How does the size of the human olfactory epithelium compare with that of dogs?

The human olfactory epithelium is significantly smaller than that of dogs, reflecting a major evolutionary difference in the importance of the sense of smell between the two species.

• Human Olfactory Epithelium: Covers an area of approximately 5–10 cm² and contains about 10–40 million olfactory receptor cells.1
• Dog Olfactory Epithelium: Can cover an area of about 170 cm² and contains approximately 1 billion olfactory receptor cells.1

This reduction in the size of the human nasal epithelium causes much of the inhaled air to bypass the sensory surface, contributing to the human nose being relatively insensitive compared to that of a dog. While humans and dogs share similar types of receptors, the vast difference in the scale of the olfactory apparatus allows dogs to detect certain chemicals at thresholds nearly a million times lower than those of humans.21