When a smell is associated with a threat, our sensitivity to it can also increase dramatically. In one study, Lundström and his colleagues showed that pairing a smell with an electric shock can make people detect that odour at much lower concentrations – a response that likely evolved to help humans react quickly to potential dangers, even when the smells are only faint. Similarly, the rotten eggs scent of hydrogen sulfide, a gas produced in sewage processing, can be perceived in concentrations as low as 0.5 parts per billion – an alarm bell for a gas that is lethal in higher concentrations.

— Read more in How bad smells affect your health at BBC.

This is what’s known as a human emotional chemosignal: a subtle chemical cue that lives in sweat that carries information about an individual’s emotional state. According to a foundational 2020 study published in Philosophical Transactions of the Royal Society B, fear can actually change the chemical composition of sweat, and in measurable ways, at that.

— Read more in A Biologist Explains How The Human Brain Evolved To ‘Smell Fear’ at Forbes.

The research shows that the volume of this endocast closely tracks the number of intact odor-receptor genes an animal carries.

Bigger bulbs point to richer gene repertoires – and stronger sense of smell. That tight anatomical-genetic link opens a reliable window into the olfactory abilities of mammals across deep time.

— Read more in Fossils reveal a powerful sense of smell in ancient mammals at Earth.com.

Pine contains compounds called terpenes, like α-pinene and limonene, which are touted in wellness circles for their stress-reducing and mood-boosting properties. Aromatherapy brands frequently promote pine essential oils with terms like “refreshing” and “uplifting,” suggesting there’s something inherently calming in the chemistry of the tree itself.

But that idea doesn’t hold up scientifically, according to [neuroscientist Dr. Rachel] Herz. She says the perception of scent occurs in the amygdala-hippocampal complex, otherwise known as the primary olfactory cortex. It is this mechanism that drives pleasant experiences around particular scents.

— Read more in Why smelling a pine tree can instantly lift your mood at Popular Science.