Guidance on the Control of Odour and Noise from ... - Defra

harm, or air that smells “clean”. Fresh air may contain some odour, but these odours will
usually be pleasant in character such as the smell of freshly mown grass or sea spray.
Different life experiences and natural variation in the population can result in different
sensations and emotional responses by individuals to the same odorous compounds.
Because the response to odour is synthesised in our brains, other senses such as sight
and taste, and even our upbringing, can influence our perception of odour and whether
we find it acceptable, objectionable or offensive.
2.1.3 Attributes of odour
There are four interlinked (sensory) characteristics that are used to describe an odorous
emission. These are as follows:
1) Hedonic tone is a judgement of the relative pleasantness or unpleasantness of an
odour made by assessors in an odour panel. Outside of a laboratory setting this
parameter can be subject to substantial variation between individuals. Some odours
may be pleasant when weak but unpleasant when strong, or when exposure is
frequent.
2) Quality/Characteristic is a qualitative attribute, which is expressed in terms of
“descriptors” (e.g. “oily”, “greasy” or “spicy”). This can be of use when establishing
an odour source from complainants’ descriptions. Alternatively, it may be possible to
identify key chemical components by a description of their specific odour.
3) Concentration is the “amount” of odour present in a sample of air. It is usually
expressed in terms of odour units per cubic metre and is determined using dynamic
dilution olfactometry.
4) Intensity may vary between faint to strong. Perceived intensity is the magnitude
(strength) of perception of an odour. Increases or decreases in concentration of an
odour do not always produce a corresponding proportional change in the odour
strength as perceived by the human nose. This can be important for control where
an odour has a strong intensity at low concentration as even a low residual odour
may cause odour problems.
Odour quality, hedonic tone and concentration influence the perceived odour intensity
(and potential for annoyance), although the response to a particular odour will vary
between individuals.
2.1.4 Effects of odour
The main concern with odour is its ability to cause an effect that could be considered
‘objectionable or offensive’. An objectionable or offensive effect can occur where an
odorous compound is present in very low concentrations, usually far less than the
concentration that could cause adverse effects on the physical health of humans or
impacts on any other part of the environment.
Effects that have been reported by people include nausea, headaches, retching,
difficulty breathing, frustration, annoyance, depression, stress, tearfulness, reduced
appetite, being woken in the night and embarrassment in front of visitors. All of these
contribute to a reduced quality of life for the individuals who are exposed.
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