Long before they land, mosquitoes are reading our body signals from afar. Find out if you feature high on the bloodsuckers’ hit list.
I am a mosquito magnet.
No matter where in the world I go on summer holiday, one thing is certain: I will inevitably be bitten by mosquitoes. Massive, itchy welts that plague me for weeks.
Meanwhile, others who are with me don’t suffer at all. Not a single bite. And those that are bitten, are often left with just a tiny red dot. My friends have long joked that my blood must be “alluringly sweet”.
It turns out they may be right. Our bodies exude numerous biological markers – including breath and body odour – which determine an individual’s susceptibility to bites. For some people, those markers are irresistibly strong.
Only female mosquitoes bite humans. They’re drawn to our blood for the protein it provides for their egg development. They use visual and olfactory cues (sight and smell) to identify their targets from around 10 metres (33ft). Among these are the plumes of carbon dioxide (CO2) that we expel in our breath and skin.
Human breath sends out a CO2 signal to mosquitoes which activates “host-seeking behaviour” in their olfactory organs. Adult humans are thus more alluring than children as they produce more CO2.
But it means mosquitoes are drawn to non-human CO2 sources too, making dry ice and bottled CO2 useful tools for mosquito traps.
Studies show that mosquitoes are also drawn to heat and moisture (and that CO2 enhances this attraction to warmth).
Pregnant women are therefore twice as attractive to mosquitoes compared to non-pregnant women. This is because pregnancy increases metabolic demand and breathing volume, which results in greater heat and CO2 being exhaled.
“You’ve got a little furnace inside you; you’re hotter,” says Steve Lindsay, a professor of public health entomology at Durham University in the UK.
People who are exercising may be temporarily more attractive to mosquitoes, especially during and just after exertion, due to the increased metabolic demand which increases CO2 production and leaves them warmer and sweatier. Larger-bodied people, who typically produce more heat and exhale more CO2, may also attract mosquitoes.
As mosquitoes close in (and are less than 10m or 33ft away), they identify victims via a cascade of cues, including skin and breath odours. “It’s smell, essentially,” says Lindsay, that determines who a mosquito bites. “Small, highly volatile chemicals make the difference. Mosquitoes live in a chemical world.”
Together with other scientists, Lindsay has debunked the myth that people with “sweet blood” are more prone to being bitten and found instead that mosquitoes are drawn to our unique “skin scent”. The skin microbiome can break down carbohydrates, fatty acids, and peptides on the skin into volatile organic compounds (VOCs), which easily evaporate into the air and mosquitoes can differentiate. There are over 500 VOCs in our skin. (BBC)
Mosquitoes are already attracted to ammonia and lactic acid on our skin and the presence of the carboxylic acids makes this attraction even stronger.
Researchers at the Rockefeller University in the US analysed the skin odour of 64 people who wore nylon sleeves for six hours. The mosquitoes could choose between the nylon samples – which acted like a “scent collection device” – and revealed a clear preference for the scent of individuals with higher carboxylic acids.
The researchers computed an attractiveness score for each person and found that the highest score was 100 times higher than the lowest. These differences remained consistent for years, regardless of lifestyle changes. “Your relative attractiveness (to mosquitoes) is largely fixed,” says Lindsay.