In people, the repulsive reaction is usually an indicator of danger.

Spoiled food, disease, rotting, and humidity invariably has a negative impact on our health and the smells associated with that can be labeled as bad or unpleasant.

Bad odors like the smells of fish or rats, have become popular phrases in our everyday language such as: something smells fishy which means something isn’t right and similar to this one it smells like there is rat around here is that something or someone isn’t right and it might be a signal for danger or something that has to be revealed.

Other scents that were perceived by the primitive man,

for example, the smell of an opponent or a deadly animal, burning tree etc. were meant to be interpreted very carefully. We aren’t born with the instinctive negative reaction to a burning tree, even though the smell might be an indication of danger for us, our ancestors associated this more often with lunch or warmth, therefore this category of smells isn’t automatically accepted as negative.

In the world of insects the smells of chemicals mean something specific and they act as a means of communication.

Isoamyl acetate is a type of aromatic chemical with a strong fruity scent and is used in pear essence and some perfumes.

In its natural state, it can be found in fruits like apples and bananas. The same chemical is being secreted by some types of bees when they are under a wasp attack it is also a signal to other bees to get outside of their hive and help them against the wasp attack. For the human mind, this smell indicated apple or pear, while for bees it means “danger, get out or come and fight.”
According to Francis Bacon, the plague was accompanied by a sweet aroma, similar to that of a freshly ripen apple. It is said that a number of other diseases are also accompanied by specific smells. In traditional Chinese acupuncture, one of the forms of diagnosing Is includes smelling the hidden aroma of the patient (not the physical one from the flesh).

Unpleasant aromas were used as a form of therapy through provoking aversion in the patient.

It is proven that some smells like the one of Sulfur can suppress the need for smoking.
And in two children that were hospitalized ammonia was used as a way of preventing a certain behavior which led to self-harm.
This type of negative aromatherapy is not a part of aromatherapy, in fact, there isn’t an essential oil that is unbearable to a large extent. After all the success of a such a type of conditionality shows us the way that scents are able to impact behavioral reactions.