Odor Intensity

Overview

A measure of how strong an odor may be based on an initial perception. Odor intensities and odour concentration have a non-linear relationship. For instance, if two distinct odours are both at a concentration of 3 OU/m3, the resultant intensity measurement may result in one being more offensive than the other.  For example, a low odour concentration of hydrogen sulphide (rotten egg smell) may have a higher odour intensity due to the nature of the smell.

VDI 3882 Qualitative Scale

The German standard Olfactometry Determination of Odour Intensity VDI 3882 Part 1 outlines a qualitative scale to measure odour intensity shown below:

0	Not perceptible
1	Very weak
2	Weak
3	Distinct
4	Strong
5	Very strong
6	Extremely strong

Unlike measurements of odour concentration that take place at the detection threshold (1 OU/m3), odour intensity measurements are taken at and beyond the detection threshold. Note that the odour concentration is known for each odour intensity measurement since the detection threshold is known a prior and can be adjusted by changing the dilution.

Once odour concentration and intensity are determined, they can be expressed as a nonlinear mathematical relationship.  Intensity increases linearly with the logarithm of the odour concentration.  Some formulae include Weber Fechner Law (exponential function) and Stevens Law (power function). 

Webner Fechner Law (Exponential Function)

Webner Fechner Law is shown below:

I = kw log(C/Co) + const

Where:
I: Odour Intensity
kw: Weber-Fechner constant
C: Odour Concentration;
Co: Concentration of odorant at the detection threshold (by definition equals 1OU when using odour units);
const: a constant which relates to the use of mean intensity levels that can be calculated from the line of best fit for each odorant.

An example calculation can be found in the German Standard VDI 3882 for the Webner Fechner Law.

Steven’s Law (Power Function)

Stevens Law is shown below:

I = k(C)ⁿ

Where:
I represents odour intensity
C:  odour concentration
K: constant
N:  exponent

A straight line in logarithmic coordinates appears if function plotted as:

log I = log K + nlog (C)