Odour monitoring and control

In this article, Jak Fazakerley and Dr Danen Appasamy look at the quality and analysis of air affected by nuisance odour.

Air pollution

Air pollution is the introduction of solid particles, liquid droplets or gases into the earth’s atmosphere that can cause harm to human health and/or the environment. There are various activities and factors that are responsible for releasing pollutants into the atmosphere. These sources can be classified into two major categories: manmade (anthropogenic) and natural phenomena.

Manmade pollutants can originate from: stationary sources, e.g. power stations; mobile sources, e.g. motor vehicles; and waste deposition, e.g. in landfills. Natural sources of air pollutants include volcanic activity, radioactive decay and digestion of food by animals.

Air pollutants can be further broken down to primary and secondary pollutants. Primary pollutants include sulphur oxides (SOx), carbon monoxide (CO), nitrogen oxides (NOx), volatile organic compounds (VOCs) and particulates. Secondary pollutants, formed when primary pollutants react or interact, include ground level ozone (O3), peroxyacetyl nitrate (PAN) and persistent organic pollutants (POPs).

Air pollution risk is a function of the pollutant’s hazard and the exposure to that pollutant, with exposure expressed for an individual or entire population. Health effects caused by air pollution are far reaching, but commonly include wheezing, coughing, asthma, cardiovascular disease, cystic fibrosis, lung disease, cancer, and death. This can, in turn, increase pressure on emergency room visits and hospital admissions. An individual’s reactions to air pollutants, however, depends on the type of pollutant, the degree of exposure and the individual’s health status.

Defining odour

Odour is the property of a substance that gives it a characteristic scent, be it pleasant or otherwise. In the UK, statutory authorities have to deal with odour complaints from a variety of sectors including, but not limited to, solid waste processes, wastewater treatment, agriculture, breweries, dairies and manufacturing. Complaints about odours are frequent, partly due to the increased encroachment of urban areas on agricultural and industrial operations, which were originally in either isolated areas or the presence of many different multi source industrial and commercial plants. Authorities have a duty under the 1990 Environmental Protection Act to investigate cases where odour may be causing statutory nuisance.

Nuisance odour

A nuisance can be any smell arising on an industrial, trade or business premises that causes annoyance, reduced enjoyment, reduced property values, or raises health concerns about exposure to harmful emissions. Statutory nuisance can result in enforcement action and ultimately, prosecution. Any activities that are potentially odourous should, therefore, have procedures in place to reduce their risk of causing a statutory nuisance.

The H4 Odour Management Guidance (2011), part of a suite of guidance notes issued by the Environment Agency (EA), is designed to help both holders and potential holders of environmental permits to understand how to comply with the 2010 Environmental Permitting Regulations, as amended. Contained within the H4 guidance is the need for appropriate Best Available Techniques (BAT) to minimise odourous emissions. Such techniques should take a systematic approach, incorporating a range of processes including managing inventory, controlling evaporation, containment and abatement, dispersion and reducing impacts, as well as plant design, operation and maintenance.

When determining what the appropriate BAT will be for a site it is important to factor in needs, costs and benefits. Sensitive receptors need to be considered when determining measures, as some receptors are more susceptible to the adverse effects of exposure than others. Domestic residences, hospitals, schools and day care facilities, for example, are likely to be more sensitive than an industrial complex. An assessment should therefore be made on the sensitivity of existing and likely future development. In addition, as the degree of pollution increases with the size of the exposed population, the more people that are affected the greater the justifiable expenditure on control measures will be.

Even if only a very small number of individuals are affected, however, the seriousness of the exposure may require further control measures. Ultimately, it is really important to engage and communicate with the people who may be affected by the activities. Many operators do this as a matter of course and have well-established procedures for interfacing with the general public.

Determining if nuisance has been caused is difficult to define and is usually subject to an individual’s olfactory perception, that is, their ability to detect and identify airborne molecules in the environment. The issue is further complicated by other factors beyond the odour itself, such as the frequency of odour detection and its duration, intensity and offensiveness.

Odour detection threshold values for individual chemicals are based on the concentration at which half of a test group can detect the odour, while the other half does not. Published values therefore vary widely.

The recognition thresholds of nuisance or annoyance can be expressed as multiples of the odour threshold concentrations. The European Committee for Standardisation, Comité Européen de Normalisation (CEN), has published a standard protocol for the measurement of odour in order to standardise both the threshold values and the way odour is measured. BSEN13725: 2003 states that a European odour unit is the point of detection of that odour (1 ouE/m3). Some contaminants and odorous compounds have very low thresholds of detection. This means they can be perceived by the general population even at low concentrations below the normal exposure limit concentration, compared with undetectable conventional pollutants that might be more dangerous to human health.


Olfactometry is the widespread method for quantifying odour emission concentrations. The H4 Odour Management Guidance (2011), along with Guidance on the Assessment of Odour for Planning (2014), recommends a number of olfactometers for measuring odour concentrations. These are portable devices that allow facility operators, regulators and local residents to conduct odour monitoring and documentation in a variety of locations or circumstances. Rather than a subjective description of odour, they allow the accurate measurement and quantification of the scale of the odour present.

Ambient odours are determined as a Dilution to Threshold (D-T) ratio, essentially mixing odorous ambient air with filtered odour-free air in a controlled volume ratio. This is achieved via two airflow pathways. There are no laboratory fees associated with the technique, making innovative environmental odour monitoring time and cost effective.

Quantifying odours can be beneficial for:

• Monitoring daily operations • Determining specific odour sources • Investigating sources and pathways • Evaluating odour mitigation methods • Creating credible and defensive evidence • Investigating odour control effectiveness • Verifying odour complaints

The results obtained from the monitoring exercise can be plotted on a contour map, providing a pictorial view of odour concentrations on and around the site at the time of the survey.

Personnel using the olfactometer are subject to an acuity test to determine their sensitivity to odours. The test will evaluate whether the user is anosmic, unable to smell, or hyperosmic, with an abnormally acute sense of smell. Either end of the spectrum can skew results and render the individual unsuitable to undertake odour detection tasks.

The odour sensitivity test is comprised of n-butanol dispensing instruments, which are organised on a dilution scale. The test individual is expected to produce a series of judgments with respect to the definable task of selecting one odorous presentation against two others that are odourless, thereby demonstrating an ability to distinguish like from unlike at a set dilution levels.


Odour modelling depends on several factors such as frequency and duration of detection, perceived intensity, chemical compounds and reactions, terrain, and meteorological parameters such as wind direction, wind speed, pressure, temperature, and precipitation. Although air modelling has improved considerably over the last decade, there remains room for improvement in the field. The Environment Agency (EA) is facing new and continued problems, with many consultants offering the service not having a profound understanding of the modelling software and the input parameters required to produce accurate models.

Data gathered through field olfactometers can be plotted on a computer system designed to capture, visualise, analyse, manipulate and manage geographical data to better understand relationships, patterns and trends.

This can then be used to compare and validate atmospheric dispersion models, principally to: • Determine the odour impact that existing or proposed units will have on the surrounding area • Calculate approximate setback distances for new units and to site the units appropriately • Estimate the maximum odour emissions permitted and which abatement techniques will prevent odour complaints occurring

Current and future air quality can then be assessed with respect to the air quality standards such as the EU Air Quality Directive, UK Air Quality Strategy and WHO guidelines.

Consultants, universities, statutory authorities and regulators such as DEFRA and the EA, are now focusing on validating models based on fieldwork data. Chemical analysis using Gas Chromatography Mass Spectrometry (GC-MS) will help to identify the wide range of compounds found in some of the emissions and, coupled with other data, will give the tools to render better models in the future. Other research is investigating the relationship between bioaerosols and odour. The bacteria in bioaerosols from composting sites, for example, can produce odour, so understanding the relationship between these different bacteria and their specific odour source can help to model the odour dispersion based on bioaerosol surveys.

There has been increasing interest in correlating the olfactory approach with an analytical approach. Since odours are made of organic compounds, they can be analysed using GC-MS and quantified. New technology now allows for detection and quantification of odours in the environment using electronic noses. While the human nose analyses the total odour sample and identifies the odour pattern from the memory bank, the electronic nose can differentiate between different sources. Research has shown that the accuracy of the electronic nose can be improved and is improving with new Artificial Neural Network (ANN) algorithms, but more work is needed to understand the non-linear relationship observed in human perceptions such as taste and odour.

The electronic nose is often also adapted for specific odours where they work best, as the models based on olfactometry can be used simultaneously to give the best results. Sampling devices of different sizes and shapes have been used to measure emission rates of volatile substances including odour. Different devices, however, cannot be expected to provide equivalent emission rate estimates due to the complex chemistry, in particular, the partitioning of volatile substances between two phases.

Fingerprinting odours

The next stage in olfactometry is the fingerprinting of odours based on modelling site data, such as from bioaerosol monitoring and olfactometer surveys. Fingerprinting odours ensures that odours are characterised through GC-MS and potentially the producers (bacteria) of these odours, allowing them to be managed accordingly. If it was found, for example, that the high odour concentration was due to anaerobic bacteria, more oxygen could be pumped into the system to reduce the anaerobic bacteria’s activity. This highlights the need to establish reliable source-term emission data, with sufficient statistical power to develop meaningful dose related effects from odour exposure. In the absence of this, the implementation of precautionary measures may prove to be unnecessarily prohibitive.

Published: 19th Aug 2014 in AWE International