About Odours

What is an odour?

The sense of smell is a primal sense for humans and animals. From an evolutionary standpoint it is one of the most ancient of senses. Smell (or Olfaction) allows us to identify food, mates, predators, and provides both sensual pleasure (the odour of flowers and perfume) as well as warnings of danger (e.g. spoiled food, chemical dangers). For both humans and animals, it is one of the important means by which our environment communicates with us.

Odour is a property of a mixture of substances capable of stimulating the olfaction sense sufficiently to trigger a sensation of odour.

Odour characterisation

The following odour components can help to define the character of an odour:

  • Odour threshold (concentration) is the minimum concentration of an odorous substance in order to provoke a given stimulus in humans. They are usually expressed at 50% or 100%, depending on the % of people that perceive the odour. Different types of odour thresholds can be defined: Odour detection threshold; Odour recognition threshold; Odour annoyance threshold

  • Intensity is a measure of the strength of the odour stimulus, and it can be related to odour concentration.

  • Diffusivity An odour can only be perceived when an odorous molecule reaches the nostril. For this reason, how far an odourous molecule can spread is a fundamental element in estimating how likely it is to cause an odour.

  • Quality is usually defined by using odour classes or descriptors. For example, an odour could be described as sweet, rancid, musty etc

  • Hedonic tone is a measure of the pleasantness/ unpleasantness of an odour.
  • Odour impact
    Odour is considered a nuisance when people can perceive it in their living environment and:
  • The perception of the smell is negative;
  • The odour is recurring;
  • It is difficult to avoid the odour;
  • The smell is connected (often incorrectly) to a negative effect on health.
  • When an individual experiences an odour nuisance, the following factors should be taken into account to determine the level of annoyance:

  • Frequency of the odour exposure;
  • Intensity of the odour;
  • Duration of exposure to the odour;
  • Offensiveness of the odour;
  • Location: Tolerance and expectation of the exposed subject.