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newSchoolWorks.com › VoTechWorks › Building & Construction › Nuclear Power Plant Technician / Operator Prep › Radiation Protection & Dosimetry › Lesson 10
BLD-NUCL84-05 · Lesson 10 · 26 clock hours · demo

Airborne Radioactivity: Sampling, Action Levels, and Respiratory Protection

Objective. Students will set up an air sampling train to measure airborne radioactivity concentration, compare results to derived air concentration values, and select the correct respiratory protection based on the calculated dose potential.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Airborne Radioactivity Matters

When radioactive material becomes suspended in air — as dust, aerosol, or gas — workers can inhale or ingest it, producing an internal dose that is harder to measure and control than external exposure. Unlike a gamma field you can walk away from, inhaled radioactive particles lodge in lung tissue and continue irradiating surrounding cells long after the worker leaves the area. For this reason, nuclear plants treat airborne radioactivity as a primary radiological hazard, with strict monitoring, defined action levels, and a hierarchy of respiratory protection.

The Air Sampling Train

An air sampling train is the combination of components used to collect a measured volume of air for radiological analysis. A basic portable train consists of:

1. Filter holder assembly — holds a collection medium (typically a cellulose or glass-fiber filter) that traps particulate radioactive material. A charcoal cartridge or silver zeolite cartridge may be added downstream to capture radioactive iodine gases.

2. Sampling pump — draws air through the filter at a controlled rate.

3. Flow meter — measures the volumetric flow rate (commonly in liters per minute). A calibrated rotameter or electronic flow meter is used; the meter must have a current calibration sticker and be zeroed before use.

Procedure highlights (always follow your plant's written procedure):

Lockout/tagout is generally not applicable to portable sampling pumps, but you must follow radiation work permit (RWP) requirements before entering any posted airborne radioactivity area.

Derived Air Concentration (DAC)

The U.S. Nuclear Regulatory Commission (NRC) defines a Derived Air Concentration (DAC) in 10 CFR Part 20 Appendix B as the airborne concentration of a given radionuclide that, if inhaled at the standard breathing rate for a working year (2,000 hours), would result in receiving the applicable annual occupational dose limit. DAC values are expressed in units of microcuries per milliliter (µCi/mL).

The key ratio used in the field is the DAC fraction (also called DAC multiple):

> DAC fraction = measured airborne concentration ÷ DAC value (from 10 CFR Part 20 Appendix B)

If the DAC fraction is ≥ 1, the worker is at or above the reference level for that radionuclide. Plants typically establish action levels well below 1 DAC (often at 0.1 DAC or lower) per their radiation protection program. When the DAC fraction meets or exceeds the plant's action level, controls must be applied immediately.

Cumulative internal dose can also be tracked using DAC-hours: DAC fraction × hours of exposure. Appendix B and 10 CFR Part 20 set the DAC-hour limit that corresponds to the annual dose limit; plants commonly post and track DAC-hours per job.

Respiratory Protection Tiers

When engineering controls (ventilation, containment) cannot reduce airborne concentrations below action levels, respiratory protection is required. The plant respiratory protection program — developed to meet NRC and OSHA requirements — specifies a tiered approach based on the calculated hazard:

| Tier | Device | Typical Use Condition |

|---|---|---|

| 1 | Filtering facepiece (disposable) | Low DAC fractions; plant-specific |

| 2 | Half-face air-purifying respirator | Moderate DAC fractions; limited protection factor |

| 3 | Full-face air-purifying respirator | Higher DAC fractions; higher protection factor than half-face |

| 4 | Supplied-air respirator (SAR) or self-contained breathing apparatus (SCBA) | Highest DAC fractions or unknown atmospheres |

Protection factors assigned in the plant program determine how much the device reduces the inhaled concentration. A higher assigned protection factor permits work at higher DAC fractions. Always confirm your respirator class against the plant's posted or RWP-specified requirement — never downgrade based on personal judgment.

User Seal Check — Required Every Time

Before entering a radiologically controlled area with an air-purifying respirator, you must perform a user seal check:

If either check fails, the respirator must not be used. Adjust the straps, recheck, or select a different unit. A failed seal check means the worker's face is not protected — this is a safety stop.

Safety Summary

Key terms9

Derived Air Concentration (DAC)The airborne concentration of a specific radionuclide, listed in 10 CFR Part 20 Appendix B, that if inhaled continuously at a standard breathing rate for a working year would result in receiving the annual occupational dose limit.
DAC fractionThe ratio of the measured airborne radioactivity concentration to the applicable DAC value; used to determine whether action levels are exceeded and which respiratory protection is required.
DAC-hoursA unit of internal dose potential calculated by multiplying the DAC fraction by the number of hours of exposure; used to track and limit cumulative internal dose for workers.
Air sampling trainThe complete set of connected components — filter holder, collection medium, pump, and flow meter — used to draw a measured volume of air through a filter for radiological analysis.
Flow meter (rotameter / electronic)A calibrated instrument used to measure and control the volumetric flow rate of air through a sampling train; must be calibrated and zeroed before each use to ensure accurate volume calculations.
User seal checkA positive-pressure and negative-pressure check performed by the wearer each time a respirator is donned to verify that the facepiece forms an airtight seal against the face.
Protection factorA number assigned to a class of respirator indicating how much it reduces the inhaled concentration of a contaminant; a higher protection factor allows use at higher DAC fractions.
Radiation Work Permit (RWP)A documented authorization issued by the radiation protection department that specifies the radiological conditions, required dosimetry, PPE, and controls for a specific work task in a radiologically controlled area.
10 CFR Part 20The NRC regulation titled 'Standards for Protection Against Radiation,' which establishes occupational dose limits, DAC values (Appendix B), and requirements for monitoring and controlling internal and external dose.

Worked examples4

1. An air sample collected in a reactor auxiliary building shows an airborne concentration of Cs-137 of 3.0 × 10⁻⁸ µCi/mL. The DAC for Cs-137 (Class D, inhalation) listed in 10 CFR Part 20 Appendix B is 6.0 × 10⁻⁷ µCi/mL. Calculate the DAC fraction and state whether a plant action level of 0.1 DAC would be triggered.
  1. Write the formula: DAC fraction = measured concentration ÷ DAC value.
  2. Substitute values: DAC fraction = (3.0 × 10⁻⁸ µCi/mL) ÷ (6.0 × 10⁻⁷ µCi/mL).
  3. Divide the coefficients: 3.0 ÷ 6.0 = 0.50.
  4. Subtract exponents (same unit cancels): 10⁻⁸ ÷ 10⁻⁷ = 10⁻¹ = 0.10.
  5. Multiply: 0.50 × 0.10 = 0.050.
  6. DAC fraction = 0.05 (5% of 1 DAC).
  7. Compare to action level: 0.05 < 0.10, so the 0.1 DAC action level is NOT triggered.
Answer: DAC fraction = 0.05; the 0.1 DAC plant action level is not triggered, but monitoring should continue.
2. A work area has a measured I-131 airborne concentration of 2.0 × 10⁻⁹ µCi/mL. The DAC for I-131 (vapor) from 10 CFR Part 20 Appendix B is 2.0 × 10⁻⁸ µCi/mL. A worker spends 4 hours in this area. Calculate the DAC fraction and the DAC-hours accumulated.
  1. DAC fraction = measured concentration ÷ DAC value = (2.0 × 10⁻⁹) ÷ (2.0 × 10⁻⁸).
  2. Divide coefficients: 2.0 ÷ 2.0 = 1.0.
  3. Subtract exponents: 10⁻⁹ ÷ 10⁻⁸ = 10⁻¹ = 0.10.
  4. DAC fraction = 1.0 × 0.10 = 0.10 (10% of 1 DAC).
  5. DAC-hours = DAC fraction × hours of exposure = 0.10 × 4 = 0.4 DAC-hours.
Answer: DAC fraction = 0.10; DAC-hours accumulated = 0.4 DAC-hours.
3. An air sample is collected for 30 minutes at an initial flow rate of 2.0 L/min and a final flow rate of 2.2 L/min. Calculate the total volume of air sampled in liters.
  1. Calculate the average flow rate: (2.0 + 2.2) ÷ 2 = 4.2 ÷ 2 = 2.1 L/min.
  2. Convert sample duration to minutes: 30 minutes (already in minutes).
  3. Total volume = average flow rate × time = 2.1 L/min × 30 min = 63 L.
  4. Record 63 L as the total sampled volume; this value will be used to calculate airborne concentration from the activity counted on the filter.
Answer: Total air volume sampled = 63 liters.
4. A health physics tech counts a filter and finds 1.26 × 10⁻⁶ µCi of Co-60 on the filter. The total air volume sampled was 63 L (0.063 m³). Express the airborne concentration in µCi/mL. (1 L = 1000 mL)
  1. Convert total volume to mL: 63 L × 1000 mL/L = 63,000 mL.
  2. Airborne concentration = total activity ÷ total volume = (1.26 × 10⁻⁶ µCi) ÷ (6.3 × 10⁴ mL).
  3. Divide coefficients: 1.26 ÷ 6.3 = 0.20.
  4. Subtract exponents: 10⁻⁶ ÷ 10⁴ = 10⁻¹⁰.
  5. Airborne concentration = 0.20 × 10⁻¹⁰ = 2.0 × 10⁻¹¹ µCi/mL.
Answer: Airborne concentration = 2.0 × 10⁻¹¹ µCi/mL.

The math in this lesson2

Learners divide airborne concentrations in scientific notation to find the DAC fraction.
8.EE.4N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstration of air sampler setup: filter holder assembly, pump connection, flow-rate verification with a calibrated rotameter or electronic flow meter, and sample collection procedure; students follow a written procedure and annotate each step. · DAC calculation exercise: using provided airborne concentration data (activity per unit volume), students calculate the fraction of the applicable DAC from 10 CFR Part 20 Appendix B and determine whether respiratory protection is required, documenting their work. · Respirator selection and fit-check practice: students review the plant respiratory protection program tiers (filtering face piece, half-face, full-face, supplied air) and, given three airborne hazard scenarios with specified DAC fractions, select the correct respirator class and perform a positive/negative pressure user seal check on a training half-face respirator.
Materials: Air sampling pump and filter holder assembly, calibrated flow meter, 10 CFR Part 20 Appendix B DAC tables, training half-face respirators, written airborne sampling procedure, calculator, PPE
Check: Students submit completed DAC calculation worksheet for three scenarios (must be arithmetically correct and include the correct action recommendation) and demonstrate the seal check; both components must be satisfactory to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Radiation Protection / NRC Non-Licensed Operator Prep
1. What does a Derived Air Concentration (DAC) value represent?
Radiation Protection / NRRPT
2. A worker measures an airborne Co-60 concentration of 4.0 × 10⁻⁸ µCi/mL. The DAC for Co-60 from 10 CFR Part 20 Appendix B is 8.0 × 10⁻⁷ µCi/mL. What is the DAC fraction?
Radiation Protection / OSHA 10
3. Before entering an airborne radioactivity area with a half-face air-purifying respirator, which action is required every single time the respirator is donned?
Radiation Protection / NRRPT / NUCP
4. An air sampling pump's flow rate drifts from 2.0 L/min at the start of a 60-minute sample to 2.4 L/min at the end. What total air volume should be recorded?
Radiation Protection / NRC Non-Licensed Operator Prep
5. A health physics calculation shows the DAC fraction for a work area is 0.4. The plant respiratory protection program requires a half-face air-purifying respirator above 0.1 DAC and a full-face unit above 0.5 DAC. Which respirator is correct for this situation?
Radiation Protection / NRRPT / NUCP
6. Which component of an air sampling train is responsible for capturing radioiodine vapor in addition to particulates?
Radiation Protection / NRC Non-Licensed Operator Prep / NRRPT
7. A worker accumulates 0.25 DAC-hours during a morning job and 0.35 DAC-hours during an afternoon job. What are the total DAC-hours for the day?
Radiation Protection / OSHA 10 / NRC Non-Licensed Operator Prep
8. During a negative-pressure user seal check on a half-face respirator, the worker covers the inhalation ports, inhales gently, and the facepiece does NOT collapse inward — it remains unchanged. What should the worker do?
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