The impact of environmental stressors on public health is now widely recognised. The World Heatlh Organization has published extensive guidance on the health risks associated with environmental noise. If we look at the contribution of odour and noise as causes of environmental quality complaints by the public, the contribution is typically equal. It is therefore remarkable that where an international binding regulatory framework for regulating environmental noise exists in the European Union, such a framework is absent for regulating environmental malodours.

Like odour, environmental noise does not cause direct damage to hearing; the health effects are stress mediated. This implies that the noise or odour is assessed subjectively by individuals in the exposed population as unpleasant, unwanted and hence annoying. Depending on the ability of individuals to cope with such annoyance this may cause physical health effects such as elevated arterial blood pressure and coronary disease, when the exposure exists over long periods of time. These effects are the physical expression of our reaction to an unwanted sensory perception in our living environment. If that exposure is unwanted and it is impossible to withdraw from it or to reduce the perception then we suffer annoyance. If this happens repeatedly, intermittently, over a long period of time, we speak of nuisance.

Exactly the same mechanisms that explain the relevance of environmental noise for public health are at play when the sensory stimulus is not unwanted noise, but unwanted odour. If the perceived odour is identified as a particular odour character, it is deemed unpleasant within the context and it is unwanted, the odour becomes malodour.

Environmental malodour regulations do exist, but typically only on a national, regional or municipal level. Even when regulatory frameworks exist on a national level it typically is implemented in the form of guidance documents for licensing authorities, such as the guidance note Environmental Permitting: H4 Odour Management in the United Kingdom, rather than in the form of environmental law.

“the one EU wide emission limit is a limit of 1500 ouE/m3 for all IPPC solid waste facility ducted emissions”

On the EU level there is no appetite for odour regulation (perhaps too closely reminiscent of the fake news EU regulation on straight bananas). The subsidiarity principle is applied: let the lowest possible level of public administration regulate environmental malodours. The advantage of this approach is that the local impacts of environmental malodours are indeed tackled by local authority, in many cases, allowing bespoke air quality criteria to be applied in the context of a particular facility.

The one EU wide emission limit was introduced in 2018 in the best available techniques (BAT) document for waste treatment facilities, where a limit of 1500 ouE/m3 was set for application to all IPPC solid waste facility ducted emissions. (Commission implementing decision (EU) 2018/1147, establishing BAT for waste treatment, Official Journal of the European Union L208/38, 2018-08-17).

This brings us to the main topic of this article: the tools we have to monitor odorous environmental gases. As you can’t manage what you can’t measure, methods to turn such a human experience as odour perception into numbers and quantities is a precondition for designing, applying and enforcing regulations for limiting the impact of environmental malodours.

Technical standards

Technical standards for quantifying odour emissions and the presence of perceptible odour in ambient air were developed by national standards organisations in the 1980s and 1990s (Germany, Netherlands, France, Japan, USA). Later, in 1995, the European Committee for Standardization, CEN, by means of its Technical Committee 264 ‘Air Quality’ decided to constitute a Working Group 2 Olfactometry, which I had the privilege to serve as convenor. The result was technical standard EN13725:2003 Air quality – Determination of odour concentration by dynamic olfactometry, which is now the widely used standardised and in many cases accredited method for measuring odour concentration in European odour units per cubic metre (ouE/m3).

“technical standard EN13725:2003 Air quality ‘Determination of odour concentration by dynamic olfactometry’ is now the widely used standard”

This method is suitable for measuring the odour concentration in emission flows, and hence the odour emission rate in ouE/s. As the European odour unit is defined on the basis of a standard stimulus quantity, the European Reference Odour Mass (EROM), the odour emission rate can be used in the same way that emissions in mass per unit time are used as emission rates for dispersion modelling.

Measurement of odour concentration and odour emission rate in emissions in the most frequently used method for enforcing compliance to emission rate limit values, which can be set in a license condition.

To assess the combined odour impact of a facility, the emission rates of all emission sources are fed into a dispersion model to calculate the spatial and temporal distribution of the ambient odour concentration, determined by the emission quantities, the height, velocity and heat content of emissions, the flow of the wind and the atmospheric turbulence. These calculations result in isopleths, contour lines representing a certain level of impact expressed as a percentile value with a limit concentration, at a certain averaging time, typically one hour.

  • Numerically expressed this looks like:
  • C98, 1-hour >3ouE/m3

Such exposure levels are associated with a certain level of annoyance which is associated with a level of ‘acceptable annoyance’ in the exposed population.

In the United Kingdom the Environment Agency provides guidance on such air quality criteria for environmental malodour exposure in guidance note Environmental permitting: H4 odour management. For more background on the basis for setting air quality criteria please refer to the background document van Harreveld, A.Ph., Stoaling, M., Jones, N. Assessment of community response to odorous emissions. R&D Technical Report P4-095, Environment Agency UK, ISBN 1857059247, which is available on the Environment Agency website:

This approach to setting and enforcing air quality criteria for malodour exposure is most common in countries in Europe and also globally.

Some countries, however, take a different approach, most notably Germany, where direct measurement of ambient air odour presence frequency is the main air quality criterion for malodour exposure. Source emission measurement and dispersion modelling are used as alternative methods, but not as a primary air quality criterion.

The method for measuring odour presence frequency on a grid of receptor points was introduced in 2006 as German national standard VDI3940, which served as the precursor of European standard EN16841 published in 2016. The measurements are done by qualified and trained panel members, on a predetermined observation schedule which evenly covers all hours of day and night, over a period of 6 months. The long duration is required to provide a representative measurement for both summer and winter conditions.

Four observation points define a measurement square, and for each square the positive ‘odour hours’ for the four defining corner points are added to one overall odour hour frequency for that square.

The German regulation on environmental malodour sets limits for acceptable exposure based on the odour hour frequency, differentiated for the usage of the exposed area:

  • •Odour hour frequency <10% for residential areas
  • Odour hour frequency <15% for commercial/mixed use areas

A comparative study comparing the two methodologies in a practical case study around a waste management facility in Spain led to the following conclusion on the equivalency between the outcomes of the two methods:

The C98p, 1hour = 3 ouE·m-3 contour coincided with the limit of 1-3% odour hour frequency observation squares to the South of Ecoparc2. To the North this contour coincided with the limit of observation squares with 7-8% odour hours, while crossing through a maximum square of 13%

This observation implies that the German regulatory air quality limit of 10% odour hours is broadly compatible with the C98p, 1hour = 3 ouE·m-3 contour obtained by modelling.

In addition to the two methods described previously, there are indicators that can be used, based on the actual level of annoyance in the population. Questionnaire methods have been used in the Netherlands and in Germany to establish the actual relation between dose and effect: exposure to malodours measured directly using EN16841 field panels or source emission measurements according to EN13725 combined with modelling; and percentage of population classified as annoyed or severely annoyed, respectively.

More recently citizen participation using web based observation registration panels have been used to provide more direct feedback on annoyance episodes.

For regulation, standardised methods are preferable to citizen complaint or observation methods, because the latter can be influenced by the interests of the observers. For enforcement of limits objective methods such as EN1375 olfactometry are indicated.

Future perspective: continuous odour monitoring

The promise of instrumental odour monitoring systems (IOMS) which allow continuous monitoring of odour in ambient air, fence line or at the source has raised significant interest. Sensor networks for detecting odour or VOC vapour emission episodes have been implemented on a large scale, for example in the industrial areas around Rotterdam harbour in the Netherlands. Globally there is a high level of interest in the application of IOMS, particularly in combination with online web based platforms, sometimes also providing online plume dispersion monitoring.

Given the demand for such IOMS applications, a CEN working group WG41 was formed by CEN Technical Committee 264 Air Quality, to establish a standard for validating IOMS that results against the reference methods using human perception, i.e. EN13725 and EN16841.

This working group is progressing and aims to produce an EU standard in 2020 or 2021, which can be used to validate the degree to which the reading provided by the IOMS is indeed indicative for the perception with a human nose.

The standard looks at the following monitoring tasks, aimed at industrial or agricultural facility odours:

  • Presence or absence of an odour under study
  • Classification (recognition) of odours under study
  • Indicating the amount of odour, compared to odour concentration measured by EN13725 olfactometry

The challenge of emulating the human olfaction with a relatively simple electronic sensor device is a considerable one. Human olfaction uses 400 types of olfactory neuron cells (ONCs) as sensor system. Each of these ONCs has a very specific affinity with several of the about 10,000 known odorants, or chemical substances that can elicit an odour perception. From the 50 to a few hundred odorants that are detected in a complex natural smell, the combined response of our human olfaction is processed and translated in our consciousness to one fragrant concept, such as a pine wood after rain.

Of course IOMS applications do not need to emulate human olfaction fully, but in environmental applications there are many confounding influences which are not odour, but which can cause the IOMS sensors to change output: temperature, humidity, non-odorous gases (e.g. methane, carbon dioxide).

IOMS have proven to be useful in highly defined situations, to detect change or difference compared to a well-defined benchmark. The standard that is being developed for validating IOMS results for monitoring environmental malodours will help in determining the value of these systems for odour regulation and enforcement under real world conditions.

Future perspective

Now we address the future perspective and predicting odour concentration from chemically measured odour composition. One would expect that with improving sensitivity of analytical methods, it should be possible to unravel the chemical composition of any odorous gas. Apply some big data analytics and machine learning and we should be able to call a rose a rose by its chemical pattern.

“analysis of odorants is possible, down to 10 ppt, this still leaves a few thousand odorants beyond the reach of detection”

The reality is more resilient. Indeed, the analysis of odorants is possible, down to around 10 ppt. However this still leaves a few thousand odorants beyond the reach of detection, at concentrations where these same odorants are perfectly smellable.

The second obstacle is that our database of human perception thresholds is incomplete. The human detection threshold of an odorant molecule is essential to convert a chemical mass concentration into an ‘odour activity value’, approximating odour units, is essential to building mathematical models of smell. We have recently started an online repository of high quality standardised odour detection thresholds,, with a crowdfunding mechanism to increase our access to such information. However, we are still far from cataloguing the detection threshold of all 10,000 known odorant molecules. This means many will not be accounted for in our models, or will be accounted for at the wrong ‘weight’.

Finally, we have no proven mathematical approach to model the known interactions between odorants in a mixture when they are perceived by human olfaction. We know there are synergetic effects, we know about masking effects, but we have no working mathematical approach to take these interactions into account in complex odorant mixtures.

There are initiatives, in particular in monitoring the effectiveness of odour reducing techniques suing scrubbers in livestock production, to move away from EN13725 olfactometry, and its significant but known uncertainty, and to use chemical odour characterisation as an alternative. It is too early to report real tangible promise for the effectiveness of this approach.

Revision of EN13725

This perspective will become reality in the near future. The result of the revision of EN13725:2003 is now published as the draft revised version. In the Enquiry phase the professional stakeholder community will be able to submit comments and questions to the working group CEN/ TC264/WG2 ‘Olfactometry’ working group. These enquiry comments need to be considered and replied to in writing, possibly leading to adaptations in the draft. The end result of the draft after enquiry will go to the Technical Committee TC264 ‘Air Quality’ for voting. With a bit of luck we can have a revised EN13725:2020 or 2021.

What has changed in the standard?

  • A new and more comprehensive method to assess uncertainty has been introduced, which takes into account the uncertainty caused by different panel compositions. The assessment is done on the basis of multiple reference odorants or on the basis of duplicate samples of complex environmental odours
  • A protocol to determine EROM values for secondary reference materials relative to the primary reference odorant n-butanol has been included. Defined odorant mixtures can be used as secondary reference materials as well. This allows laboratories to choose reference materials that are closer to the odours measured by a specific laboratory
  • The health and safety recommendations have been significantly elaborated with assessors, laboratory staff and sampling technicians in mind
  • The suitable materials and required recovery for sampling materials and the materials for the olfactometer have been reviewed and modified
  • The issue of sampling passive area sources using hood methods has been discussed in an informative way, but not resolved in a standard protocol

If you want to contribute to the enquiry, please contact BSI for the official draft and the process for submitting comments.


In spite of the absence of one European approach to regulating odours, which does exist for the other environmental stressor, noise, a large variety of regulations for limiting exposure to environmental malodours exists on national, regional and local level.

Standardised methods for measuring odour concentration (in emissions) and odour hour frequency (in ambient air) have been introduced on EU international level. These methods are widely accepted.

The widely used EN13725 Olfactometry standard revision draft has now been published and is available for commenting in the CEN Enquiry process.
New opportunities are being pursued, such as continuous odour monitoring and chemical characterisation of odours.

The regulation of exposure to environmental malodours as an air quality attribute is well established in many developed jurisdictions, after 40 years of experimenting and adjusting with such regulations after pioneering regulator initiatives in the 1980s.

Noise and odour have much in common, both affecting public health as environmental stressors, causing annoyance and stress mediated physiological health effects in susceptible individuals in the general population.