Odour management: comply with your environmental permit

Assessing the impact of odour

How to assess the impact of odour from your permitted activity on the public.

Reports of odour from the public

The only direct way to assess if people have experienced offence to their sense of smell is through reports from people who have been affected.

It is not necessary for Environment Agency officers to personally perceive odours to classify odour pollution incidents.

The Environment Agency will:

• record, collate, analyse, and investigate reports
• use reports to help assess the severity of the impact of any odour pollution

People reporting odour pollution may describe:

• odour intensity
• odour offensiveness
• how long an incident lasted
• any emotional or perceived health impacts
• what they have done to cope with the odour, such as closing doors and windows

Within any community there will be people with different sensitivity to odour. Some people will interpret a particular odour as intense, offensive and unacceptable. Others will perceive it as unremarkable or not even notice.

Our officers will consider this variation when carrying out their regulatory duties.

Other indications

Some odour pollution incidents happen in rural areas where relatively few people are affected. Other incidents may happen near schools, prisons, or care homes, where people are less likely or unable to report odour pollution. In these cases, indirect indications of odour pollution may be more important.

Indirect methods for assessing the magnitude of offence to people’s senses include changes to their behaviour. Strong but indirect indications of serious odour pollution include:

• cancelling community events
• sheltering
• avoiding community resources that are affected
• reports of nausea
• reports of seeking medical attention

Other indirect methods may include ambient air monitoring or odour observations by Environment Agency officers.

Sniff testing investigations outside the site boundary

Consultants sometimes use grid and plume sniff testing investigations (for example BS EN 16841, 1 or 2) to systematically observe and assess:

• the dispersion of offsite odours
• their potential for community exposure

These methods are limited by real world conditions of:

• short assessment times compared to residents
• varying emissions
• inconsistent dispersion conditions
• limited access

Investigators also need to be aware of adaptation to odour and how this will affect their observations.

These constraints mean that there is still a risk of missing peak odour exposure times. Investigations need to carefully consider how representative their observations are of community experience.

There are instruments which allow people to do a form of dilution olfactometry in the field with ambient air. The principle is similar to that used in BS EN 13725:2022 laboratory dilution olfactometry but uses portable equipment. This type of device could potentially be used to assess exposure in the community. But an Environment Agency trial found users were unable to obtain useful results. In part, this is due to:

• the dynamic nature of odour detection
• highly variable ambient air odour concentrations
• an inability to maintain stable detection limits for users
• human factors, such as distraction

Development of these methods and their implementation by skilled users may lead to useful data on relative odour intensity in communities in the future. But these results will not be directly comparable to BS EN 13725:2022 laboratory dilution olfactometry.

Operator sniff testing investigations

The Environment Agency does not require you to carry out routine offsite sniff testing, even when specified in older permit conditions.

You and your staff have the same constraints which apply to our officers. You are likely to:

• be less sensitive than many individuals in a community
• spend much less time in offsite neighbourhoods
• experience both short and long-term olfactory adaptation

Due to adaptation, people who work on site and are exposed to higher levels of odorous emissions, often for extended periods, will have reduced sensitivity to those odours. This may not be clear to them because their sensitivity to other odours is unaffected.

You and your staff may sometimes detect other odours in the community which you are not adapted to. You may conclude, often incorrectly, that odours from those sources are causing problems instead.

Finally, you and your staff are less likely to find odours from your own activities offensive. People can have a positive emotional connection with their workplace, which may affect their perception of the offensiveness of an odour. People who find certain odours highly offensive are also likely to choose to work in other industries.

Instrument monitoring of odour surrogates in ambient air

Instruments can sometimes be used to assess a community’s exposure to odorous chemicals and, indirectly, likely levels of odour pollution. Some instruments can monitor nearly continuously, providing a pattern of potential exposure over time. They may also provide average concentrations of chemicals or other indicators over time or across a path between 2 points, providing a more detailed picture of exposure patterns.

However, it is not possible for instruments to measure odour pollution directly. This is a function of the human olfactory sense as interpreted by individual observers. Instead, instruments measure chemicals or chemical properties which are somehow unique and present as a fixed proportion of odorous emissions.

Establishing a stable relationship between odour surrogate measurements and odorous chemicals:

• requires detailed characterisation of emissions
• is often too much of a barrier to effective odour monitoring with instruments

While some instruments can provide valuable insight into odorous releases or exposure, it is incorrect and misleading to refer to them as ‘electronic noses’.

Hydrogen sulphide levels are sometimes used as a surrogate for odour when conducting ambient air quality monitoring, but there are limitations to this approach. The World Health Organisation (WHO) air quality guidelines for Europe states:

On the basis of the scientific literature, it is not possible to state a specific concentration of hydrogen sulfide at which odour nuisance starts to appear. Half-hour average concentrations exceeding 7 μg/m³ are likely to produce substantial complaints among persons exposed. A reduction in the concentration of hydrogen sulfide does not guarantee a substantial reduction of the odour nuisance, since hydrogen sulfide in many effluents provides only a small contribution to the odour strength of the total effluent.

The qualifications in this WHO statement are important. The concentration of hydrogen sulphide is only likely to be relevant where it is the dominant odorous chemical. Otherwise, hydrogen sulphide monitoring on its own may severely underestimate levels of odour pollution. This potential limitation applies to monitoring for any surrogate chemicals which might be measured in ambient air.

Classification of odour pollution incidents by severity

The Environment Agency uses both direct reports and indirect indicators to assess the severity of odour pollution incidents. This includes whether there is serious pollution.

The classification will depend on site specific circumstances and the nature of the data which is available at the time. They also use pollution incident severity classifications, in part, to inform permit compliance assessments.

They use the following criteria to determine the appropriate category for odour pollution.

Category 1 – major, serious, persistent or extensive impact or effect on people

This classification would apply where there was a serious medical impact on individuals or distress to groups of sensitive individuals such as hospitals or schools. There may be widespread reports, changes in behaviour or other effects on a local community. Large public events might need to be cancelled.

Category 2 – significant impact or effect on people

This would be indicated by a noticeable change in behaviour or widespread effects on the community. It might also be indicated where more minor impacts persist for an extended period. The use of public or private amenities may also be restricted.

Category 3 – minor or minimal impact or effect on the people

Odour pollution which is at least transitory, or low intensity, might be given this classification. This would not restrict the use of amenities such as public footpaths.

Category 4 – substantiated incident with no impact.

This is not generally applicable to odour pollution incidents.