Introduction

Olfaction, the sense of smell, is the least understood of the five human senses. This, among other factors, makes the task of reducing livestock odours a considerable challenge. This factsheet explains the terminology used to describe odorants and odour, how the human olfactory system works, and how humans respond to odour.

Objectives

The objectives of this factsheet are to:

• define odour and how perception is quantified
• describe the human olfactory system and how it responds to odour, and
• describe how the human olfactory system adapts to odour.

Odour Terminology and Perception

An odorant is a substance capable of eliciting an olfactory response whereas odour is the sensation resulting from stimulation of the olfactory organs. Odours play an important part in our everyday life, from appetite stimulation to serving as warning signals for disease detection. A number of diseases have characteristic odours including gangrene, diabetes, leukemia, and schizophrenia. Odours have been implicated in depression and nausea as well.

Detectable odours can have a significant impact on people by affecting human emotions as well as having physiological impacts on the olfactory system. People associate odours with past experiences and, from those experiences, involuntarily assess the odour as likable, dislikable or indifferent. Effects on individuals, however, vary from one person to another.

Odour threshold is a term used to identify the concentration at which animals respond 50 per cent of the time to repeated presentations of an odorant. This term is reserved, primarily, for use in research with animals. Most often, however, odour threshold is interpreted as detection threshold, which identifies the odour concentration at which 50 per cent of a human panel can identify the presence of an odour or odorant without characterising the stimulus. Detection threshold is the term most frequently used when discussing odour research results associated with livestock operations. The recognition threshold is the concentration at which 50 per cent of the human panel can identify the odorant or odour, such as the smell of ammonia or peppermint.

Although the detection threshold concentrations of substances that evoke a smell are slight (Table 1), a concentration only 10 to 50 times above the detection threshold value often is the maximum intensity that can be detected by humans. This, however, is in contrast to other sensory systems where maximum intensities are many more multiples of threshold intensities. The maximum intensity of sight, for instance, is about 500,000 times that of the threshold intensity and a factor of one trillion is observed for hearing. For this reason, smell often identifies the presence or absence of odour rather than quantifies its intensity or concentration.

The ability to perceive an odour varies widely among individuals. More than a thousand-fold difference between the least and the most sensitive individuals in acuity have been observed. Differences between individuals are, in part, attributable to age, smoking habits, gender, nasal allergies, head colds or other illnesses. Non-smokers over the age of 15 show greater sensitivity than smokers in general. Furthermore, females tend to have a keener sense of smell than males, a finding that has been substantiated in work at Iowa State University.

Generally, the olfactory sensory nerves atrophy from the time of birth to the extent that only 82 per cent of the acuity remains at the age of 20; 38 per cent at the age of 60 and 28 per cent at the age of 80. Consequently, olfactory acuity and like or dislike of an odour decrease with age.

Infants appear to like all classes of odorous materials, perhaps because of the lack of previous experience and because of their innate curiosity. Children younger than five years old rated sweat and faeces as pleasant but above that age, as unpleasant.

Like and dislike of a particular odour can change with odour concentration or intensity. Generally, humans can distinguish between more than 5,000 odours but some individuals experience anosmia (smell blindness) for one or more odours. In this situation, the individual apparently has a normal sense of smell but is unable to detect one particular odour regardless of its intensity. For example, because methyl mercaptan has an odour recognition threshold of only 0.0021ppm (Table 1), it is often mixed with natural gas as an indicator of leaks; however, approximately one in 1,000 persons is unable to detect the strong odour of this mercaptan. An estimated 30 per cent of the elderly have lost the ability to perceive the minute amount of this mercaptan used in natural gas.

Table 1. Examples of varying threshold measurements of odorous substances (odorants) [1]

Odour Physiology

Olfaction depends upon the interaction between the odour stimulus and the olfactory epithelium. The olfactory membrane is a sensitive area, covering 4 to 6 square centimetres in each nostril (Figure 1).

Top: Figure 1. Nasal cavity and detail of nerve fibres from olfactory
Below: Figure 2. The olfactory system

Beneath the membrane is a mucous layer. The nerve cells or peripheral receptor cells that primarily sense odours and fragrances are located in the epithelium. Cilia extend from the nerve cells into the mucous layer, which greatly increases the potential receptor area. The cilia are thought to contain the ultimate olfactory receptors, which are specialised protein molecules. Specific anosmia may result from the inability to synthesise the appropriate protein. The receptor cells transmit impulses to the olfactory bulb located at the base of the front brain (Figure 2).

At the bulb, fibres from the nose contact with other nerves, which travel on to various parts of the brain. In order for there to be a sensation the following are important:

1. the substance must be volatile enough to permeate the air near the sensory area
2. the substance must be at least slightly water-soluble to pass through the mucous layer and to the olfactory cells
3. the substance must be lipid-soluble because olfactory cilia are composed primarily of lipid material, and finally
4. a minimum number of odourous particles must be in contact with the receptors for a minimum length of time.

Many theories have been proposed to describe the mechanism of smelling odours. Most can be classified into one of two groups: a physical theory or a chemical theory. The physical theory proposes that the shape of the odourant molecule determines which olfactory cells will be stimulated and, therefore, what kind of odour will be perceived. Each receptor cell has several different types of molecular receptor sites, and selection and proportion of the various sites differ from cell to cell.

The chemical theory, which is more widely accepted, assumes that the odorant molecules bind chemically to protein receptors in the membranes of the olfactory cilia. The type of receptor in each olfactory cell determines the type of stimulant that will excite the cell. Binding to the receptor indirectly creates a receptor potential in the olfactory cell that generates impulses in the olfactory nerve fibres. Receptor sensitivity may explain some of the variation in detection thresholds exhibited by different compounds. For example, ammonia has an odour threshold of 0.037ppm whereas the corresponding values for hydrogen sulphide and sulphur dioxide are 0.00047 and 0.009ppm, respectively (Table 1).

Odour Responses

Odour adaptation is the process by which one becomes accustomed to an odour. The adaptation time needed is greater when more than one odour is present. When adaptation occurs, the detection threshold increases. The detection threshold limits change faster when an odour of high, rather than low, intensity is presented. Besides, adaptation occurs differently for each odour. Odour fatigue occurs when total adaptation to a particular odour has occurred through prolonged exposure. This situation would apply to swine production workers or managers who are exposed to the smell of swine manure on a daily basis and appear virtually unaware of the odour.

While ammonia and hydrogen sulphide are odorants, and not odours per se, they are produced through processes often associated with odour, including municipal sewage treatment systems, coal burning, industries and factories, and livestock operations.

Both ammonia and hydrogen sulphide can cause olfactory losses as a result of chronic or prolonged exposure. Ammonia also can affect the central nervous system. A number of other chemical pollutants, including some insecticides result in losses in olfaction by damaging olfactory receptors. The use of medications may exacerbate chemosensory disorders.

On average, olfactory receptors renew themselves every 30 days. Pollutants may alter this turnover rate or disrupt the integrity of the lipid membranes of olfactory receptors. Threshold levels have been identified for a number of pollutants, above which odour or irritation occur. Unfortunately, however, knowledge of the exact mechanisms by which pollutants alter olfaction is limited.

Summary

Odour science is complex and olfaction is the least understood of the five senses. Odour is the sensation resulting from an odorant stimulating the olfactory system.

The four factors that influence when a stimulation occur are the volatility of the substance, the water and lipid solubility of the substance, and the number of particles in contact with the odour receptors over time. However, age, gender, exposure and experience also impact the response to odorants or odour. Human response to odour is quantified by the detection and recognition thresholds, which occur when 50 per cent of a human panel can detect the presence of, or recognise the odorant or odour, respectively.

References cited and additional resources

1. Water Environment Federation. 1978. Odor Control for Wastewater Facilities. Manual of Practice No. 22. Water Pollution Control Federation, Washington D.C.

2. Powers-Schilling, W.J. 1995. Olfaction: chemical and psychological considerations. Proc. of Nuisance Concerns in Animal Management: Odor and Flies Conference, Gainesville, Florida, March 21-22.

This paper is adapted from 'The Science of Smell Part 1: Odor perception and physiological response' by Wendy Powers, Iowa State University Extension Publication PM 1963A, May 2004 and can be found on the Air Quality and Animal Agriculture Web page.

February 2013