How do air fresheners really work?
The effect of air fresheners is based on cheating the sense of smell. Our nose can be cheated to ignore foul-smelling substances, if it is given extra stimulation by dispersing aromatic scents into the air. The technique is the same as the ample use of perfumes in the French Court in the seventeenth century. Since baths were not appreciated, the alternative was to cover up the smell of body odour by masking it with perfumes.
Antagonising and covering smells
To antagonise smells one has tried to discover “pairs” in the sense that a certain odour can be blocked (antagonised) by another specific odour. Examples of such pairs are skatole (causing the unpleasant smell of faeces) and coumarin (a fragrance in several plants), and butyric acid (smell of rancid butter) and juniper oil. These substances have no effect on each other, and the foul-smelling partner exists in the air at exactly the same concentrations as before, but one odour prevents the nose from recognizing the other’s smell. The physiological mechanism behind this phenomenon is unknown, and therefore the finding of pairs is simply trial and error. If the substance used to antagonise the smell has its own clear odour, we speak of masking the smell. In practice, there is probably little difference between antagonising and masking the smell.
There are devices in industrial use for spreading the air fresheners, but in homes, sprays are mostly used to disperse the freshener into the air. Also gels or tablets with evaporating compounds may be purchased. The latter are often seen in toilets as toilet fresheners.
The substances may be terpenes, plant oils or compounds used as fragrances for other purposes, often containing alcohol, aldehyde or ester structures. In industrial environments, rather simple compounds may be used such as acetophenon, benzaldehyde or methyl benzoate. Sometimes also fake odours are created, e.g. plastic furniture may be purposefully made to smell of leather. In the U.S., second hand car dealers sometimes scent an old car with the “odour of a new car”.
Air fresheners should not be confused with personal deodorants or antiperspirants. Deodorants are antiseptic substances that are aimed at preventing microbial growth e.g. in the armpits to keep sebum and sweat from going rancid and producing an unpleasant smell. One historically used antiseptic compound was hexachlorophene, which is a neurotoxic compound and its use is limited to surgical washing and other special uses. Nowadays, deodorants often contain quaternary ammonium compounds. Antiperspirants are aluminium salts which prevent sweating by obstructing the ducts of sweat glands.
Air fresheners may be allergenic, and therefore proper hygiene is a better way of treating the odour problems of homes.
Air fresheners do not remove the odours, they only mask them. Therefore it is not reasonable to take any risks in their use.
One level up: The air that we breathe
Next chapter: Why is it worth investing in environmental protection?