One faint but familiar example of odor is the telltale of a slight gas leak. The major constituent of natural gas, however, is quite odorless methane. Humans can detect mercaptan even when its molecules are outnumbered a billionfold by plain air. But in each breath we take, molecules pass in daunting multitudes. The total population of a sniff is a billion trillion molecules, nearly all of them normally in the mix we call air.
Dogs are proverbially and in fact much better at odor detection than humans are.
The K-9 Corps sniffers can detect many substances reliably at molecular counts thousands of times smaller than our best, a billion molecules or so. Yet the wonderfully brushy antennae of certain large moths are chemical sensors more sensitive by far. The specialized moth antenna fibers respond to an airstream marked by only a few hundred scent molecules. Such virtuosity must be highly prepared at both the transmitter and at the receiver. The signal molecule of the sphinx moth, for instance, is a specific chain of 16 carbon atoms, generated in a female moth gland.