5.1 Table of reference values
The table below collects the foundational numbers of human olfaction, together with an assessment of reliability. This is the table to use when a number needs to be cited in internal documents or website content.
| Quantity | Value to use | Reliability | Notes |
|---|---|---|---|
| Proportion of inhaled airflow reaching the olfactory cleft | ~1–15%, lower in older adults | Low–moderate | All from simulations; do not use “10%” as a constant |
| Optimal sniff parameters | ~30 L/min, ~200 cm³, ~0.4 seconds | Moderate | One study, n = 21 |
| Nasal cycle | ~2 hours awake, ~4.5 hours asleep | Fair | 33/33 people had a cycle |
| Olfactory epithelium area | No modern value | Very low | Only measurement: 2 cadavers in 1892 |
| Total number of sensory neurons | No counted value | Very low | “6–20 million” is an estimate |
| Intact receptor genes | ~350–400 | Fair | Per Volume 3 |
| Glomeruli per bulb | ~5,000–8,000 | Fair | Not 1,100–1,200 |
| Glomeruli per receptor gene | ~16 (human) versus ~2.5 (mouse) | Moderate | Human count on 7 bulbs |
| Mitral cells per bulb (young people) | ~40,000–51,000 | Moderate | Fall sharply with age |
| Olfactory bulb volume | Compare only within the same protocol | Moderate | 2.5-fold difference between laboratories |
| Olfactory bulb neuron turnover | < 1% after 100 years | Fair | One carbon-14 study; a rapidly renewing subpopulation is not excluded |
| Piriform volume | Compare only within the same definition | Moderate | ~66–510 mm³ depending on definition |
| Enantiomer pairs discriminated | 3/10 at group level | Moderate | One study, n = 20 |
| Odour-threshold variance explained by physicochemistry | ~76% | Fair | 353 compounds |
5.2 Data conflicts traced to their source
| Topic | Values | Cause / conclusion |
|---|---|---|
| Number of glomeruli | 1,100–1,200 / 5,568 / ~8,000 | Resolved: the first figure has no count in humans; the latter two are compatible once specimen age is taken into account |
| Proportion of olfactory airflow | ~10%, <10%, <15%, 1.2–7.7%, 3–15%, 11.1 ± 7.4% | Differences in regional definition, nasal geometry, age, simulated flow rate |
| Retronasal route | Model: more sensitive / Measurement: less sensitive | Unresolved; the model is based on only one person |
| Olfactory bulb volume | 67–71 mm³/bulb; 54 mm³ both bulbs; 50 mm³/bulb | Different MRI and segmentation protocols |
| Piriform volume | ~66–188 / 228–252 / 461–510 mm³ | Different regional definitions |
| Piriform laterality | Right larger by 17.7% / 5% / left larger / histology: uncorrected rightward lateralisation in one area | Unresolved |
| Location of the olfactory epithelium | Includes middle turbinate / superior turbinate only | Unresolved; possibly due to specimens, differing criteria and individual variation |
| Mitral cells in young people | ~40,000 / ~51,000 | Counting method and sample (one all-female study) |
| Deuterated musk | Humans discriminate ~90% / receptors do not discriminate | Unresolved, but the behavioural result stands alone |
| α-synuclein in normal older adults | 7% / 20% | Possibly mainly due to the definition of the control group |
| Mediodorsal thalamic lesions | Discrimination normal / impaired | Unresolved; only about 10 cases in total |
5.3 The field’s genuine gaps
The gaps below are not due to limits on access to the literature; they are measurements that nobody has made:
- Olfactory epithelium area, and the total number and density of sensory neurons in humans — by modern methods and by age.
- The lifespan of olfactory sensory neurons in humans.
- The thickness and renewal rate of human olfactory cleft mucus.
- Kinetic constants of human olfactory enzymes with odorants, and the extent of metabolism within a single sniff.
- The functional role of odorant-binding proteins in humans.
- The atomic structure of a human odorant receptor in its native form.
- Quantitative transduction biophysics in human olfactory neurons.
- Combinatorial code size at physiological odour concentrations.
- Three-dimensional glomerulus counts on a large sample spread evenly across ages.
- The number of axons in the olfactory tract; the volume and neuron count of the anterior olfactory nucleus.
- Neuron counts and a transcriptomic atlas for the human piriform.
- The cytoarchitectonic borders of the olfactory area of the orbitofrontal cortex.
- Evaporation rates of aroma compounds from wood, wax and paper.
5.4 Implications for fragrance design and products
The implications below are the editorial team’s inferences from the data in this monograph.
- What customers smell may already have been transformed by their own noses. In vitro, many aromatic esters and aldehydes are substantially metabolised by mucus enzymes, whereas tertiary esters are almost untouched; in the breath, metabolites appear within a few seconds. When a note “smells different on each person”, part of the cause may lie in the chemistry inside the smeller’s nose rather than in the formula. For formulas that need to be stable across users, tertiary esters such as linalyl acetate are a less variable choice.
- Older customers smell with a different “nose”. According to simulations, airflow to the olfactory cleft falls from about 13–15% in children to about 3–5% in older adults; mucus volume falls by about 60%; the number of glomeruli falls by about 10% per decade. Products aimed at older people may need higher intensity — but this should be tested on that very group rather than inferred from a young panel.
- Sniffing instructions. According to one model, the main resistance lies in the mucosal phase, so for the same volume a longer sniff may be more effective than a stronger one — a hypothesis not yet tested in humans. The optimal parameters measured in humans are about 0.4 seconds at about 30 L/min.
- Two nostrils. The nasal cycle makes the two nostrils differ in flow at any given moment, and one experiment suggests that this alters the perception of mixtures. For serious sensory evaluation, smell with both nostrils, and repeat at a different time.
- Passive diffusion products need to be measured in-house. The collection strands found no peer-reviewed data on evaporation rates from wood or wax. For essential-oil-infused wood products, the release curve over days can only be known by weighing mass loss over time, or by measuring with headspace analysis equipment — and this is also valuable proprietary data.
- Do not assume that “chirality” makes a difference. Of the 10 enantiomer pairs tested, only 3 were discriminated at group level, although some individuals could discriminate as many as 6 pairs. Before paying a premium for a pure enantiomer, test each material separately on your own panel.
- Be cautious with poorly purified synthetic materials. In one large study, only 12/50 chemical samples had an odour genuinely caused by the nominal compound itself. For materials used at high concentration, impurities may determine the odour.
5.5 Correction to Volume 2
Section 3.3 of Volume 2 originally used the figure of 1,100–1,200 glomeruli per bulb in humans. This monograph establishes that the figure is not based on any count in humans; Volume 2 has been corrected to about 5,000–8,000, with the reason recorded in the errata log.
A monograph compiled from 259 sourced facts collected under a common protocol. Practical-implication sections are the compilers’ inferences. Diagrams are newly drawn from published data. Scientific reference material, not medical advice.