Understanding Odour and Human Sense of Smell

All odour results from a blend of substances and their interactions.

"Over 165 volatile compounds have been reported in odour from swine production," says Perih, an Agricultural Engineer-in-Training."Most of the odorous substances can be grouped into different classes of chemical compounds, such as volatile fatty acids, phenols, nitrogen derivatives and reduced sulphur compounds. The chemical compounds stem from the degradation of plant fibre and protein as well as the anaerobic degradation of more complex compounds."

This complexity at the source is matched by the complexity in human reaction to odour.

Human sense of smell
The human sense of smell is a primary factor in the human sensation of comfort. "Research shows that smell perception is unique to each individual and varies over time because of changes to the physical condition of the individual and the individual's memory of exposures to similar odours," says Perih.

Smell reaction from an individual is the result of the stimulus created by the olfactory bulb, located at the top of the nasal cavity, coming in contact with odorous molecules. Signals are sent to the olfactory centre in the brain through five nerve fibres. The odour impression is created and compared with the person's odour "memory," based on individual perceptions and social background.

Psychological response to odours is more complex and less understood than the physiological response. "Evidence suggests that each of us learn to like or dislike certain odours," says Perih. "Children like most smells, but it is only as we mature and begin to talk about odours that we develop a sense of which smells we like and dislike.

"This is one reason why reaction to odour emissions from livestock production may appear subjective and can vary from one community to another," she says. "There are examples from rural communities where people have complained about odour emanating from a hog facility when it was actually empty."

The human nose reacts to the odorous mix and not to a single compound in that mix, says Perih. Research is attempting to relate odour measurement to specific chemical components, such as ammonia, hydrogen sulphide and fatty acids.

FIDO analysis
"Research linking chemical levels to odour levels is developing, but it is difficult to relate a specific chemical concentration to odour levels," she says. "Extreme variability of sources, environmental factors and human responses make it difficult to measure odour intensity or determine an acceptable limit for livestock odours."

The response to odours does not always correspond to its intensity or odour concentration. Therefore odour nuisance is generally defined by the FIDO factors: frequency, intensity, duration and offensiveness.

• Frequency. How often an odour occurs.
• Intensity. The strength of an odour.
• Duration. The length of time the odour is encountered.
• Offensiveness. The unpleasantness or character of the odour.