Odour monitoring: measure, identify and locate sources to guide action
An odour can arise from a single compound or a complex mixture of substances in the air. Its perception depends on their concentration, their combination and the sensitivity of the people exposed. Outdoors, weather conditions also influence how odours travel and which areas are affected.
Measuring a few gases is therefore not always enough to understand an odour episode. Effective monitoring combines continuous instrumental measurements, sensory references, contextual data and, where relevant, feedback from the people exposed.
The aim is to answer four questions: when does an odour episode occur, how can it be characterised, what is its probable source, and which actions can reduce its impact?
Why monitor odours at a site?
Odour nuisance can affect residents’ quality of life, building occupants’ comfort and the relationship between a site operator and the surrounding community.
Episodes can be intermittent: unloading operations, process changes, an open door or a shift in wind direction can cause a brief event that is difficult to characterise during a one-off visit.
Continuous monitoring helps operators:
- Detect odour episodes, including outside staff working hours.
- Document reports by comparing perceptions, measurements and observed conditions.
- Investigate probable sources and operations that may contribute to emissions.
- Trigger investigations or corrective actions according to rules agreed with the operator.
- Assess the effectiveness of actions taken, for example after changes to air treatment or operating practices.
- Build a usable historical record to support site monitoring and dialogue with stakeholders.
The value of the system lies in its ability to turn environmental data into operational decisions.
Concentration, intensity and nuisance: what are we measuring?
Several concepts must be distinguished to interpret the results correctly.
| Concept | What it describes |
|---|---|
| Odour concentration | The dilution required to reach the detection threshold of a sample using an olfactometric method. |
| Perceived intensity | How strongly an odour is perceived: weak, moderate or strong, for example. |
| Odour character | Its sensory description: sulphurous, fermentation-like, solvent-like, etc. |
| Odour annoyance | The discomfort experienced, which depends on factors including intensity, frequency, duration and the context of exposure. |
Odour concentration is not the same as perceived intensity or the level of annoyance. These complementary aspects help explain a situation and guide appropriate action.
How are odours measured?
Dynamic olfactometry: a sensory reference method
EN 13725 specifies the determination of odour concentration in a gaseous sample using dynamic olfactometry with human assessors.
The sample is presented at different dilutions to determine its detection threshold. The result is expressed in European odour units per cubic metre: ouE/m³.
This method provides a reference for characterising emissions and supporting certain studies. However, it relies on discrete sampling: it does not, on its own, describe the variations occurring between two measurement campaigns.
ASTM E679 also addresses the determination of sensory thresholds, using a different protocol. The choice of reference methods depends on the project’s objectives.
Instrumental systems: tracking variations continuously
Instrumental odour monitoring systems, often called “electronic noses”, use multiple sensors whose responses are analysed together.
Depending on the configuration, they can combine sensors targeting specific gases with metal oxide sensors, or MOX sensors, that respond to different families of compounds. Together, these responses form a profile known as an instrumental signature.
Following a training and validation phase, models can use these signatures to:
- detect changes associated with odour episodes;
- recognise previously learned odour profiles;
- estimate odour concentration when the model has been developed using appropriate reference measurements;
- help investigate probable sources.
These results depend on the selected configurations, the available references and the models’ validated operating range. A new or insufficiently documented profile may require further investigation.
Ellona’s outdoor WT1 Pro and WT1 Lite solutions can be incorporated into this approach, depending on the site’s requirements and the project configuration.
Tracer compounds: useful indicators that require interpretation
Hydrogen sulphide (H₂S) and ammonia (NH₃) can be valuable indicators for certain activities, particularly wastewater and waste treatment.
However, measuring these compounds does not necessarily describe the entire odorous mixture. A change in odour may result from other compounds or a change in their combination.
An appropriate approach therefore combines targeted measurements, instrumental signature analysis and suitable sensory references.
Learn more: Are NH₃ and H₂S measurements enough to assess an odour?
How do models learn to recognise odours?
Training involves linking sensor responses to sufficiently documented episodes.
References may include olfactometry results, field observations, information about site operations or perceptions collected using a defined protocol.
A verified source allows an instrumental signature to be linked to a known origin. Without this reference, a model may recognise an emission profile without necessarily identifying where it comes from.
Validation must then assess performance using data separate from the training data. This establishes the conditions under which the results can be used.
Once trained and validated, models can recognise learned profiles in new measurements without a new human assessment for every episode. Additional references can enrich this learning, particularly when processes or emissions change.
Identifying and locating outdoor odour sources
Source location relies on combining several types of information:
- the signatures measured by the monitoring stations;
- the positions of the monitoring points;
- wind direction and speed;
- the timing of episodes;
- site activities and operational events;
- atmospheric dispersion modelling results, where included in the project.
The monitoring network may include points close to potential sources, at the site boundary and, where feasible, in exposed areas. Its layout must take account of prevailing winds, buildings, terrain and monitoring objectives.
The EllonaSoft platform brings data together to analyse episodes, visualise available information and configure alerts.
Attribution to a source remains a conclusion that requires supporting evidence. Similar signatures, multiple simultaneous sources or complex wind conditions can limit the certainty of the assessment.
Learn more:
Incorporating feedback from residents and building occupants
Instrumental measurements provide information about phenomena detected at the monitoring points. Human feedback adds another perspective: what was perceived, where, when and with what level of annoyance.
Questionnaires accessible through QR codes can collect these observations in identified areas. Comparing them with measurements and contextual information helps document episodes and guide investigations.
To be useful, reports need sufficient detail, such as the date, time, location, odour description and perceived intensity.
They can also contribute to model training where the protocol allows. However, a report alone does not establish the origin of an odour.
Monitoring indoor odours
In buildings, odours may be associated with toilets, drainage systems, cleaning products, kitchens, materials or occupants’ activities.
Their movement depends on factors including ventilation, openings and airflow between spaces. Monitoring must therefore take account of the building’s layout.
The POD2 can be configured to monitor odour signatures and other environmental parameters relevant to the analysis.
For example, the system can help investigate recurring episodes after cleaning or examine nuisance in a poorly ventilated area. CO₂ measurement, where included in the configuration, provides additional context about air renewal; it does not measure odour.
Learn more: How can indoor odour sources be measured, identified and located?
Which sectors are affected?
| Sector | Examples of situations to monitor |
|---|---|
| Wastewater treatment | Preliminary treatment, sludge handling, containment and odour treatment. |
| Waste treatment | Reception, storage, composting and air treatment. |
| Industry | Process emissions, materials handling, cleaning and discharges. |
| Ports | Storage and handling of bulk materials or odorous products. |
| Buildings and public facilities | Toilets, kitchens, cleaning and the movement of odours between spaces. |
Turning data into action
An alert is useful when it is linked to an appropriate response procedure.
Depending on the context, teams may inspect odour treatment equipment, examine a handling operation, check that doors or covers are closed, or investigate a ventilation fault.
Historical analysis then makes it possible to compare the frequency, duration and characteristics of episodes before and after an intervention, taking operating conditions and weather into account.
Indicators may include:
- the number and duration of detected episodes;
- recognised odour profiles;
- conditions associated with events;
- response times;
- changes in reports and monitoring results following corrective action.
Examples of Ellona deployments
Explore different applications of odour monitoring:
How do you set up an odour monitoring system?
1. Define the objectives and assess the site
Identify potential sources, exposed areas, relevant operations and existing reports. Clarify which decisions the system should help inform.
2. Design the monitoring network
Select sensor configurations, locations and the necessary contextual data. Plan installation, maintenance and data quality checks.
3. Develop and validate the models
Organise reference measurement campaigns and document representative episodes. Develop the models, assess their performance and define their limits of use.
4. Organise day-to-day operation
Configure dashboards and alerts, assign responsibility for investigations and define the associated actions.
5. Assess and adapt the system
Review the results, check the effectiveness of actions and update the models when site conditions change.
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Frequently asked questions about odour monitoring
Does an electronic nose replace olfactometry?
The two approaches are complementary. Olfactometry provides a sensory reference at a particular point in time; the instrumental system tracks variations continuously and uses models developed from appropriate references.
Can a source be identified using a single sensor?
A monitoring point can detect an episode and recognise a learned profile. Locating a source generally requires additional information: weather data, other monitoring points, source references and operational data.
Does a strong odour indicate a health hazard?
Odour perception alone is not enough to assess a health risk. The assessment must consider the substances present, their concentrations and exposure. Odour assessment and health risk assessment serve different purposes.
How many stations need to be installed?
There is no universal number. Network size depends on the site’s area, potential sources, weather conditions, exposed areas and project objectives.
What happens when a new odour appears?
A profile that was not represented during training may require investigation and additional references. The system should support the examination of unusual episodes and allow models to be adapted where necessary.
Building a system suited to your site
Every site has specific sources, constraints and challenges. Ellona combines measurement and analysis solutions to help operators understand odour episodes, investigate their causes and track the actions taken.
Tell us about your site and your monitoring objectives so we can develop a tailored proposal.