What Makes Smells Good or Bad?

 by Sabiha S

Why are some smells good and others bad? This question was raised whilst I was talking to my friend who has no sense of smell, wondering how it was possible to differentiate between smells. Though the answer might seem simple, there are actually many factors influencing our perception of smell, helping us find a difference between the sickening odour of mould and the fresh scent of flowers.

Being exposed to many smells, humans can detect and analyse more than 1000 billion different odours, so naturally different people react diversely to the same scents. This is largely dependant on our olfactory system which works to activate neurons so it can send information to the brain. However, it was found that 30% of these receptors are different in each person, radically altering our perception to certain odours and so explaining why we receive smells at varying intensities. This is also created by the interaction of scent molecules with our brain. Whilst inhaling air, the dissolved scent molecules can react with smell receptors on nerve endings in the olfactory epithelium found near the top of our nose, signalling to the olfactory bulb in our brain. As this is directly connected to the amygdala and hippocampus (related to emotion and memory), it is quite easy, and perhaps inevitable, that scents are quite inextricably linked to the emotions and memories created, thus perceived differently by all those who experience it.

Moreover, smell receptors are unique as they are created by specific genes and bind to specific classes of scent-creating molecules. Sometimes, people may have the genetic condition ‘specific anosmia’ where the gene for a receptor isn’t expressed, causing their inability to perceive certain smells, in turn affecting what they recognise to be good and bad smells.

Though it is arguable that our sensitivity to smells increases with bad experiences, some scents remain innately abhorrent, such as cadaverine and putrescine which are produced by decaying corpses, producing a revolting smell even if you have never smelt a dead body before. This is shared by many animals and has evolved at least 420 million years ago.

A part of why we experience smells differently could also be due to evolution. Humans have naturally evolved a higher sensitivity to odours that indicates poison or danger, being able to distinguish this from the good odours – important signals to look out for when animals are in search for food or a mating partner. For example, our body has evolved to warn us from food that are potentially dangerous such as rotten fish, which would make us ill as it is packed with bacteria, resulting in the disgusting smell.

However, our perception of smell is not entirely due to scientific factors but could be due to cultural interpretation. If you were with a baby, you would find that they have no positive or negative interpretations of the world, as it depends on their education and how they learn from their surroundings. This is a clear case in France, where they find cheese having a cheesier smell to be better, while this would be questionable in other countries.

In the end, the smells you take in are neither good or bad. It’s just information. We may have evolved to smell things because the information is useful to us and due to the nature of the olfactory system, or it could be majorly influenced by our culture and surroundings. Either way, how we perceive smell is different to one another and is always susceptible to change, making our experience of the world unique.