

References

U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.1101: Radiation protection programs. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.1201: Occupational dose limits for adults. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 20.2203: Reports of exposures, radiation levels, and concentrations of radioactive material exceeding the constraints or limits. ecfr.gov
U.S. Nuclear Regulatory Commission. Regulatory Guide 8.10, Revision 2: Operating Philosophy for Maintaining Occupational Radiation Exposures As Low As Is Reasonably Achievable. 2016. ADAMS ML16105A136. nrc.gov
U.S. Nuclear Regulatory Commission. Regulatory Guide 8.7, Revision 4: Instructions for Recording and Reporting Occupational Radiation Dose Data. 2018. ADAMS ML17221A245. nrc.gov
U.S. Nuclear Regulatory Commission. NUREG-1556, Volume 9, Revision 3: Consolidated Guidance About Materials Licenses — Program-Specific Guidance About Medical Use Licenses. nrc.gov
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