By Daniel Leonard 

As you may have noticed, mosquitoes don’t attack everyone equally. Scientists have known that the pests are drawn to people at varying rates, but they have struggled to explain what makes certain people “mosquito magnets” while others get off bite-free.

In a new paper published on October 18, researchers suggest that certain body odors are the deciding factor. Every person has a unique scent profile made up of different chemical compounds, and the researchers found that mosquitoes were most drawn to people whose skin produces high levels of carboxylic acids. Additionally, the researchers found that peoples’ attractiveness to mosquitoes remained steady over time, regardless of changes in diet or grooming habits.

“The question of why some people are more attractive to mosquitoes than others—that’s the question that everybody asks you,” says study co-author Leslie Vosshall, a neurobiologist and mosquito expert at the Howard Hughes Medical Institute and the Rockefeller University. “My mother, my sister, people in the street, my colleagues—everybody wants to know.” That public interest is what drove Vosshall and her colleagues to design this study, she says.

Scientists have put forth some theories to explain why mosquitoes swarm to some of us more than others, including one idea that differences in blood type must be to blame. Evidence is weak for this link, however, Vosshall says. Over time, researchers began to coalesce around the theory that body odor must be a primary culprit in mosquito attraction. But scientists have been unable to confirm which specific odors mosquitoes prefer.

To answer this question, Vosshall and her colleagues gathered 64 participants and had them wear nylon stockings on their arms. After six hours, the nylons were imbued with each person’s unique smell. “Those nylons would not have a smell to me or, I think, to anyone really,” says Maria Elena De Obaldia, a senior scientist at the biotech company Kingdom Supercultures and lead author of this new study, which she conducted while at Rockefeller. Still, the stockings were certainly odorous enough to entice mosquitoes.

The researchers cut the nylons into pieces and placed two (from different participants) into a closed container housing female Aedes aegypti mosquitoes. Did they migrate to subject number one’s sample en masse or prefer the scent of subject number two’s? Or were both equally appealing? The researchers continued these head-to-head battles over several months, Vosshall says, collecting new samples from the participants as needed. When the tournament was over, the team had clear proof that some people were more attractive than others. Subject 33 had the dubious honor of being the biggest mosquito magnet; they had an attractiveness score “over 100 times greater” than that of the least attractive subjects, 19 and 28, the study authors wrote.

The researchers analyzed the subjects’ scent profiles to see what might account for this vast difference. They found a pattern: the most attractive subjects tended to produce greater levels of carboxylic acids from their skin while the least attractive subjects produced much less.

Carboxylic acids are commonplace organic compounds. Humans produce them in our sebum, which is the oily layer that coats our skin; there, the acids help to keep our skin moisturized and protected, Vosshall says. Humans release carboxylic acids at much higher levels than most animals, De Obaldia adds, though the amount varies from person to person. The new study had too few participants to say what personal characteristics make someone more likely to produce high levels of carboxylic acids—and there’s no easy way to test your own skin’s carboxylic acid levels outside of the laboratory, Vosshall says. (She muses, however, that sending people skin swabs in the mail could make for an interesting citizen science project in the future.)

But we do know that skin maintains a relatively constant level of carboxylic acids over time. This, in turn, leads to a consistent odor profile. (Mosquitoes could also be attracted to skin bacteria digesting the carboxylic acids we produce, Vosshall suggests.) When Vosshall and De Obaldia ran their tournament multiple times several months apart, they found that people’s attractiveness rankings remained largely the same. Any personal factors that may have changed over those months—from what each subject ate to the kind of soap they used—didn’t seem to make a difference.

“This property of being a mosquito magnet sticks with you for your whole life—which is either good news or bad news, depending on who you are,” Vosshall says.

“This study confirms, in a very careful way, that it is true that some people are more attractive [to mosquitoes] than others,” says Omar Akbari, a cell and molecular biologist at the University of California, San Diego, who was not involved with the study but whose recent work focuses on mosquitoes. He adds that the study’s identification of specific carboxylic acids as a key determinant of mosquito attraction is a new contribution to biologists’ understanding of the insects’ behavior. Akbari suspects that the results of this study—which focused on A. aegypti mosquitoes—are probably generalizable to other species of mosquitoes that also primarily prey on humans.

But if you think you might be a mosquito magnet, all hope is not lost. Akbari says the study could help researchers develop more effective mosquito repellents in the future. The secret might be in adding new bacteria to the skin’s existing microbiome to alter its scent profile.

Akbari is working on a Department of Defense–funded project called ReVector that seeks to develop repellents that can be applied once and remain active for multiple months. “The idea is taking human-colonizing skin bacteria ... and engineering them in such a way that they can either express a repellent compound or be able to degrade something that’s attractive,” Akbari says. Now that Vosshall and De Obaldia’s team has identified specific carboxylic acids that might be powerful attractors, researchers may try to engineer bacteria-based skin creams that are specifically targeted to break down those compounds, Akbari says.

Still, one question remains: Why do mosquitoes love the smell of carboxylic acids or their attendant bacteria so much? De Obaldia has an answer for that—but it’s a bit speculative, she admits. She notes that A. aegypti mosquitoes evolved to prey specifically on humans (possibly because we often have containers of clean water nearby, which is a perfect breeding place for them). Thus, A. aegypti became extremely adept at differentiating the smell of humans from the smell of other animals. Carboxylic acids are compounds that humans emit in spades, while other animals do not. So, De Obaldia says, mosquitoes probably grew to love carboxylic acids because they’re an excellent indicator that the insects have located a human.

If you find yourself covered in more bites than all of your friends, you can take solace in the idea that mosquitoes love your scent profile because it’s so distinctly human.