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Jon Pullen on new guidance from IAQM on odour impact assessment for planning
The national planning policy framework (NPPF) in England requires the effects of pollution on health, the natural environment or general amenity to be taken into account in planning decisions.
The NPPF definition of pollution specifically includes odour. “Pollution can arise from a range of emissions, including smoke, fumes, gases, dust, steam, odour, noise and light,” states the framework.
However, until now, there has been limited guidance on how best to carry out an odour impact assessment and none at all on how to assess the significance of the effects for planning purposes.
The Institute of Air Quality Management (IAQM) – the UK professional body specifically for air quality practitioners – has now issued guidance that addresses both of these needs.
What is perceived as odour is actually a person’s physiological response to one, or (more often) to a complex mixture of airborne chemicals that they detect in their olfactory organ at the base of the brain when they breathe air through their nose.
Their evaluation of the intensity, character and pleasantness of the detected odour takes place quickly, over a matter of seconds, and involves many socioeconomic and psychological factors – for example, particular memories and associations. Adverse effects of exposure to odour can be disamenity, annoyance or nuisance.
There are two general scenarios where an assessment of the impact of odour may be required for planning applications:
- the proposed land-use activity is potentially a significant source of odours, such as wastewater treatment, waste management, food and drink or industrial and agricultural activities; or
- a sensitive use – such as a residential development – is being proposed near an existing odorous activity.
The air quality section of the national planning practice guidance (NPPG) for England advises: “Assessments should be proportionate to the nature and scale of development proposed, and the level of concern about air quality, and because of this are likely to be locationally specific.”
The IAQM guidance helps to put some flesh on these bones by summarising what an odour impact assessment for planning purposes should cover and the different assessment tools that can be used, highlighting their applications and limitations.
Typical assessment tools include qualitative predictive assessments, detailed atmospheric dispersion modelling and odour monitoring by so-called “sniff tests”. The IAQM guidance requires the air quality professional to justify that the assessment approach is suitable and proportionate.
The impact of the odour – exposure determined by the amount, pattern and character of the odour available for perception by an individual – can lead to an adverse effect on people. The magnitude of this will depend partly on the sensitivities of the receptor – that is, how responsive the surrounding land users are to the smell.
The adverse odour effect of most relevance to planning is disamenity, which is “impaired amenity”. The government’s planning portal defines this as a negative element or elements that detract from the overall character or enjoyment of an area, while the Oxford English Dictionary defines disamenity as “the unpleasant quality or character of something”.
Properly categorising receptor sensitivities is crucial to a robust assessment of odour effects. The IAQM has developed a sensitivity classification scheme based on the concept of reasonable expectation of amenity (see panel, below).
For something as subjective as odour, the significance of the effect is a matter of judgment that cannot easily be defined by scientific methods alone. On the one hand, a high sensitivity receptor subject to a large odour exposure will experience a substantial adverse effect.
On the other, a low sensitivity receptor subject to a small odour exposure will experience a negligible effect. However, between these extremes the various combinations will give rise to a gradation of effects for which no descriptor terms have been universally agreed.
The IAQM guidance proposes a general framework of descriptors for the magnitude of effects resulting from the odour impact on a receptor of a particular sensitivity.
This general relationship between the level of odour exposure (impact) experienced by a receptor of a given sensitivity and the magnitude of adverse effect that is likely to result will hold irrespective of the particular tool or method – for example, modelling, qualitative assessment or sniff-test monitoring – that has been used to estimate that odour exposure.
Where the overall effect is greater than “slight adverse”, it is likely to be considered significant.
Concluding that the odour effect is significant should not mean a development proposal is unacceptable and the planning application should be refused. Rather, it should lead to careful consideration against the wider environmental, social and economic benefits that the proposal would bring.
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