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Smell habituation and anosmia

This section describes fundamental properties of the Human Olfactory System (HOS) when interacting with olfactory stimuli at various concentrations. Temporal atten- uation of olfactory perception is a phenomenon that happens in a daily basis even though we might not realise it. Permanent loss of olfactory perception is rare and can be caused due to a damage to the olfactory bulb and/or the brain.

The HOS is able to adapt to the concentration levels of an odour present in an environment over time. The phenomenon of smell habituation or smell fatigue is the temporal inability to distinguish an odour after a prolonged exposure to this smell at concentrations that can be easily perceived. When an intense smell is present for a long time in an environment the HOS temporarily suspends the signals that are sent to the brain in an attempt to protect it [SP04]. Exposure to strong

smells can cause headaches that can be partially prevented by olfactory adaptation. Smell sensitivity is restored over time when the smell intensity fades or it is masked by another smell that needs to get identified and perceived by the HOS. When the concentration of the smell fluctuates over time the HOS adapts slower to it depending on the fluctuation level and the exposure time [CMA+10].

The loss of smell, known asAnosmia, is di↵erent than smell adaptation as it refers to someone’s permanent inability to smell. According to the National Health Service (NHS) in UK around 6000 people were a↵ected by anosmia at 2012 [HSN12]. Another form of anosmia is the inability to recognise a particular smell or family of smells which is known as Parosmia. Also there are cases of people that su↵er fromPhantosmia where they smell an imaginary smell that does not actually exist in the environment. Loss of smell can happen due to simple causes such as colds or obstruction of the nostrils or it can be the result of more rare symptoms like brain and/or nervous system damages.

Odour assessment

Smell assessment refers to the process of evaluating an odour using human perception criteria and not physical measurement units. Odour assessment is a procedure undertaken in the context of health and safety precautions at various environments were people live, work or get entertained [CEN03]. According to Nicell [Nic09] there are five criteria that are used for the perceptual assessment of an odour. These are: frequency, intensity, duration, o↵ensiveness and location of a smell stimulus.

How frequently someone is exposed to smell stimuli defines the frequency of the presented odour and depends on the way smell is presented to him. For example, smell of constant concentration or bursts of smell. Intensity of smell refers to how strong a smell is perceived. Intensity can be considered as the perceptual analogue of concentration and can be estimated using Steven’s power law (see equation 2.14). Writing this law in terms of smell intensity and concentration gives:

logI = logk+↵logC, (2.21)

where I is the perceived intensity, C is the concentration, and k,↵ are constants specific to the odour. According to Jiang et al. [JCT95] odour intensity can be divided to a scale of seven levels ranging from “no-odour” (level 0) up to “intolerable” odour (level 6).

Duration of smell refers to the time needed until humans adapt, on average, to the presented smell and again depends on the presentation method. O↵ensiveness

of smell is a subjective evaluation of how o↵ensive an odour is. This is connected to the hedonic assessment of the presented smell and varies in a person to person basis although there are smells that are perceived as pleasant by the majority of people (e.g. chocolate smell) and others that are characterised as distasteful or unpleasant (hydrogen sulphide or the smell of rotten egg). Finally, location refers to the source of smell. Health standards set strict restrictions for the distance of unpleasant smell sources near work places, residential areas or leisure facilities.

Odour classification

Many studies have proposed classifications of odours into di↵erent groups depend- ing on their perceptual impact on panels of human subjects. This process is highly controversial for many reasons. The sense of olfaction is connected to previous memories and experiences thus a classification is mostly based on subjective criteria rather than objective techniques [HCL03]. In addition, people from di↵erent de- mographic groups of the population might not even be familiar with the presented smellper se as they were never exposed to it previously. Amongst other factors that a↵ect smell classification is the intensity of the presented smell, the way of present- ing smells (continuous flow smells or pu↵s) as well as the impact of the surrounding environment when presenting a smell stimulus [WOC00]. An odour may not have the same perceptual impact to someone when is delivered in an experimentation room instead of the environment that is usually sensed. In the following, a brief overview of di↵erent classification schemes is presented. For more information the interested reader is referred to [HEL68, RSD+_02].

Ammore [Ern82] has proposed an odour classification into 7 odours that are frequently found in nature amongst a range of 600 organic compounds. These are: camphor, musk, floral, peppermint, ether, pungent, and putrid. Another scheme, as proposed by Rimmel [Rim10], considers a wider set of 18 main smells while in the same way, Zwaardermaker [Zwa95] proposed a more technical classification scheme that comprises 9 main smell categories that are subdivided into many subcategories. In another study by Henning [Hen16], the author proposes a classification scheme similar to the way the monochromatic RGB values are used in the description of colour perception. This classification scheme is composed of an odour prism where every smell can be given as a mixture of 6 main components in appropriate con- centrations. The main components are: flowery, putrid, fruity, spicy, burnt and resinous. In a more recent study, Weiss et al. [WSY+_{12] report evidence that the}

HOS can perceive a “white” smell in the same way as the HVS perceives the white colour as a neutral colour that results from mixing the highest possible values of

Red, Green and Blue. In their study, participants were asked to identify the com- ponents of an odour mixture where every component co-existed in the mixture at equal concentration proportions. The results indicated that at a mixture of around 30 smells, participants were no longer able to distinguish the di↵erent components. The authors argue that the prerequisites to obtain the olfactory “white” are firstly, equal proportions of the smells that compose the mixture and secondly, odours that can be easily perceived as di↵erent from each other.

Recent attempts to classify odours are based on the use of verbal descriptions for smells [Ste01]. This process assumes that all the participants have been presented with a particular set of smells in the past and are able to make use of specialised vocabulary in order to describe the displayed odour.