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NRSB RMS: Radon Measurement: Physics, Protocols, QC, and Interpretation

NRSB RMS · Work through your exam, one topic at a time.

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Radon Measurement: Physics, Protocols, QC, and Interpretation

This domain is the measurement core of the NRSB Radon Measurement Specialist exam. It tests whether you understand what a radon number actually represents, how devices must be deployed and controlled while deployed, and how to interpret a result into a defensible recommendation for a client.

Decay, units, and what you are measuring

  • Radon-222 is a noble gas with a 3.82-day half-life, produced from radium-226 in soil and rock. Its short-lived progeny (Po-218, Pb-214, Bi-214, Po-214) are the alpha emitters that deliver dose.
  • 1 pCi/L = 37 Bq/m³. The EPA action level is 4 pCi/L (about 148 Bq/m³); 2 pCi/L is a consider-action level, and no exposure level is considered risk-free.
  • Working level (WL) measures progeny, not gas. At equilibrium, 1 WL = 100 pCi/L; indoor equilibrium ratios are commonly 0.4–0.5.

Test conditions and device placement

  • Closed-house conditions: exterior doors and windows closed, and no forced air from outside, for 12 hours before and throughout the test. Central HVAC runs normally; brief entry is allowed.
  • Place the device in the lowest livable level, at least 20 inches above the floor, 4 inches from walls and objects, and 12 inches from exterior walls. Avoid bathrooms, closets, drafts, heat, and direct sunlight.
  • Short-term tests run 2–90 days; long-term tests exceed 90 days and better represent the annual average.
  • QC: roughly 10% duplicates, 5% blanks for lab-analyzed devices, and 3% spikes; continuous monitors need annual calibration plus periodic calibration checks and cross-checks.

Worked example

A 48-hour continuous monitor in a finished basement averages 150 Bq/m³. Convert: 150 ÷ 37 = 4.05, about 4.1 pCi/L, which is at or above the action level. At a 50% equilibrium ratio the progeny exposure is 4.1 × 0.50 ÷ 100 = 0.02 WL. Correct reporting: confirm with a follow-up test under closed-house conditions before recommending mitigation, and state the conditions actually achieved.

Traps that cost marks

  • Reporting pCi/L as if it were WL, or converting with 1 pCi/L = 3.7 Bq/m³.
  • Accepting a result where the 12-hour precondition was not met, or omitting conditions from the report.
  • Ignoring a blank above the device's detection limit; the batch is suspect, not correctable by subtraction.
  • Duplicate relative percent difference outside the QA plan criterion without investigation.