Hedonic Tone Odour Assessments – Measuring Pleasant & Unpleasant Odours
People do not perceive all odours in the same way. We naturally consider some pleasant or familia, while others are immediately unpleasant, even at relatively low concentrations.
Unlike other odour tests, hedonic tone odour assessments measure how pleasant or unpleasant an odour is perceived to be. In contrast, odour concentration measures the strength of an odour, and odour intensity describes how strong it is.
Hedonic tone assessment results can help organisations, consultants and regulators better understand the potential impact of odours on the surrounding community.
How the Hedonic Tone Scale Works
Hedonic tone is a subjective assessment of an odour’s pleasantness or unpleasantness. During testing, a trained odour panel scores an odour using a standard scale ranging from +4 (extremely pleasant) to -4 (extremely unpleasant).
Typical scores include:
| Score | Perception | Example Odours |
|---|---|---|
| +4 | Extremely pleasant | Freshly baked bread, blooming flowers |
| +2 | Pleasant | Chocolate, fresh coffee |
| 0 | Neutral | Clean air, distilled water (very weak or non-distinct odours) |
| -2 | Unpleasant | Stagnant water, sour milk |
| -4 | Extremely unpleasant | Cat urine, burnt rubber |
Trained odour panellists carry out hedonic tone assessments. They are selected and screened according to the requirements of BS EN 13725:2022 for odour sensitivity, supporting a consistent and repeatable approach.
Why is Hedonic Tone Important?
Two odours can have exactly the same concentration but produce very different reactions.
People are generally more tolerant of pleasant or familiar odours than unpleasant industrial or waste-related odours. This means that understanding how an odour is likely to be perceived can provide valuable context when assessing its potential impact.
A strong odour is not always unpleasant, and an unpleasant odour does not necessarily need to be strong to cause concern.
For example, a bakery and a wastewater treatment works could produce odours of a similar concentration. However, the bakery odour is likely to receive a much more positive hedonic tone score than wastewater odours. This helps to explain why the latter is more likely to generate odour complaints.
Hedonic tone data can also contribute to planning and permitting work by providing additional evidence, alongside other odour assessment techniques. In addition, it can help operators evaluate mitigation measures and improve communication with regulators and local communities.
Hedonic tone is only one part of a complete odour assessment. It is often helpful alongside other odour testing measurements to build a more complete understanding of an odour source and its potential impact.
For example:
| Odour Test | Output |
|---|---|
| Odour Concentration | How much odour is present |
| Odour Intensity | How strong the odour is |
| Odour Characterisation | What the odour smells like |
| Hedonic Tone | How pleasant or unpleasant the odour is |
When are these Assessments Useful?
A hedonic tone odour assessment can support a range of environmental and planning applications.
These include:
- Understanding the potential for community annoyance.
- Comparing odours from different processes, materials or sources.
- Evaluating changes following odour abatement or process improvements.
- Supporting odour impact assessments for planning purposes.
- Informing odour management plans.
- Research and product development.
Hedonic tone is rarely used in isolation. It provides additional context within a comprehensive odour assessment, helping to understand how odours could be perceived by nearby receptors.
Need Advice on Odour Assessments?
Silsoe Odours provides UKAS-accredited odour services to help organisations understand odour characteristics, perception and potential impacts.
Contact the team to discuss your project or learn more about our odour services.
Originally published 2019. Last updated August 2026.
