Something not to be sniffed at

This article outlines a few basic facts about odours and the anatomy and physiology of the nose, and seeks to define what constitutes a nuisance odour - looking at relevant legislation and enforcing bodies, highlighting the industrial processes which can produce odours and the management strategies that can be used to prevent or mitigate their occurrence.

The article highlights some important elements of government guidance relating to managing odours, including management plans and monitoring techniques that are used for carrying out risk assessments and testing the effectiveness of odour control measures on site.

Some basics about the human nose

Humans have a sensitive sense of smell (olfaction) and can detect odours even when some chemicals are present at very low concentrations. Humans can detect odours using their sense of smell when chemicals in the atmosphere interact with air molecules. These molecules are then detected by receptor cells located in the nose. The odour creates a nerve impulse in the olfactory bulb which sends a signal to the memory and emotional centres of the brain. This leads to the experienced impression of an odour. Characteristics of odours An odour can be generated due to the presence of a single chemical species in the air, where dominant odorous chemical species are among many other essentially non-odorous substances. Alternatively, it may be a mixture of many substances, some or all of which may be odorous. There are various aspects to the sensory properties of odours - it is not a simple case of whether there is an odour present or not - there are other considerations to be made: • Concentration This is the amount of odour present in a given volume of air. Most odours are complex mixtures of compounds requiring a specific measure of concentration which are described in British Standard BS EN 13725:2003 Incorporating Corrigendum No. 1 Air quality - Determination of odour concentration by dynamic olfactometry. There are no mandatory numerical standards set in the UK for odour concentration units for ambient air, although some “custom and practice” guideline values have been used • Intensity The intensity of an odour relates to how an individual perceives its’ strength or magnitude, ranging from faint to strong • Odour character Odour character or quality is what the odour smells like, in other words it is the property that identifies an odour and differentiates it from another. Odour is characterised by either the degree of its similarity to a set of reference odours or the degree to which it matches a scale of various “descriptor” terms such as “fishy”, “fruity” or “cabbage-like”. The result is an odour profile that can be useful for pinpointing an odour’s source from a complainant’s description • Hedonic tone, unpleasantness and relative-offensiveness Hedonic tone is the degree to which an odour is perceived as pleasant or unpleasant which can differ widely from person to person and is strongly influenced by previous experience and emotions at the time of odour perception. Odours may be pleasant when weak but unpleasant when strong, or when exposure is frequent. Odour quality also relates to the emotional response and associations that humans have with certain odours, for example, flowers, sewage, coffee etc.

“there are various aspects to the sensory properties of odours - it is not a simple case of whether there is an odour present or not”

These characteristics should be taken into account when designing a monitoring strategy and it is common practice to adopt the FIDOL strategy. FIDOL is an acronym for the steps needed to adequately assess the extent of an odour release: • Frequency • Intensity (and therefore concentration) • Duration • Offensiveness (hedonic tone/character) • Location (and context), along with any aggravating characteristics

Environmental legislation governing odours

The Department for Environment, Food and Rural Affairs (Defra) is responsible for setting legislation, policy, regulations and guidance for environmental issues. There are also several European and international laws and policies that apply. The legislation that applies to industry and commerce regarding odour pollution is listed below and is enforced by the Environment Agency (EA) and Local Authorities (LA): • Environmental Protection Act 1990 (EPA) • Town and Country Planning Act 1990 • Environment Act 1995 • Pollution Prevention Control Act 1999 (PPC) • Environmental Permitting (England and Wales) Regulations 2007 (EPR)

Industrial processes producing odours

There are a number of industrial processes that have the potential to produce pollution, including odours, on a scale that could have a significant impact on the immediate environment. These processes are subject to closer scrutiny and tighter regulation from the Environment Agency and Local Authority through an Environmental Permit, depending on whether they require an A1 or A2 permit. These processes include (but are not limited to): • Sewage treatment works • Landfill & composting • Animal & vegetable processes, for example: • Maggot breeding • Production of fish meal and fish oils • Pet food manufacturing • Animal feed production • Drying of vegetable matter • Animal rendering • Paper mills • Food industries

Statutory nuisance

If a situation should arise whereby an odour is produced by either industrial or domestic activities, this may constitute a statutory nuisance as described in Section 79 of the Environmental Protection Act. If the odour is emitted from a business premises that is not regulated by an environmental permit, then the Local Authority would be called upon to investigate whether the odour constituted a nuisance under the EPA and would make necessary interventions with the producer so that appropriate action is taken to mitigate the odour.

Guidance to prevent or mitigate odours

Defra and the Environment Agency have published several industryspecific codes of practice and guidance notes containing good practice techniques for monitoring and managing odours. One particular code of practice held up as best practice is the Defra Code of Practice on Odour Nuisance from Sewage Treatment Works. This may be used by other industries as the general principles and methods outlined for preventing, monitoring and controlling odours can be applied to other processes that may generate odour.

“odour from business activities may cause annoyance beyond the site boundary, the operator will be required to have a written odour management plan approved by the relevant regulator”

The EA have published a suite of permitting guidance as part of the PPC/EPR regime. The top level in this suite is ‘Getting the Basics Right’ which covers a large proportion of what an operator needs to know. In addition there are ‘sector specific guidance’ notes that cover issues specific to particular business sectors and ‘horizontal guidance’ notes that go into more detail on a particular topic such as risk assessment, noise or odour. The Environment Agency Technical Guidance Note H4 Odour Management is currently being revised and a public consultation on the draft replacement has recently closed. The Environmental Permitting Regulations require the control of pollution including odour for A1 and A2 processes. Guidance note H4 outlines regulatory requirements with regard to odour, advice on the management of odour and, in particular the aspects that should be dealt with in an odour management plan. Depending on the age of a permit in use, it may express an odour condition using different terms. For example, it may say that the operator must not cause nuisance, annoyance, offensive odours, offence to man’s senses, interference with amenities, pollution etc. It will require the use of Best Available Techniques (BAT), appropriate measures, due diligence, all reasonable precautions, odour management or working plans etc. to minimise odour.

Odour management stratagies & monitoring

If it is likely that odour from business activities may cause pollution or annoyance beyond the site boundary, the operator will be required to have a written odour management plan (OMP) which will have to be submitted to and approved by the relevant regulator, either the Environment Agency or the Local Authority. Once an OMP is considered acceptable to the regulator, operators are required to comply with it and may be in breach of their permit conditions if not, which would be an offence under the environmental permitting regulations. If an OMP does not adequately control the odour, then it should be revised and submitted to the regulator for approval. An OMP should be reviewed at least annually or sooner if there have been complaints or if there are significant or relevant changes to a site’s operations or infrastructure. All OMPs should, as a minimum, contain the following elements: • A risk assessment of normal and abnormal situations, including worst case scenarios, for example of weather, temperature, or breakdowns, as well as accident scenarios • The control measures needed to manage those risks • Suitable monitoring provisions • Actions, contingencies and responsibilities when problems arise • Regular review of the effectiveness of the odour control measures • Emission limits where appropriate Odour management plans should also include clear statements that the operator understands and accepts its responsibilities. In particular, it should show that the operator: • Understands what is the best available technique for their process • Will ensure that any odour control equipment is designed, operated and maintained such that it operates effectively to control odour at all times. This can be either directly or through their contractors or subcontractors • Is familiar with the characteristics of the processes and equipment on site and has identified the areas where there is a risk of odorous emissions occurring • Will reduce or cease operations if necessary to avoid serious odour pollution • Will engage with their neighbours to minimise concerns and complaints • Will respond to complaints


In order for a site operator to assess risks and review the effectiveness of an OMP, it may be necessary to undertake odour monitoring. Qualitative and quantitative techniques (or a combination of these) may be considered appropriate depending on the industry or process. An adequate monitoring plan should include the following: • Why and how monitoring will take place, for example: • Monitoring methodologies, i.e how it will be carried out, including details of monitoring locations etc • Steady state monitoring to confirm that odour is under control, such as regular sniff tests and if appropriate, continuous monitors or process surrogates • If an odour problem arises, the monitoring that will be carried out to establish what needs to be done and how to confirm that control measures have resolved the problem • How to interpret the results including, whenever feasible, trigger values for further monitoring or remedial action • If the terrain is complex, or if odours come from many different sources, how the monitoring strategy will be designed to take account of this • Record-keeping and reporting

Qualitative techniques Complaint data

This is probably the most direct and reliable way to determine whether odours are causing a problem. It is important that an operator records complaints, responds to them and communicates with the complainants. In assessing qualitative data, it should be noted that it can be vulnerable to systematic bias. When reviewing complaints it should be remembered that people tend to under-report odour problems. This may be because they: • Don’t know who to complain to • Fear, rejection, indifference or ridicule • Feel that it is too much effort to register a complaint • Don’t believe that anyone will act on their complaints • Feel concerned about secondary consequences (e.g. reduced property values) • Do not see that their complaints have been properly dealt with, recorded or reported in the past

Sniff testing

Sniff testing is the most common form of odour monitoring. While the factors mentioned in this section need to be taken into account in order to minimise inconsistencies, it can provide perfectly good evidence of an odour problem. A distinct advantage is that while sniff testing may fail to detect odours (underestimate the problem), it is very unlikely to indicate odours which are not there (overestimate). The credibility of operator self-monitoring can suffer for a number of reasons; the public may perceive self-interested bias, many sites are not staffed at night when dispersion conditions can be poor and the odour problem may be at its worst, and staff working at the site get used to odours from the site (olfactory fatigue) and this adaptation means that even if you try to assess your site objectively, you may not be the best person to do so. Anyone who has become used to a smell may not even be aware of the fact, because their sense of smell continues to respond normally to other odours. You should therefore consider whether using external, independent contractors is appropriate, ensuring that their own sensory perception has been calibrated as per the British Standard.

Quantitative techniques

Dynamic Dilution Olfactometry

The standard method for measuring odour is Dynamic Dilution Olfactometry (BS EN 13725:2003). It reduces the subjectivity of sniff testing by an individual. This involves diluting a grab sample in the laboratory to a point where a panel can just begin to detect the odour. The sample could be from an emission point, or it could be a sample of ambient air. This laboratory method is very useful but can prove to be expensive.

Field dilution olfactometry

Equipment such as the Scentometer or the Nasal Ranger may assist investigators in their characterisation of ambient odours. This provides immediate quantitative results. These techniques are subject to the same limitations as sniff testing (olfactory sensitivity of the user, short term adaptation, the need for the tester to be physically present during peak exposures as well as poor usage technique).

Chemical monitoring

Techniques can be used under some circumstances. For example: • Non-specific instruments, either photo-ionisation detectors (PID) or flame ionisation (FID) • The Jerome gold foil instrument or instruments based on metal-oxide semi-conductors can measure extremely low levels of H2S • Gas chromatograph mass spectrometer (GCMS) can be used to give speciation or a finger print of a particular chemical combination • Electrochemical detectors (electronic noses) used in arrays can be useful to detect a change of state in operating conditions as process controls

Issues with monitoring

Whether using sniff testing or taking samples, those developing plans or undertaking the monitoring should take account of the following: • It is often difficult for investigators to witness odour incidents (particularly peaks) that are episodic and short-lived • Emissions are greatly diluted from their point of release, and are often below the detection limits of instruments but can still be detected by people • Peaks in exposure may be due to changing dispersion conditions (wind direction, turbulence) or variable emissions (doors opened) • Emissions from elevated stacks may ground well beyond the monitoring point • Sources that are less obvious, episodic or otherwise inconvenient should be considered • It is important that those people undertaking odour monitoring have a relatively standard sense of smell as some people are hypersensitive to odours whereas others have a relatively low threshold for odours. In addition, people’s sense of smell may alter if they are unwell e.g. with a cold or if they have recently consumed strongly flavoured food or drink or have applied a strong perfume etc In conclusion smells represent the human sensory and subsequent psychological response to an odour, which can be perceived differently by everyone. This makes the measurement, control and enforcement of odour pollution a difficult one, yet it is not impossible to manage. Although responses to odour are subjective, there is still a place for formally regulating and monitoring environmental odours. Monitoring odours is an important element of odour management plans that are essential for key industries to operate. The law can be complex, but there are a number of documents in the public domain that can assist business operators in complying with the law. In the past these have been fragmented, but information is now being streamlined in updated guidance.


1. British Standard BS EN 13725:2003 Incorporating Corrigendum No. 1 Air quality - Determination of odour concentration by dynamic olfactometry. 2. Defra guidance document ‘Code of Practice on Odour Nuisance from Sewage Treatment Works’, PB 11833, 2006 3. Environment Agency Technical Guidance Note H4 Odour Management 4. ‘Best Available Techniques for Assessment and Control of Odour’, Environment Agency, R&D Technical Report P4-079/TR/2, July 2002 ISBN 1 857059 352


Michelle Twigg BA(Hons) MSc CMIOSH Michelle is a health and safety consultant for Sypol Limited specialising in chemical and asbestos management. Michelle began working with Sypol in 1998 after working for Magnox Electric in the chemistry team. She began working on Sypol’s CMS (COSHH Management System) team progressing to a field consultant focusing on occupational health and hygiene strategies in construction and asbestos management. She went to work as health and safety advisor for British Waterways in 2004 before returning to Sypol last year. Published: 10th Dec 2009 in AWE International