There are a variety of aspects and themes to consider when looking at odours. This Odourpedia is a non-exhaustive glossary which may help understand some of the most important aspects related to odours.

Each entry of the Odourpedia defines a word, aspect or topic which pertains to odours. Where available, helpful resources are linked to provide further information on the topic explained.

Do you think an entry is missing? You can make suggestions for new definitions to be added by filling the contact form at your disposal at the bottom of the page.


  • Citizen Science

  • Citizen Science is a scientific approach and a scientific field which aim is to bring science into the non-academic realm. In Citizen Science, non-expert audiences participate to the scientific process. Citizen involvement may vary depending on the Citizen Science approach chosen. Ideally, citizens taking part in citizen science projects should be involved in all stages of the scientific process: from research design to data collection, analysis and conclusions. This is also called 'Extreme Citizen Science'1.

    1 More information on Citizen Science on the International Odour Observatory.


  • Discomfort

  • A set of human reactions that occurs as a result of immediate exposure to an environmental stressor (e.g. odour) that, once perceived, causes a negative cognitive assessment that requires a certain degree of coping.

  • Dispersion

  • Set of complex processes of transport, mixing and chemical transformations that give rise to a variable distribution (spatial and temporal) of the concentration of a species.


  • Emission factors

  • Relationship between the amount of pollutant emitted into the atmosphere and a unit of activity (hours of operation of the source, surface area involved; or in data such as number of employees or others)1.

    1 Data correspond to bibliographic source.

  • European Odour Unit (ouE)

  • Quantity of odorous substance(s) that, when evaporated in 1 cubic meter of a neutral gas under normal conditions, causes a physiological response of a panel (detection threshold) equivalent to a quantity of odourous substance which is at the origin of a Reference Mass of Odour (RMO) evaporated in a cubic meter of a neutral gas under normal conditions.


  • FIDOL Protocol

  • A tool to estimate odour impact or the degree of nuisance caused by odours. The FIDOL parameters are frequency, intensity, duration, offensiveness and location1.

    1 See more on the International Odour Observatory.


  • Odour

  • Organoleptic property perceptible by the olfactory organ when inhaling certain volatile substances.

  • Odour Concentration

  • Number of European Odour Unit (ouE) in one cubic meter of gas, under regular conditions.

  • Odour Flow or Odour Emission Rate

  • Number of European odour units (ouE) that pass through a given surface per unit of time. It can be defined as the product of odour concentration, exit velocity and emission area, or the product of odour concentration and corresponding volumetric flow rate.

  • Odour Impact

    Impact that an odour has on one or more individuals who perceive it. It is standardly described using odour concentrations' frequency, duration, note, intensity and hedonic tone1. It is usually compared to a standard threshold devised per industrial sector. Thresholds often are country-specific.

    An example of how to understand odour impact is given by the British Environmental Agency (2011)2, as follow:

    • High odour impact

    • One or several odours emitted represent(s) a serious contamination, or are(is) likely to cause annoyance to identified recipients, regardless of whether appropriate odour control measures are already being used. You must take additional measures or you may have to reduce or discontinue operations.

    • Medium odour impact

    • The odour contamination generated is likely to disperse outside of the perimeter of the facility and reach the closest receptors. Appropriate measures must be taken to minimise odour. If proper measures are used, the residual odour should be tolerated by the community. For some operational activities, appropriate measures will ensure that the odours do not leave the perimeter of the facility or, failing that, that they do not reach the closest receptors.

    • Low odour impact

    • There is no odour outside the perimeter, however, a preventive approach must be maintained and additional odour control operations may still be considered.
  • 1Please refer to the 'FIDOL' factors on the International Odour Observatory.

    2 Environmental Agency (2011). How to comply with your odour permit - H4 Odour Management.

  • Odour Management Plan

  • An Odour Management Plan (OMP) is a framework developed by an odour emitting stakeholder which outlines their commitment(s) and strategy to manage odours. These commitment(s) and strategy, undertaken by the stakeholder, enable them to comply with their environmental regulatory permit and/ or prevent or minimise industry-related odour nuisances.

    You can find Odour Management Plan guidelines on the International Odour Observatory.

  • Odour Modelling

  • Used as a forecasting tool usually implied in impact evaluations. Odour models use an equation or set of equations (i.e. models) that describe the relationship between odour concentrations in an area with the emission rate of a facility, and the factors that affect atmospheric dispersion and dilution of the odours.

  • Odour Nuisance

  • An odour becomes a nuisance when the episode is so offensive and prolonged that it impairs the well-being of the affected individual(s). The frequency and offensiveness of an odour episode may be understood differently from country to country.

  • Odour Unit

  • An odour unit is the amount of (i.e.a mixture of) odorous substances present in one cubic meter of odorous gas (under normal conditions) at the threshold of the panel.

  • Odour Quality

  • Odour quality or character is that property that identifies an odour and differentiates it from another odour of equal intensity. Basically this parameter provides information about what the substance smells like.

    The odour character is described by a method known as multidimensional scaling or profiling. In this method, the odour is characterized 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. The result is an odour profile.

  • Odour Wheel

  • Illustrations listing standardised descriptors for reporting odour quality.


  • Perception

  • Awareness of the effect of a simple or complex sensory stimulus.


  • Receiver

  • Any individual who lives in or visits an area, be it a private or public space (e.g. a home, a workplace, an outdoor area), who is or may be exposed to odours generated by an emitting source.


  • Source

    The source of an odour can be defined as the point, area or process during which an odour is emitted. Typically, a distinction is made between conveyed sources and diffuse sources, but some sources are also characterised as fugitive or passive sources.
    • Conveyed sources (or point sources)

    • Emission sources of odorous gases through canalised ducts of defined dimension and flow rate (e.g. chimney, vents) Diffuse sources: generally not conveyed sources, which do not have a defined waste air flow, whereby the emission of odours occurs by diffusion/ convection from odorous surfaces exposed to the atmosphere (‘area sources’), tanks or buildings (‘volume sources’), or from gas leaks from non-airtight ducts or equipment.

    • Diffuse sources

      Generally not conveyed sources, which do not have a defined waste air flow, whereby the emission of odours occurs by diffusion/ convection from odorous surfaces exposed to the atmosphere (‘area sources’), tanks or buildings (‘volume sources’), or from gas leaks from non-airtight ducts or equipment. Typically, a distinction is drawn between active and passive diffuse sources.
      • Active diffuse sources
      • Diffused sources with forced aeration (e.g. biofilters, extended aeration pool, other).

      • Passive diffuse sources
      • Diffuse sources without forced aeration (e.g. sludge piles, sedimentation ponds, other).

    • Fugitive sources

    • Elusive or difficult to identify sources that release indefinite amounts of odorous substances (e.g. valve and gasket leaks, passive ventilation openings, others).

    • Passive sources

    • Sources with defined dimensions (area sources, volume sources) that do not have a defined outlet air flow, such as waste basins, ponds, fields after spreading manure, non-aerated compost piles, buildings.
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