Weekend Reads | What the Nose Knows: How Your Sense of Smell (or Lack of One) Impacts Your Health

How important is our sense of smell? In Western culture, the default answer seems to be “not very,” especially in comparison with our vision or hearing. But in this weekend’s read, a group of researchers from Israeli research institutions argue that smell is far more important than we give it credit for.

It turns out that somewhere between 1.4% and 15% of people don’t have a sense of smell, a condition called “anosmia.” There are various potential causes for this: viruses such as COVID-19 (many of us experienced this temporarily), toxins, disease, and head trauma. And about 4% of anosmia cases — about 1 in 10,000 people — have congenital anosmia: They were born without a sense of smell. The researchers note that anosmia is “poorly documented,” which is why the estimate of how many people have it is so imprecise.

That said, having no sense of smell is associated with several health issues, both physical and mental: depression, feelings of personal isolation and emotional blunting, social difficulties, and dietary challenges. It also deprives us of an important warning system for nearby dangers. But the mechanisms linking these issues with anosmia is unclear. We do know, however, that the two functions of the nose, smelling and breathing, are intertwined and affect each other. Breathing moves air through the nose, increasing the exposure of airborne chemicals to the cells in our nose that detect them. And it works the other direction as well, through what is known as the “sniff response”: The intensity and pleasantness of a smell affects our breathing pattern — even when we’re asleep. To that end, the researchers asked, could not having a sense of smell change our breathing enough to affect our health?

We certainly know that how we breathe affects our health. Anyone who has ever bruised or broken a rib is aware that the pain tends to make us breathe more shallowly, which reduces the regular exchange of fresh air into our lungs enough to risk developing pneumonia. Medical researchers have also identified that inhaling through our nose on a cold day warms the air significantly before it reaches our lungs. And in the mid-20th century during the polio outbreak, when some victims were placed in “iron lung” machines to keep them breathing, it was discovered that “sighing” every five minutes or so was critical to keeping lungs healthy (and patients alive). But are there specific differences in how anosmic people breathe, compared with people who retain a normal sense of smell — “normosmics” — that make a difference to their health?

To begin, the researchers found that people with congenital anosmia and normosmics breathe at approximately the same rate: They take about the same number of breaths per minute under the same circumstances. But they discovered that there were differences in how they took those breaths. In particular, normosmics take more “sniffs” during a breath, which look like tiny peaks in the airflow in and out of the nose:

It turns out, though, that they only do this when they are awake, and only in the presence of odors: When sitting in a room that has been thoroughly scrubbed of airborne scents, the peaks go away and their breathing much more closely resembles that of anosmics.

An interesting aside: The researchers wondered why, in the decades of research that has been done on human lungs and breathing, no one has ever noticed this before. It turns out that the most common medical equipment for monitoring breathing uses a “low-pass filter” to smooth out any tiny fluctuations that last less than a third of a second. The peaks generated by the tiny sniffs are smaller than that — so they were effectively erased from the data.

The researchers found a few other differences in how anosmics breathe. They tend to pause more in the middle of inhaling, and they exhale slower when they are awake (but not when they are asleep). Anosmics also have more variation in how they inhale when they sleep, including the volume and speed of the breath. 

Together, these are significant differences — and consistently different enough from “normosics” that the researchers were able to diagnose with 83% accuracy which test subjects had anosmia solely from data on these breathing differences.

The researchers don’t yet have experimental evidence linking these breathing changes to specific health issues, but they feel they have some strong leads based on other researchers’ previous work, particularly on cognitive and mental health issues. According to the researchers, “nasal airflow orchestrates volleys of neural activity throughout the brain,” or put another way, every time we breathe in through our nose, several areas of our brain light up like a Christmas tree as they process all of the data coming in. The breathing changes for anosmics translate into about 240 fewer of these “neural waves” every waking hour, a significant reduction in certain brain activity. Scientists have also found links between nasal airflow and memory function, cognitive function, and mental imagery (which explains why meditation practices often involve controlled breathing).

There are some important limits to this research that require follow-up studies. For one, they only studied congenital anosmics, people born without a sense of smell. Do people who lose their ability to smell later in life see the same changes in their breathing? If so, how quickly do the changes happen, and does their breathing revert back later if they regain their sense of smell? They also didn’t attempt to gather data on the amount people were breathing through their mouth, what those patterns might look like, and whether they complemented or aligned with nasal breathing.

Yet clearly they found that people born without a sense of smell breathe differently, and the researchers believe they have enough evidence to further conclude that the changed breathing patterns in the anosmics they studied may have broad effects on their health. If they’re right, we may just need to adjust our thinking about how important it is to be able to smell.

Humans without a sense of smell breathe differently