Why can a blocked nose reduce the ability to smell?

A blocked nose reduces the ability to smell because it obstructs the flow of air required to deliver odor-emitting particles to the olfactory system.21

The sense of smell relies on the olfactory epithelium, a specialized region within the nasal cavity containing sensory nerve cells. For these cells to function, microscopic particles carried by inhaled air must reach the olfactory region and dissolve into the thin layer of moisture covering the cilia of the receptor cells.21

When the nasal passages are congested, the volume of air reaching this sensory area is significantly restricted. Without a consistent flow of air, fewer odor molecules can reach the receptors, and the fluid covering the receptors may not effectively capture and dissolve the particles necessary to trigger a chemical response. Consequently, the brain receives less information about the scents present in the environment, leading to a diminished sense of smell.132