What is Odor Concentration (ouE/m³)?
Odor concentration expresses how “smelly” an air sample is, independent of which compounds cause the smell. In Europe it’s reported as ouE/m³ (European odor units per cubic metre) according to EN 13725. It’s determined with dynamic olfactometry: a panel of trained assessors smells diluted sample air and we record the dilution where the odor becomes just detectable.
Definition
By convention, 1 ouE/m³ equals the detection threshold for an odor sample: at this concentration, 50% of a trained panel can just detect the odor. If a sample needs to be diluted 200 times to reach that threshold, the original odor concentration is 200 ouE/m³.
How dynamic olfactometry works (EN 13725)
- Collect a representative air sample in an inert sampling bag.
- Use an olfactometer to mix the sample with neutral (odor-free) air in controlled steps.
- A trained panel indicates from which dilution they can just detect the odor.
- The dilution-to-threshold yields the odor concentration in ouE/m³.
Sampling basics
- Use clean, inert bags and lines suitable for odor work; avoid contamination.
- Record temperature, humidity, flow, and sampling position and conditions.
- Analyse promptly after sampling and follow your lab’s holding time rules.
Interpreting results
Odor concentration is a screening metric for nuisance risk. Local permits or guidelines define acceptable levels at sources or receptors. Combine periodic olfactometry (ouE/m³) with process monitoring (e.g. H₂S or NH₃ in ppm) to understand both nuisance and root causes.
ouE/m³ vs ppm (can I convert?)
No general conversion exists. ppm measures the concentration of a specific gas; ouE/m³ measures human perception of a whole odor mixture. For mixtures, many compounds interact, so a simple factor won’t hold.
Reducing odor concentration
Odor is typically reduced by capture and treatment. For soluble or reactive compounds (e.g. H₂S, NH₃), wet gas scrubbers are a proven option. For poorly soluble VOCs, consider pre-cooling, chemical stages or polishing (e.g. carbon).
FAQ
Is 1 ouE/m³ always the same everywhere?
The unit is standardised (EN 13725) and labs calibrate their olfactometers with a reference odor. Still, results depend on sampling and panel performance; use accredited labs and consistent procedures for comparability.
How many samples do I need?
For compliance, follow permit or guideline requirements. For troubleshooting, take multiple samples over time and operating conditions to capture peaks and variability.
Can I use H₂S ppm as a proxy for odor?
H₂S correlates with odor in many processes, but odor is a mixture. Use gas analyzers to manage process and olfactometry (ouE/m³) to verify nuisance risk at the source or stack.
What reduction can a wet scrubber achieve?
For soluble/reactive compounds, well-designed scrubbers often reach high removal. Final performance should be confirmed by measuring odor concentration (ouE/m³) against site limits.
Need help?
We design and size odor control systems and can coordinate accredited measurements. See odor nuisance & emission control or contact us.