Sweat
People who workout outside are big targets for mosquitoes because when people sweat, there are substances that get extracted, such as lactic acid, uric acid, and even ammonia which help mosquitoes become attracted to them.
Carbon Dioxide
Mosquitos can smell you away from up to 50 meters and people who emit large quantities of carbon dioxide are more prone to get bit. This includes large people and even pregnant women who produce 21% more carbon dioxide than others. This explains why small children get bit the least.

How to Avoid Mosquitoes
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