Sense of smell

science and technical writing

IF IT STINKS, IT WONT FLY IN SPACE ( . . . at least it shouldn’t.) Some notes on the nose .

By Rob Crimmins

At the Analytic, Chemical and Environmental Lab at NASA’s White Sands Test Facility, 20 volunteers have sniffed it all. Since 1967, they’ve smelled every item carried in the crew cabins of American spacecraft. To be sure that the astronauts aren’t affected by unpleasant odors everything is sniffed. Each item undergoes machine analysis or tests for toxicity but the human nose is orders of magnitude more sensitive than any machine and noses make the final check for harmful or unpleasant odors.

There’s a bit of a giggle factor associated with this work, but it is important. Should a flight crew or even one astronaut develop motion sickness as a result of an objectionable odor the cost could be very high. In the most notable example a Soviet spaceflight in 1976 ended early when a harsh odor in the cabin of Salyut5/Soyuz21 sickened the cosmonauts.

For some substances, very low concentrations can have serious effects so protecting the astronauts and their missions by screening cabin materials in this manor is necessary. Human absolute sensitivity varies from person to person and from odorant to odorant. A substance that is perceptible and objectionable to one person may be imperceptible

to another. For this reason a panel of sniffers, rather than a single subject, make the checks.

Age affects sensitivity. People in their seventies are about a tenth as sensitive as young adults. Gender is not a factor. Men and women are equally sensitive although older women have a better sense of smell than older men and sensitivity changes slightly during the menstrual cycle. Contrary to popular belief, smokers are no less sensitive than nonsmokers.

As everyone knows, species is a very important determinant of olfactory abilities. Rats and dogs have the best sense of smell of all land vertebrates with sensitivity a thousand times better than humans. Some insects are incredibly sensitive, particularly to pheromones. Pheromones are produced by organisms to induce a behavioral or physiological response in others of the same species. The silkworm moth responds when as little as 40 molecules of sex attractant contact sensory hairs on its antennae.

Hundreds of thousands of substances in airborne molecules stimulate the olfactory receptors in our nostrils. Usually, these are organic chemicals but several inorganics such as ammonia and hydrogen sulfide (rotten eggs) have

distinct odors. Chemical analysis of perfumes, wine, cigarettes and coffee reveal that they contain hundreds of substances that are odor-relevant. It’s rare that a single constituent forms an odor and the weaker constituents of a mixture can influence the aroma. A pleasant fragrance can have some rather unsavory ingredients.

For example, ambergris, a waxy substance found in the intestines of some sperm whales, is very valuable to perfumers. It slows the evaporation of perfume oils and adds longevity to the scent. Alone it has a musky, unpleasant odor but the plant oils and other, pleasant smelling constituents mask it’s scent.

The concentration of an odorant is important and the level of concentration required for it to be perceived varies greatly between substances. People can detect green bell pepper, a very potent odorant, at concentrations as low as 1 part per trillion parts of air. To smell methanol (wood alcohol) the concentration needs to be one hundred million times greater.

Once detected, the perceived magnitude of a scent changes only slightly even when concentration increases. On average it takes a tenfold increase in concentration to yield a twofold increase in perceived magnitude. A process called olfactory adaptation occurs after exposure to an odor relevant substance.

Soon after exposure, perceived magnitude drops to about one third its initial value and if the concentration falls below what it was originally the scent will be lost. Remove the substance and recovery from olfactory adaptation begins. Complete recovery takes just a few minutes.

The human olfactory system is a complex sensory system which, unlike the other senses, has two neural pathways. The first carries information from the olfactory receptor cells in the nasal cavity to the pyri-

form cortex. The path continues to a sensory relay in the thalamus and finally to the brain’s frontal cortex.

This pathway is strictly sensory. The other pathway is a branch from the pyriform cortex to the lateral hypothalamus, which regulates body temperature, breathing, sleep, hunger, thirst, urination, sexual drives and emotions. This is the path responsible for the strong emotional and motivational affects of many odors. Smells are often more memorable than visual images or sounds because of this emotional connection.

The space agency’s testers use their noses and all the complex sensory devices at their disposal to answer a simple question; Does it stink?

On Earth new air comes with the wind or by opening a window and bad smells are washed away. The space traveler takes a tiny bubble of precious atmosphere into the void. He can’t open a window and he doesn’t giggle about the sniffers at White Sands.