Researchers from the University of Porto and the Max Planck Institute of Chemistry recruited 24 volunteers (12 men and 12 women) for the project.
They were hooked up to a mask which records the chemicals in their breath at the parts per trillion level and the participants were monitored with sensors geared specifically to identify signs of piqued sexual interest.
The researchers then played three 10-minute long videos to the willing volunteers. One was of a sporting event, one a horror movie clip and the other an erotic film.
They found that as soon as the erotic film was shown, there was a noticeable spike in three chemicals called indole, cresole and phenol. There was also a significant drop in the level of CO2 and another molecule called isoprene.
'Quite remarkable'
Prof Jonathan Williams, an atmospheric scientist who is more accustomed to recording levels of pollutants in the stratosphere or in the Amazon rainforest than unpicking the biological mysteries of sex, said these changes were seen "almost immediately".
"These chemical signals could be good indicators for sexual arousal in breath," he told the Telegraph.
"It's quite remarkable because it seems to switch on and off really quickly. Most hormonal things in the body tend to happen reasonably slowly but there must be something here which is happening quite quickly."
The results were more obvious for men than women, the researchers found, but this may be a result of the stimuli used and the small study size, with larger follow-ups needed to determine if there is in fact a gender divide.
Prof Williams said that the "golden question" thrown up by this research is whether or not the signals play a role in human courtship.
'Golden question' outstanding
"What we've established here is that there definitely is a specific signal broadcasted," he said.
"We know that the human brain can respond to a chemical signal even if it's below our smelling threshold. You can be exposed to a chemical and you won't smell it, but your brain will react to it.
"We've established that chemicals are broadcast but does it have an effect on someone else? We don't know yet, but at least now we know which chemicals to test.
"We see the person we are kissing, we hear them, we feel them, but is there also a chemical communication going on invisibly between us? That is indeed the golden question."
It is a question the researchers will now try and answer with fresh studies building on the new findings.
"A neuroscientist with brain scanning equipment could expose people to these chemicals and see if there's a strong reaction or what part of the brain reacts to the stimuli. That would be a nice follow up," Prof Williams said.
The scientists say that the findings could revolutionise how sexual arousal is investigated by scientists going forward.
Very few studies 'touch the taboo of sexual arousal'
Currently, invasive apparatus means many people are unwilling to take part in such studies and being able to detect arousal in a more palatable way via breath, instead of with a penile gauge or vaginal probe, may help reduce hesitation.
Prof Williams says his colleagues at the University of Porto's so-called SexLab are constantly working to overcome inherent societal reservations.
"It's difficult for them to map out human sexuality, something as simple as that, because it's taboo. Culturally, the topic is still taboo. Therefore, research funds are conservative in nature," he said.
"There are hundreds of thousands of studies on fear, but very few have dared to touch the taboo of sexual arousal despite the fact is part of everyone's lives to some extent. It's a reflection on society a little bit."
The findings are published in the journal Scientific Reports.
