Roger Grainger looks at the monitoring and regulation of odour.

Odour is recognised by the human brain in response to an individual chemical or group of chemicals in the air, known as odorants. The average person can detect these chemicals at very low concentrations down to parts per billion (ppb). Very often, the human nose can register odours at levels lower than is possible using sophisticated chemical analyses in a specialised laboratory. Some of us, however, are not capable of detecting any odours whatsoever. This condition is known as Anosmia, whereas an increased ability to detect odours is known as Hyperosmia.

The whole subject of odour is highly complex and an individual’s response is very subjective. Their reaction will depend upon matters such as frequency, intensity, duration, offensiveness and location (FIDOL) of the odour.

Further complications in the assessment of odours include the following: • Odours can arise from a single substance or from a combination of substances • Odours can be pleasant when dilute for a short period, but can cause offence if concentrated or the period is prolonged • Odours that are pleasant for one individual can be offensive to another

Familiarity with odours causes individuals to adapt to an odour so that they do not smell it These are known as nesting odours, which your nose/brain does not register. When a homeowner has a pet dog, for example, there is usually an odour present within the house. The owner does not smell it, but a visitor will normally detect the odour as it is unfamiliar to them. The unfamiliar odour could be offensive to the visitor or it could be pleasant – this will vary between individuals.

Odour annoyance

The major concern with odour is the fact that it can cause an effect that is offensive or objectionable to a person and can give rise to annoyance, nuisance or actual harm to human health. Many symptoms can be experienced by people exposed to odour annoyance, including insomnia, nausea, vomiting, tiredness, headaches, palpitations and shortness of breath. Obviously, in the case of a ‘normal’ healthy person, one or a combination of the symptoms can be serious and have an effect on a person’s daily activities and their short term wellbeing. If an individual has an impaired immune system then the effects can be potentially far more serious, and could lead to increases in their normal medication doses, or even hospitalisation.

When people try to describe odours they often compare them to known, common substances which their brains have remembered from past examples.

Comparisons include: • Rotten eggs – Hydrogen sulphide • Sour milk – Lactic acid • Rotten cabbage – Dimethyl sulphide • Garlic – Dimethyl disulphide • Vomit – Mix of valeric and butyric acids • Sewage – Mercaptans

It is important to remember that a person is actually detecting a chemical or a mixture of chemicals when they recognise an odour. Through complex analytical techniques it is possible to detect and measure the chemicals that may be of interest to a professional chemist. This is not, however, important to someone who is affected by odour annoyance and wants a regulator to prevent, or minimise, the release of the source of the odours from the offending site.

Emission patterns

The pattern of odorant emissions from a source can be highly variable, as air dispersion is dependent on many factors including wind speed and direction, height of release, size of emissions area, surrounding topography, and buildings and vegetation. Ideal air dispersion can be created via the use of a stack 2.5 times greater in height than the surrounding infrastructure, which can allow for the plume of odours to rise.

Issues are more likely to occur from surface emissions, or in situations where the stack height does not sufficiently exceed the surrounding infrastructure or terrain features, which can result in the plume of odorous gas descending.

Monitoring methods

There is a scale for expressing an opinion on the pleasantness or unpleasantness of the odour, which ranges from minus ten for the most unpleasant, to ten for the most pleasant. At zero the odour is classed as neutral; for example, the odour from a cut potato is usually classed as neutral. This scale, which is often used by regulators, is known as the Hedonic Tone Scale.

For determining odour strength there is also a scale based from zero to six as shown below: • 0 – No odour • 1 – Very faint • 2 – Faint • 3 – Distinct • 4 – Strong • 5 – Very strong • 6 – Extremely strong

Despite the simplicity of the above scale, regulators do record the numeric descriptors on their inspection/record sheets when investigating odour incidents in the field. There are, however, more robust methods of measuring the intensity of odours in the field, such as using a portable field olfactometer.

A portable field olfactometer measures a dilution to threshold ratio where the odorous, ambient air is diluted with a known amount of carbon filtered, odour-free air. An immediate dilution factor is recorded by the user, who can move around a site to observe the more odorous areas, or identify high risk operations.

In a second method of determining odour intensity, samples of odorous air are grabbed and contained within bags. These samples are then transported to an odour laboratory, where an odour panel consisting of six professional panellists smell the bagged odours at different clean air dilutions. The point at which half the panellists smell the odour is known as the ‘detection threshold’, and represents one European odour unit (EOU). This test, BS EN 13725:2003, is used in the UK and Europe and is often known as ‘grab samples and dilution olfactometry’.


Local authorities in the UK regulate statutory nuisance and clean air under part three of the Environmental Protection Act 1990 (EPA 90), and can take enforcement action against any premises or processes that are considered a harm to human health, or a nuisance. Harm is defined in section 29 of EPA 90 as “offence to any of his senses or harm to his property.” Offensive odours constitute an offence to mans’ senses, and homeowners often state that an ongoing odour problem that causes a change to their daily lives is causing harm to their property.

Statutory nuisance is defined in the EPA 90 as “any dust, steam or smell arising on industrial, trade or business premises and being prejudicial to health or nuisance.” Under EPA 90, any existing sites producing odorous emissions would only have to apply Best Available Techniques (BAT) to any process on site that led to odorous releases, to comply with the Act and remove the possibility of prosecution by the regulators. Odour complaints, therefore, could be made to the regulators over a long period. If BAT was being applied at the site nothing could be achieved by the regulators without the cooperation of the site owners or tenants, who would have to voluntarily fulfil obligations under their corporate and social responsibilities as ‘good neighbours’.

Planning for emissions

While it is possible to rectify unsatisfactory situations once nuisance is caused, it is far better to plan in advance to reduce the risks of offensive processes at the outset. This is where planning officers can have an important role. Drafting appropriately worded conditions in planning permissions for odour emissions requires an understanding of how effective odour control solutions can be, and what conditions will be effective in protecting the amenity of residents in the area.

Badly worded conditions can damage the relationship between the local authority, the industry and the community, and can incur lengthy legal proceedings commensurate with expensive costs.

Local authorities dealing with planning applications examine the suitability of the application site for the intended land use being proposed, rather than looking into the control of emissions such as odours, which are normally regulated by the pollution control authorities. It is important, however, that potentially odorous processes and suitable treatment systems are addressed at the design and planning stages by all parties, rather than acting to abate a nuisance that occurs once the process at the site is taking place.

Where odour is likely to give rise to nuisance, it is important that the applicant consult at an early stage with the regulators to ascertain risks from odours and assess the adequacy of mitigating measures proposed. If no risk of odour nuisance has been declared by the applicant but regulators identify a risk on submission, this should be addressed early in the planning process. If it is not appropriate to use planning conditions directly, in the United Kingdom a planning obligation could be drawn up as an alternative under section 106 of the Town and Country Planning Act 1990, to ensure odour impacts are controlled in the short and long term.

Where a development proposal is considered to be an Environmental Impact Assessment (EIA) development, under the Town and Country Planning (EIA) Regulations 2011 in England, or under the equivalent legislation for the other UK regions, it is possible that the impact of odours will need to be assessed specifically in relation to effects on the environment and included as part of the formal Environmental Statement (ES).

One example of this is a recent development in the South of England for an integrated waste management facility, which had an onerous odour condition included in the planning permission at all sensitive receptors. There is a requirement to carry out a survey once the plant has achieved full capacity to demonstrate compliance with the condition; a maximum of 1.5 European Odour Units (EOU) at all sensitive receptors was imposed in the condition to avoid public complaints.

In England, when a major infrastructure such as a large sewage treatment works is consented under the Planning Act 2008 (as amended by the Localism Act 2011), the impact of known odours is required to form a key part of an assessment under the Infrastructure Planning (EIA) Regulations 2009 (as amended). The local planning authority will play an important role during the consultation process, and under section 60 of the Act will also have a key role during the production of a local impact report.

Odour awareness

It is important for local authorities to be aware of the proximity of new odour sensitive developments such as hospitals, schools, residential and respite homes to existing odorous facilities. Encroachment of odour sensitive developments around existing sites can lead to many complaints, which can generate serious problems when there may not have been one for many years prior to the new odour sensitive development.

Planning officers need to be aware of the industries that can lead to odour nuisance and of assessments previously undertaken. While sewage treatment works are an obvious example, the following also need to be assessed closely: biowaste sites including anaerobic digestion plants, composting, landfills, petrochemical plants, oil refineries, animal renderers, waste treatment sites, intensive pig farming and poultry units, fat and grease processors, sugar beet processing, breweries, and food manufacturing plants. The list is not exhaustive, but it is representative of many ongoing issues that regulators are currently dealing with in the UK.


The regulators in the UK vary by region: • England – The environmental health officers of local authorities, or environment control/compliance officers of the Environment Agency (EA) • Scotland – The Scottish Environment Protection Agency (SEPA) • Wales – Natural Resources Wales (NRW) • Northern Ireland – The Northern Ireland Environment Agency (NIEA)

Regulation is carried out in the UK either by using the powers contained in EPA 90 or the Environmental Permitting Regulations 2013, as amended. The EA use the powers contained within EPR 2013 and the other regulators use EPA 90.


Prosecutions for proven offences under either statute are similar, as a maximum fine of £50,000 can be imposed in a Magistrates Court. As an alternative to imposing a fine, a Magistrates Court can impose a custodial sentence of up to six months for serious offences. If a case is deemed sufficiently serious whereby it needs to be dealt with in a Crown Court, fines can be unlimited and a custodial sentence of up to two years can be imposed.

In the last two years in the UK there have been many prosecutions for odour offences and typical sites include sewage treatment works, pet food manufacturers, landfills, pig rearing facilities and a range of biowaste treatment sites including industrial composters. The defendants have not only had to pay huge fines and court costs, but active solicitors using a no win, no fee strategy have acted for nearby residents to take litigation for compensation costs. Many cases have been settled for compensation outside of court decisions, as defendants realise they have directly caused nuisance beyond their site boundary over a prolonged period and would lose the legal case easily by the number of witness statements being made before a judge.


The monitoring and regulation of odour annoyance is a serious matter and regulators must ensure their staff are competent and fully trained to carry out their duties. Planning officers should make important, sustainable decisions at an early stage to minimise or prevent potential problems.

Published: 05th Sep 2013 in AWE International