Fresh air or foul odour? How Covid can distort the sense of smell

Deirdre likens her body odour to raw onions; Deepak says his favourite aftershave smells foul, and coffee like cleaning products; Julie thinks coffee and chocolate both smell like burnt ashes.

Most people are aware that a cardinal symptom of Covid-19 is loss of smell, or anosmia. It may last for weeks or even months. Increasingly though, those who have recovered subsequently develop another disorienting symptom, parosmia, or a distorted sense of smell. This typically results in things that once smelled pleasant smelling bad or rotten.

Covid-19 isn’t the only cause, head injuries and other types of infection can also trigger it, but Sars-CoV-2 appears particularly adept at setting off this sensory confusion. According to one recent international survey, around 10% of those with Covid-19 smell loss experienced parosmia in the immediate aftermath of the disease, and this rose to 47% when the respondents were interviewed again six or seven months later.

Based on current infection estimates, there could be 7 million people worldwide with parosmia as a result of Covid-19, the researchers calculated.

“This is on a scale that we’ve never seen before,” says Dr Duika Burges Watson at Newcastle University, who has been studying the psychological impact of parosmia.

Nor is it just a problem of the nose. “It can have a profound impact on your quality of life, from how you eat to how you socialise or engage with significant others, down to the level of whether you actually feel safe going out of your house or not,” Watson says.

Lesley Matthews, 52, of Bolton, lost her sense of smell after catching Covid-19 in January. It started coming back in August, but most toiletries and foodstuffs smell alien to her. “I have two main distorted smells. The first is a chemical-type smell which is present in most toiletries and carbonated drinks. All fragrance and aftershaves have the same disgusting smell, which makes even passing people when shopping intolerable,” she says.

“The second is what I can only liken to the awful smell of a baby’s nappy. All meats, cooked or otherwise, smell of this, along with anything toasting, roasting and frying.”

Because so many foods trigger her parosmia, Lesley’s diet is currently restricted to a handful of “safe foods”, including porridge, scrambled eggs, poached salmon, grapes and sultanas, and she feels nauseous within seconds of someone switching on a toaster. “I can’t go into a coffee shop, and I am constantly making excuses not to socialise as it is no longer a pleasant experience,” she says.

Restricted eating and weight loss is common among those with parosmia, Watson says: “Other people start overeating, because their altered sense of smell leaves them feeling unsatisfied after meals.”

Also common is an altered perception of body odour, both one’s own and other people’s. “That can lead to a loss of social intimacy, either because you are too scared to be in the company of others, or you find the company of others triggers your parosmia,” says Watson. “I have seen cases of people feeling that they had to leave their partners because they couldn’t stand the smell of them. How do you tell the person you love that you find the smell of them disgusting?”

One of the worst cases she recently encountered was a person whose parosmia was triggered by the smell of fresh air. “They literally couldn’t even move from room to room in their house. If they walked outside, they felt the disgusting smell of the air permeated everything.”

The good news is that scientists are beginning to unpick the molecular mechanisms of parosmia, which could eventually lead to better ways of treating it.

On the roof of the nasal cavity, about 7cm behind the nostrils, is a thin membrane studded with specialised cells called olfactory sensory neurons, which capture odour molecules from the air we breathe in and out, and send electrical signals to the brain area that processes scent.

Infections such as Covid-19 can damage these neurons. The current leading theory is that as they regenerate, miswiring and disordered signalling can occur, resulting in parosmia.

“If there is anything amiss with the whole chain of command among the olfactory nerves then the brain cannot receive a complete signal,” says Chrissi Kelly, founder of the smell loss charity AbScent, who has suffered from parosmia since developing a sinus infection in 2012. “That’s when you get these people reporting strange smells that they can’t really describe, that are difficult to pin down.”

Triggers vary from person to person, but many of the same substances often crop up: coffee, meat, onion, garlic, egg, chocolate, shower gel and toothpaste.

Separate research by Dr Jane Parker at the University of Reading and colleagues is beginning to shed light on why these substances are so problematic. In recent experiments, they broke the aroma of coffee down into its constituent molecular parts, and ran them under the noses of people with parosmia and unaffected volunteers.

This showed that parosmia is not linked to a person’s ability to smell. Rather, there are certain compounds that evoke feelings of disgust in many people with parosmia but which unaffected people tend to describe as pleasant. Many contain sulphur or nitrogen, although not all such compounds are triggers. They also tend to be detectable by the human nose at very low concentrations.

The most frequently reported trigger in coffee was 2-furanmethanethiol, which unaffected participants described as roasty, popcorn or smoky-smelling. “Many people [with parosmia] described it as just ‘new coffee, that’s how my coffee smells now’,” says Parker. “Others described it as awful, disgusting. They’ve never smelled anything like it before.”

The fact that there’s a common set of triggers suggests people are not imagining the unpleasantness they are experiencing. It also supports the miswiring hypothesis – although if this is occurring, it seems not to be happening at random. Further research may determine why these triggers elicit such a strong parosmic response, and possibly inform future treatment.

For now, Watson recommends that anyone suffering from parosmia write a list of all their triggers and stick it somewhere other household members can see it, so they can help them avoid these substances or find alternatives. For instance, many of the compounds that Parker and her colleagues have identified are created during the chemical reaction that gives roasted, fried or toasted food its distinctive flavour. Different cooking techniques might render the same foods less offensive.

Another unanswered question is how long those recovering from Covid-19 can expect their parosmia to persist. “The people that had it pre-Covid were taking anything from six months to two or three years to recover, so it is a long process,” Parker says.

For some individuals, certain objects may never smell precisely how they remember them, but that doesn’t mean their quality of life won’t dramatically improve, says Kelly. “You have to look for healing, and for a quality of life that makes you feel good about your day-to-day experiences,” she says. “That’s got to be the yardstick for recovery.”