Some coronavirus patients lose their sense of smell for 30-plus days — and may never regain it. Scientists are beginning to understand why.

• Many coronavirus patients lose their sense of taste and smell — it's a relatively common symptom of COVID-19, according to the CDC.
• Short-term loss of smell may stem from "cleft syndrome," a condition in which swelling prevents aromas from reaching the olfactory neurons.
• But patients with aggressive immune responses may experience direct damage to these neurons, leaving them without their sense of smell for 30 days or more.

Kelsey Meeks sprayed pine-scented air freshener in her office last week — then started to cry. For the first time in months, she could smell it.

Meeks, a 36-year-old attorney who lives outside New Orleans, Louisiana, has been sick with the coronavirus since March 30. Within a week of the disease's onset, she noticed that her senses of taste and smell were gone. Her floral, fruity perfume seemed odorless. The Tom Yum soup she prepared with fresh Thai chilis had no flavor.

Nearly three months later, Meeks said she can tell whether something is salty or sweet, or perhaps catch a whiff of a foul odor, but for the most part, her senses are still gone.

"Before COVID-19, my ability to smell and taste was equivalent to watching an action movie in a Dolby Atmos theater," she told Business Insider. "Now it's like I'm looking at black and white photos instead of watching a high-def film."

Scientists are beginning to understand why the virus has this effect: In a recent article in The Conversation, Dr. Jane Parker, an associate professor of flavor chemistry at the University of Reading, and Dr. Simon Gane, a rhinologist at the University of London, explained that coronavirus patients may experience "cleft syndrome." That's when swollen tissue and mucus block the olfactory cleft — the part of the nose responsible for smell.

In these cases, aromas can't reach olfactory neurons. Once a patient's swelling goes down, the pathway to their olfactory neurons opens and they should start smelling again a week or two later. But a more aggressive inflammation response can cause tissue damage, leaving patients without their sense of smell for 30 days or more.

Patients with more severe infections may take longer to recover their sense of smell

The coronavirus invades the body by latching onto cell receptors called ACE2. Research suggests that people with more ACE2 receptors have a higher risk of severe coronavirus infection.

Since ACE2 receptors are found in the nose — as well as the throat, gut, lungs, and heart — scientists initially thought that the coronavirus might use these receptors to destroy olfactory neurons.

But in an article that's still awaiting peer review, a group of researchers from the UK and US discovered that wasn't the case. Instead, the virus seems to invade nearby cells that support the olfactory neurons. Damage to these cells triggers swelling in the nose that can inhibit a patient's sense of smell — even when that person isn't congested.

In severe cases, the body's immune response can also lead it to attack healthy tissue. That can cause direct damage to olfactory neurons. The more aggressive the immune response, the more severe the damage could be. In some cases, some loss of smell may be permanent.

A risk of permanent smell loss

The US Centers for Disease Control and Prevention lists the loss of taste and smell as a COVID-19 symptom, but it's still unclear how common it is. An April study of more than 200 hospitalized coronavirus patients in Wuhan, China, found that only 5% had loss of taste and smell. But another study of 50 coronavirus patients the same month found that 98% had at least some "smell dysfunction."

The reality is likely somewhere in the middle: A May review found that around 53% of coronavirus patients had smell dysfunction. A Spanish case study also found that nearly 40% of patients with COVID-19 developed smell and/or taste disorders, compared to just 12% of patients with the flu.

Scientists still aren't sure whether the smell loss is what causes coronavirus patients to have trouble tasting, since the two senses are closely linked, or whether the virus affects taste pathways as well.

Some coronavirus patients may start to regain their sense of smell as their olfactory neurons regenerate over the course weeks or months. These patients often develop "parosmia" — a distorted sense of smell — as they recover, which can turn otherwise pleasant aromas into foul odors like chemicals or burning.

"I don't wear perfume anymore because it's just too depressing to spray it on every morning and not be able to smell it," Meeks said. "Sometimes I can pick up a whiff of it, but the part I can smell now smells awful."

Some patients may suffer permanent smell loss if their olfactory neurons are destroyed. In those cases, there's evidence that training exercises, like daily inhalation of essential oils, could help people regain some of the sense.

Meeks said she's treasuring the little victories when it comes to regaining her sense of smell — but the idea of missing out on the joys of food, especially in a culinary city like New Orleans, has taken a mental toll.

"The impact of not being able to experience those things anymore feels like I've suffered a much bigger loss than just my senses," Meeks said. "It's a reminder multiple times a day that I had COVID-19 and that it still has me."