Combined measurement uncertainty per ISO GUM framework. Sampling, analytical, and variability components for OEL comparison.
ISO GUM approach — occupational exposure measurement
Enter the uncertainty components for your measurement. Each component is expressed as a relative standard uncertainty (%). The combined standard uncertainty uc is calculated by root-sum-of-squares (RSS). The expanded uncertainty U (k=2, ~95% confidence) is used for OEL comparison. Inputs follow EN 482:2021 / EN 13936:2014 framework.
Measurement result
Uncertainty components (relative standard uncertainty, %)
Day-to-day and between-worker variability. Enter 0 to exclude (single-sample assessment).
Temperature, humidity, substrate, blank corrections. Include if relevant.
Uncertainty budget
Component
u (%)
u² (%²)
Contribution
Enter components above to build the uncertainty budget.
Combined and expanded uncertainty
Combined standard uncertainty uc
uc = √(us² + ua² + uv² + uw² + ux²)
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Expanded uncertainty U (k=2)
U = k × uc
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Measured value
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Measurement interval [C − U, C + U]
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OEL
—
Enter measured value and OEL to assess compliance with uncertainty.
Combined standard uncertainty: uc = √Σui² (relative). Expanded uncertainty U = k × uc. Compliance interpretation per EN 13936:2014 and EN 689:2018: if C + U ≤ OEL, the measurement is fully compliant; if C − U > OEL, the OEL is exceeded with high confidence; if the interval straddles the OEL, further sampling is required. The workplace variability component (uw) accounts for the random variation in exposure between shifts and workers — include it when a single sample is used to characterise a group or when long-term compliance is assessed. Standard: ISO/IEC Guide 98-3:2008 (GUM); EN 482:2021; EN 689:2018; EN 13936:2014.
Typical uncertainty values for occupational sampling methods
Values below represent typical combined standard uncertainties for published NIOSH/OSHA methods under controlled conditions. Actual values should be taken from method validation data or laboratory accreditation certificates. Workplace variability is excluded from these figures.
Method reference uncertainty values
Method / analyte
Sampling us
Analytical ua
Volume uv
Combined uc (without variability)
NIOSH 1500 — Hydrocarbons (GC-FID)
5%
3.5%
3%
~7%
NIOSH 2010 — Ketones (GC-FID)
6%
4%
3%
~8%
NIOSH 2531 — Formaldehyde (HPLC)
8%
5%
3%
~10%
NIOSH 5000 — Inorganic dust, gravimetric
7%
2%
4%
~8%
NIOSH 7400 — Asbestos fibres (PCM)
10%
20%
3%
~22%
NIOSH 9000 — Endotoxin (LAL assay)
15%
25%
5%
~30%
OSHA 7 — Mercury vapour (AAS)
5%
5%
3%
~8%
IOM inhalable dust — gravimetric
9%
2%
4%
~10%
GSP respirable cyclone — gravimetric
8%
2%
4%
~9%
Direct-reading instruments (calibrated)
10–25%
—
—
10–25%
EN 13936 / EN 689 compliance decision rules
Condition
Decision
Required action
C + U ≤ OEL
Compliant
No immediate action; maintain monitoring programme
C − U ≤ OEL < C + U
Inconclusive — further sampling required
Collect additional samples; consider 8-sample strategy per EN 689:2018 Annex G
C − U > OEL
Non-compliant
Implement controls; re-measure after controls to verify compliance
References: ISO/IEC Guide 98-3:2008 (GUM); EN 482:2021 General requirements for the performance of procedures for the measurement of chemical agents; EN 689:2018 Workplace exposure measurement — strategy; EN 13936:2014 Procedures for measuring airborne chemical agents — use of measurement uncertainty when comparing with limit values. NIOSH manual: www.cdc.gov/niosh/nmam/