What is olfactometry and for what purposes is it used?

Olfactometry is a psychophysical method based upon the olfactory responses of individuals sniffing diluted odours presented by an olfactometer to determine odour strength or odour concentration. Recent developments in the methodology of olfactometry have dramatically improved the repeatability and reproducibility of olfactometry measurements. Developments of particular importance include refined dilution instrument calibration and panelist management techniques. Olfactometry can provide an effective approach to the measurement of the odour concentration of complex odours.

Olfactometry employs a panel of human noses as sensors. A human nose can detect odour at concentrations well below the sensitivity levels of chemical analytical methods. In the olfactometry testing procedure, a diluted odorous mixture and an odour-free gas (as a reference) are presented separately from two sniffing ports at 20 l/min to a group of eight panelists in succession. In comparing the gases emitted from each port, the panelists are asked to report the presence of odour together with a confidence level such as guessing, inkling, or certainty. The gas diluting ratio is then decreased by a factor of two (ie chemical concentration is increased by a factor of two). The panelists are asked to repeat their judgment. This continues for five - six different dilution levels, resulting in a total of 8x6x2 = 96 judgments (sniffings) from eight panelists. Using panelist responses over a range of dilution settings, odour concentration expressed as odour unit per cubic meter can be calculated from individual threshold estimates.

Currently, the preferred and internationally standardized methods of measuring odour are the Dutch NVN2820 and the more recent draft CEN standards. A joint Australia New Zealand standard based on the draft CEN standard is in the course of preparation.

There are some limitations in odour measurement:

• Odour concentration is only one of the four dimensions that are used to express odour sensation as experience by humans. The odour concentration is determined in an odour-free environment and does not reflect the actual perception of the odour. The odour concentration may only be linked to the detectability of an odour. In a real environmental situation, a person exposed to ambient air may describe his or her responses ranging through terms such as " I smell something ", "there is a strong smell", " this is a bad smell" and "the smell is like ?. Measurement of odour intensity can provide information on the likely degree of impact of an odour.
• Common standardized instrument calibration and panel selection procedures are a prerequisite to comparison of odour concentration data reported in the literature. In the absence of standardized procedures, odour concentration levels reported might simply reflect the experience of the operator, the design of the olfactometer, its operational mode (manual or automatic), its mixing method, the flow rate presented to panelists and the number of panelists employed.

Odour measurement data can be used to:

• Predict odour impact in the vicinity of an operation for odour impact assessment purposes.
• Provide information on the strength and intensity of odours.
• Identify the causes of an odour problem and quantify the scale of odour emission from a particular source.
• Measure the performance of a pollution reduction program implemented by a company.
• Evaluate the removal efficiency of odour control technology.