Olfaction and odor
Olfaction and odor
This domain tests how humans perceive odorous volatile compounds and how those perceptions are quantified. You must distinguish sensory measurements from instrumental concentration readings, because the exam often asks why a low-concentration mixture can smell stronger than a high-concentration single compound.
Core rules
- Odor has four practical dimensions: detection threshold, recognition threshold, intensity, and hedonic tone.
- Perceived intensity usually follows a logarithmic or power relationship, not a linear one. Doubling concentration does not double perceived strength.
- Adaptation, or olfactory fatigue, reduces sensitivity after prolonged exposure; cross-adaptation changes the response to one odor after exposure to another.
- Mixtures can be additive, synergistic, antagonistic, or masking. A masking agent changes perception without removing the odorant.
- Threshold odor number (TON) is a dilution-to-threshold measure. It is not a concentration in ppm.
Worked example
A panel uses dynamic olfactometry. A sample is diluted 1:300 and 50% of panelists detect odor at that dilution; the odor concentration is reported as 300 ouE/m3. If the sample were diluted 1:50 instead, the result would be 50 ouE/m3. The reasoning: the reported value is the dilution factor at the detection threshold, not the mass of odorant.
Traps
- Treating TON or odor units as ppm.
- Assuming a portable H2S meter captures all odors.
- Ignoring panel variability and adaptation.
- Confusing recognition threshold with detection threshold.