One of the big things that came out of the pandemic was that many people who got Covid also lost their sense of smell. Some of them have got it back, others have got it partly back and others haven't got it back at all. Dr Emily Crofton from Teagasc, a researcher specialising in the area of sensory science, and Declan Casidy from the Tasteless Cuisine support group joined the Ray D'Arcy Show on RTÉ Radio 1 recently to discuss this. (This piece includes excerpts from the conversation which have been edited for length and clarity - you can hear the discussion in full above).

Crofton talked about the role of smell in our lives. "Our sense of smell is extremely important and we often don't realise just how important it is until we've lost it or it's gone. As humans, we detect smells through two separate systems. The first is through something called orthonasal olfaction, which allows us to detect smells from the world around us, which are literally just sniffed up through the nostrils in our nose.

"This allows us detect the smells in food, for example, the smell of freshly baked bread, or the smell of freshly brewed coffee, which in turn then stimulates our appetite and increases our desire to eat or drink. But this system's also really important because it's almost like a warning system of any potential dangers around you. So for example, gone off or spoiled food, potential gas leak, or smoke or fire.

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"The second system we have for smell is something called retronasal olfaction. This has a significant impact on how we perceive flavor in food and drink. When we eat food, it is completely natural for us to assume, as people, that all the different flavor sensations we are perceiving, so chocolate, nutty, strawberry, apple, milky for example, are all being detected through the taste buds on our tongue, but this actually isn't the case.

"Our tongue has taste buds and receptors within those that can only detect five different sensations: sweetness, sour, saltiness, bitter, and this kind of savory taste called umami. If you were to hold your nose really, really tight and eat a strawberry, for instance, you will detect sweetness and a sourness, depending on the variety, but you will not be able to tell me whether that is a strawberry. Obviously if you're blindfolded, you wouldn't be able to tell me."

So what do we know about how Covid affects all of this? "In the early stages of the pandemic, there was anecdotal evidence emerging across the world that people in fact who had Covid were suddenly losing their ability to smell or taste", explains Crofton. "Now, there's nothing new there. Viruses like the common cold or the flu virus always can impact our sense of smell.

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"There was a surge of reports of people experiencing a sudden loss of smell, sometimes in the absence of any other symptoms and they didn't really know why this was happening. Now they know, they think Covid was attacking the sensory neurons in the nose. So they were either damaging them, which caused smell loss, or killing them completely, so people were not really regaining their sense of smell.

"The research is now very clear. A loss of smell or change in smell and taste is one of the strongest predictors of Covid-19 infection. Now, thankfully, earlier variants like the Alpha and Delta, about 50% of people that were infected with Covid-19 would've lost their smell or taste. As the virus has mutated over time, and now that Omicron is the more dominant version, the impact of smell and taste seems to be less. There's only about 18% of people are affected, which is a good thing.

"However, there's been over 500 million confirmed cases globally of Covid-19 infection, and in a conservative estimate, we probably have millions of people worldwide with lingering smell problems. And some people cannot smell anything at all.

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"We did a study because we really wanted to capture what was happening in Ireland, because you Googled Ireland and Covid and smell loss, nothing came up. We wanted to get a sense of what type of smell loss that people were suffering. You can have something called anosmia, which is where all sense of smell is gone, and you can have hyposmia, which is a reduced sense of smell. You might walk in and say, 'geez, why can't I smell... That is not as strong as it was before.'

"Then you can have something called parosmia, which is very prevalent in Covid-19 and this is basically an altered or distorted sense of smell and unfortunately it's often associated with unpleasant smell. There's certain food products like coffee, green peppers, chicken, onion, and garlic, they are the main foods that can trigger parosmia. We'd tons of reports of someone's morning coffee smelling like sewage, or rubbish, or something like that.

It's almost like physiotherapy for your nose

"Then, there's phantosmia, which is phantom smells. This is where you can walk into a room and, say, smell burning cigarette smoke. When we ran this survey in the initial weeks of the pandemic, we asked people who were suffering from this, 'how has it impacted your daily life?' and we had so many comments from people saying it was so emotionally distressing, that they would basically have a permanent smell of cigarette smoke up their nose. They had never smoked in their lives. They couldn't sleep with it. And that onions and garlic just tasted so revolting, or smelled so revolting that they still, even as their sense of smell has come back, they still can't eat them."

Crofton then talked about one of the recommended treatments which is smell training. "It's almost like physiotherapy for your nose", she said."Basically, you can purchase these very strong smelling substances like rose or say lemon, for instance, and you retrain. You smell them about three or four times a day, and you try to just basically relearn and say, 'Well, I'm smelling lemon. I'll smell it. I can't smell it, but I know what it should smell like.' You're trying to regenerate, or respark, or rewire your neuro pathways."