Presenting the volunteers with each odor a number of times, the researchers measured the reactivity of the olfactory bulb noninvasively using electroencephalography.

The presence of both of these waves near our nasal neurons implies the very moment a fragrance tickles our olfactory bulb, it's preparing other parts of the brain – such as our motor cortex – to get cracking.

Putting the data together, it's clear our olfactory bulb processes pleasant and threatening odors at different speeds.

Within 250 milliseconds of an odor arriving within the nose, the two different brain waves are 'coupling' to coordinate a response.

If the smell is deemed a threat, a signal is sent earlier, taking around 150 milliseconds to hit the motor cortex.

"It was clear that the bulb reacts specifically and rapidly to negative smells and sends a direct signal to the motor cortex within about 300 milliseconds," says Johan Lundström, a biologist at the Karolinska Institute's Department of Clinical Neuroscience.