The findings highlight the role that expectations can play in health-related outcomes, said researchers from the Monell Chemical Senses Center in the US.
"Asthmatics often are anxious about scents and fragrances.When we expect that an odour is harmful, our bodies react as if that odour is indeed harmful," said study lead author Cristina Jaen, a Monell physiologist.
Asthma is a chronic inflammatory disorder of the lungs. The airways of asthmatics are sensitive to 'triggers' that further inflame and constrict the airways, making it difficult to breathe.
The current research was conducted to determine whether odour-triggered asthma symptoms can be elicited or worsened by associated negative expectations.
In the study, 17 individuals characterised as moderate asthmatics were exposed to the odour phenylethyl alcohol (PEA) for 15 minutes.
Often described as rose-smelling, PEA is regarded as a 'pure' odourant with no associated physiological irritant qualities.
Eight subjects were told that the odour had potential therapeutic properties, while nine were told that it potentially could cause mild respiratory problems.
During odour presentation, the subjects rated the odour's sensory properties, including intensity, irritancy, and annoyance.
Measures of lung function and airway inflammation were collected before and immediately after exposure and again at two hours and 24 hours post-exposure.
Subjects' beliefs about the odour, specifically whether it was potentially harmful (asthma-triggering) or therapeutic, influenced both their psychological and physiological responses to odour exposure.
Individuals who were told that the odour was potentially harmful rated it as more irritating and annoying as compared to those who thought it might be therapeutic.
In addition, airway inflammation increased immediately following odour exposure in subjects who believed the odour might be harmful and remained elevated 24 hours later.
"Introducing a negative bias led to a rapid change in airway inflammation," said senior author Pamela Dalton, a cognitive psychologist at Monell.
"What really surprised us was that this response lasted for over 24 hours. The increased inflammation during this period likely makes asthmatics more sensitive to other triggers," Dalton said.
There was no increase of inflammation when the odour was characterised as therapeutic, even in individuals who described themselves as sensitive to perfumes and other odours.