Translating theory into practice, however, will come with many challenges. Smell tests that can reliably identify people who have the coronavirus, while excluding people who are sick with something else, are not yet widely available. (Dr. Hopkins pointed to a couple of smell tests, developed before the pandemic, that cost about $30 each and remain in limited supply.) Should they ever be rolled out in bulk, they would inevitably miss some infected people and, unlike tests that look for the actual virus, could never diagnose disease on their own.
And smell loss, like fever, is not exclusive to Covid-19. Other infections can blunt a person’s sense of smell. So can allergies, nasal congestion from the common cold, or simply the process of aging. About 80 percent of people over the age of 75 have some degree of smell loss. Some people are born anosmic.
Moreover, in many cases of Covid-19, smell loss can linger long after the virus is gone and people are no longer contagious — a complication that could land some people in a post-Covid purgatory if they are forced to rely on smell screens to resume activity, Dr. Yan said.
There are also many ways to design a smell-based screen. Odors linked to foods that are popular in some countries but not others, such as bubble gum or licorice, might skew test results for some individuals. People who have grown up in highly urban areas might not readily recognize scents from nature, like pine or fresh-cut grass.
Smell also is not a binary sense, strictly on or off. Dr. Reed advocated a step in which test takers rate the intensity of a test’s odors — an acknowledgment that the coronavirus can drastically reduce the sense of smell but not eliminate it.
But the more complicated the test, the more difficult it would be to manufacture and deploy speedily. And no test, even a perfectly designed one, would function with 100 percent accuracy.
Dr. Ameet Kini, a pathologist at Loyola University Medical Center, pointed out that smell tests would also not be free of the problems associated with other types of tests, such as poor compliance or a refusal to isolate.