If you’ve ever been to an outdoor party or barbecue after dark and haven’t personally noticed an increase in mosquitoes, you’ll probably hear someone complaining about it. This is because mosquitoes are selective insects and some people are more likely to be bitten than others.
Several factors could explain why this happens: In a controlled study in the Journal of Medical Entomology, microbes landed on people with blood type O almost twice as frequently as those with type A. The researchers noted that this was due to the secretions we produce, which tell mosquitoes what blood type a person has.
More research needs to be done on mosquitoes’ potential preference for certain blood types over others, said Jonathan F. Day, a professor of entomology at the University of Florida. But he recognized that mosquitoes pick up on certain signals we give off that make them more likely to land on certain people.
“These signals let them know they’re heading toward a blood source,” Day said. “CO2 is perhaps the most important. The amount of CO2 you produce, such as in people with a high metabolic rate (genetic or other factors), increases the amount of carbon dioxide you give off. The more you give off, the more attractive you are to these arthropods.
But what makes us different from non-living entities that emit carbon dioxide, like cars? Mosquitoes look for primary signals in conjunction with what Day calls “secondary signals.”
Lactic acid – the substance that causes muscle cramps during exercise – is one such secondary signal, for example. Lactic acid is released through the skin, signaling to mosquitoes that we are a target, Day said.
Mosquitoes also have other qualities that help them detect secondary signals.
“Mosquitoes have excellent vision, but they fly low to the ground to stay out of the wind,” Day said. “They are able to contrast you against the horizon, so how you are dressed matters. If you wear dark clothes you will be more attractive because you will stand out from the horizon, while those who wear light colors will not be as much.
A mosquito also picks up “tactile signals” once it lands on you.
“Body heat is a very important tactile signal,” Day said. “It comes into play with genetic differences or physiological differences. Some people tend to run a little hotter: when they land, they look for a place where the blood is close to the skin. This means that those with slightly higher temperatures are more likely to be bitten.
Lifestyle or other health factors may also play a role, said Melissa Piliang, a dermatologist at the Cleveland Clinic.
“If your body temperature is higher, if you exercise and move around a lot, or if you drink alcohol, you are more attractive to mosquitoes,” Piliang said. “Being pregnant or being overweight also increases metabolic rate.”
One study showed that people who drank just one can of beer were more likely to attract mosquitoes than those who didn’t. Of course, drinking outside is a popular activity in the summer and fall.
“If you’re moving around all day doing yard work and then you stop at dusk and have a beer on your patio, you’re definitely at risk for a bite,” Piliang said.
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How to prevent mosquito bites
Just because you’re more prone to bites doesn’t mean they have to be inevitable.
“One of the best things you can do is avoid periods of peak activity (for mosquitoes),” Day said. “There are very, very few species active in the middle of the day. They are very selective. Sunrise and sunset are when you will see maximum activity. Switching from your morning run to an after-work run might help.
Of course, this tip won’t help you if, for example, you’re hosting a barbecue for friends later that night. Try to cover as much skin as possible in these cases, Day said, especially in areas or times when mosquitoes are most likely to be present.
“I love fishing shirts and long-legged outdoor pants that are breathable, but keep out mosquitoes,” Day said. “A repellent that has a good duration of protection – defined as the time from when you apply it to when you receive the first sting – is also excellent. Sprays of around 5% DEET give you 90 minutes of full coverage.
DEET is a common ingredient in insect repellents, and sprays containing DEET are probably the way to go if you know you’re at risk of a sting, Piliang said. Despite controversy over the health effects of DEET, a 2014 study by the Environmental Protection Agency concluded that normal use of DEET products does not present a health risk, including that of children, women pregnant and breastfeeding women.
“DEET is the most effective,” Piliang said. “If you’re going to be in a place that’s prone to mosquitoes, knowing that they carry disease, this is your best option. Take a shower later to wash it off and put on just a little bit. Always read the directions on a spray before using it and help children apply products following guidelines from the U.S. Centers for Disease Control and Prevention.
And while you may have heard that candles containing natural repellents like citronella oil can be helpful, there’s still no research to confirm this. Instead, leave the fan on or hang out in a ventilated area.
“Mosquitoes can’t move around much,” especially in windy conditions, Piliang said. “You can run a fan to keep the air moving.”
If you end up with a bite, it may or may not be bothered. “It all depends on how allergic you are to the chemicals in the mosquito’s saliva, and this can vary depending on the type of mosquito or how reactive you and your skin are to elements in the environment,” Piliang said .
If it itches, the worst thing you can do is scratch it.
“If you do this, more histamine is released and the itching gets worse,” she said. “If you scratch it, you’re also more likely to break your skin. You may experience bleeding, scabbing, and put yourself at risk of infection and scarring.
But there are a few simple things you can do to relieve the itch, like putting an ice cube on it. “The cold sensation travels on the same nerve as the itch, so you can’t feel both at the same time,” Piliang said. “Even a drink with ice will help relieve the itch immediately.”
If you’ve received multiple bites after a morning or evening out, she also recommended over-the-counter anti-itch creams with a mild topical steroid like hydrocortisone. “You can apply it two to three times a day to reduce the itching,” she said.
“And the last thing you can do if you’re really bitten is take an antihistamine,” she added. “That may counteract the reaction a little bit.” Although over-the-counter types that make you drowsy — like Benadryl — are more common, you can take non-drowsy antihistamines like Zyrtec or Allegra for daytime relief.
Of course, prevention is always better than cure, so use these tips to avoid bites in the first place as you head to the season’s remaining barbecues and tailgates.