Odours are one of the most difficult forms of environmental impact to quantify: subjective by definition, varying in type and intensity, and dependent on the assessor’s sense of smell as well as instantaneous meteorological conditions.
There is a standardized method, recognized at the European level, to assess the odour impact of a plant or process in a given area by monitoring the surrounding ambient air: the Field Inspection – Grid Method, described in UNI EN 16841-1:2017.
It is the only method prescribed for this type of monitoring according to the European Commission JRC document on best available techniques for IED (Industrial Emissions Directive 2010/75/EU) installations and the Implementing Decision (EU) 2018/1147 on BAT for waste treatment.
How Field Inspection works
The principle is simple in structure and rigorous in execution.
The area around the source to be monitored is divided into a grid of square cells, whose vertices (called nodes) represent the measurement points. At each node, a certified assessor performs 60 consecutive olfactory measurements, one every 10 seconds, for a total duration of 10 minutes per point.
The certification of assessors is a fundamental step in the procedure: the suitability of those carrying out the measurements is verified through olfactory tests in dynamic olfactometry, according to UNI EN 13725:2004.
If odours are perceived in at least 6 out of 60 measurements at a given node, i.e. in 10% or more of the observations, the entire hour in which the measurement takes place is classified as an odour hour.
What “odour hour” means
The 10% value defined in this way is not a regulatory threshold. It is an operational definition used to classify a measurement hour as positive or negative. The standard itself is explicit on this point: it has an informative value. It allows the characterization of odour impact in a representative way but does not include limits on maximum allowable frequency.
For assessing the acceptability of odour impact caused by any installation on an external receptor, reference can instead be made to the indications provided in the recent SNPA Council Resolution no. 268/25 of 23.01.2025, “Odour emissions: reference elements and methodological approaches for monitoring”: this resolution considers acceptable a number of odour hours per year equal to 10% for residential or mixed areas, and 15% for industrial or agricultural areas.
The importance of plausibility verification
The raw data of odour hours for each grid cell is not sufficient to attribute responsibility to a specific source. The standard acknowledges this and requires an additional step: plausibility verification.
For each detected odour hour, it is necessary to verify whether the hypothesized source can reasonably be responsible for the perceived odour. The tool used is the correlation with meteorological data recorded near the plant.
The logic is as follows:
· if the wind speed, averaged over the 10 minutes of measurement, is lower than 1 m/s, the odour hour is always considered potentially attributable to the source. With weak wind, odours can spread by diffusion, without a prevailing direction.
· if the wind speed exceeds 1 m/s, the vector connecting the source to the measurement node is evaluated: the correlation is considered plausible only if the wind direction falls within a 120° cone (60° on each side) relative to that vector.
This step transforms a frequency-based odour dataset into a contextualized one. A high percentage of odour hours under dominant wind conditions coming from an opposite or different direction than the investigated source conveys a very different meaning compared to the same percentage under favorable wind direction.
Odour type: a frequently overlooked parameter
The standard does not require specifying the type of odour detected. Any deviation from a neutral background smell is classified as “odour”, regardless of its origin. In principle, traffic, grass cutting, or manure odours can all contribute to the frequency of odour hours attributed to an industrial plant.
However, the standard explicitly allows, at the discretion of those conducting the odour monitoring campaign, the differentiation of measurements by odour type using the same detection procedures. This choice has a direct impact on the quality of the final interpretation: knowing the type of odour perceived is the only way to separate the contribution of the source of interest from that of other activities in the area.
The importance of Field Inspection
Field Inspection does not replace dispersion modeling or instrumental measurement of concentrations. It operates on a different level: it captures actual perception at ground level, under real exposure conditions, using the human nose as the measuring instrument.
Its limitations are well known and explicitly stated in the standard: it does not set thresholds, does not automatically establish causality, and requires a significant number of sessions to achieve statistical robustness. However, when correctly applied, with certified panels, reliable meteorological data, differentiation by odour type, and plausibility verification, it provides a representation of ground-level odour impact that no other currently available method can offer with the same level of territorial representativeness.
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