Cutting an onion makes its smell airborne
An intact onion keeps many of its sulfur-containing ingredients separated inside its cells. Slicing or crushing it breaks those cells, allowing enzymes to start reactions that produce pungent molecules. Some evaporate readily, so they leave the cut surface and travel through the room as odor.
That is why the smell can linger even after the onion is set out. The pieces keep releasing volatile compounds—chemicals that readily become vapor—until they dry out or are removed.
A new odor is not cleaner air
Onion molecules do not act like a sponge for other smells or a filter for harmful gases. They join the air already in the room. Because smell is perceived as a blend, a powerful onion odor may make another odor less noticeable, but that is masking, not removal.
The distinction matters: an onion can change what a room smells like without changing what else is in its air. The old trick’s most immediate result was therefore the one a visitor could notice at once—an unmistakable onion scent.
