glossary and terms of odours


the reduction in degree or intensity of pollution.


ambient air usually means outdoor air (as opposed to indoor air).


people who participates in odour testing.

background level

a typical level of a chemical in the environment. Background often refers to naturally occurring or uncontaminated levels. Background levels in one region of the state may be different than those in other areas.

detection threshold

the lowest odorant concentration which has a probability of 0.5 of being detected under the conditions of the test.

diffuse sources

sources with defined dimensions (mostly surface sources) which do not have a defined waste air flow, e.g. waste dump, lagoons, fields after manure spreading and un-aerated composting piles.

dynamic olfactometer

an apparatus to delivers a flow of mixtures of odorous and neutral gases with known dilution ratios to assessors.


release of pollutants into the air from a source.


contact with a chemical by swallowing, by breathing or by direct contact (such as through the skin or eyes). Exposure may be either short term (acute) or long term (chronic).

forced choice

an olfactometric method in which assessors are forced to make a choice out of tow or more air flows of which one is diluted sample, even if there is no difference in between.


elusive or difficult to identify sources releasing undefined quantities of odorants e.g. valve and flange leakage, passive ventilation apertures, etc.

hedonic scale

the appreciation of a smell. There are good smells and bad smells. Time and location of observation might lead to a different appreciation of smell at the same concentration. The Hedonic scale varies between 4 and -4. The latter being a very unpleasant sensational smell


breathing. People can take in chemicals by breathing contaminated air.

mobile source

a moving source of pollution, such as a car or truck.

odorous gas

gas containing odorants


the property of a substance affecting the sense of smell; any smell; scent; perfume.

odour concentration

the concentration of an odour in terms of OU per cubic meter. A concentration of 1,000 OU/m3 means that the sample requires a dilution with clean air 1,000 times in order to become odour free.

odour measurement

presenting the diluted samples and odour-free air to assessors (panelist) in a series dilution ratios to calculate the odour concentration for a sample.

odour nuisance

odour is affecting observers to an extend that complaints are reported. Odour nuisance is sometimes divided in classes. In industrial area authorities might use a term like ‘acceptable odour nuisance’.

odour panel

a group of panelists.

odour panelist

an assessor qualifying to assess odour concentration level of odour samples, using dynamic olfactometry.

odour threshold

the lowest concentration of a substance in air that can be smelled. Odour thresholds are highly variable because of the differing ability of individuals to detect odours.

odour unit

1 OUE/m3 is defined at that concentration of an odour in an air sample that can be perceived by 50% of the observers. One OUE/m3 equals the odour perception of 40 ppb n-butanol according to the CEN standard.


apparatus used to dilute the diluted odour sample, and present it with odour-free air to the panelists.


measurement of the response of assessors to olfactory stimuli.


pertaining to the sense of smell.

panel screening

procedure to determine if the performance of a panel candidate is in compliance with selection criteria.

recognition threshold

the odour concentration which has a probability of 0.5 of being recognised under the condition of test.

static olfactometer

an apparatus to dilute an odour sample by mixing it with odour-free gases in a fixed volume ratio.

stationary source

a non-moving source of pollution, such as a factory smokestack.

Volatile organic compounds (VOCs)

Organic chemicals containing carbon, oxygen, hydrogen, chlorine, and other atoms. Volatile chemicals produce vapors readily. Volatile organic chemicals include gasoline, industrial chemicals such as benzene, solvents such as toluene and xylene, and tetrachloroethylene (perchloroethylene, the principal dry cleaning solvent). Many volatile organic chemicals are also hazardous air pollutants.

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