Which molecular features predict how strongly something smells? You might expect the answer to involve receptor binding affinity, chemical reactivity, or electron configuration — the kinds of properties that dominate drug design. Instead, the dominant predictors are molecular size, polarity, ring structure, and branching (arXiv:2512.08683).
An ordinal dataset of over 2,000 molecules classified into four odor strength categories — odorless, low, medium, high — revealed through SHAP analysis that olfactory intensity is governed primarily by physical transport properties, not chemical specificity. The features that predict strong smell are the same features that predict whether a molecule can (a) evaporate, (b) survive transit through nasal mucus, and (c) physically reach the receptor.
The through-claim: smell strength is a logistics problem, not a chemistry problem. The receptor doesn't care how reactive a molecule is if the molecule never arrives. Volatility and solubility — the mundane physics of getting from source to nose — dominate the signal.
This explains a puzzle. Professional perfumers have long known that structurally similar molecules can differ radically in perceived intensity. The molecular similarity is chemical; the intensity difference is physical. Two molecules with identical functional groups but different molecular weights will have different vapor pressures, different diffusion coefficients through mucus, and therefore different perceived strengths — even if they bind the same receptor with equal affinity.
The broader insight: in any sensing system, the transport step between source and sensor can matter more than the interaction at the sensor itself. The bottleneck is often the journey, not the destination.