Mosquitoes locate bloodhosts by scent, sight and heat. From 100 feet away (30 meters) mosquitoes can smell your scent, especially the carbon dioxide (CO2) you exhale. Biting flies are even better; they can smell their prey from 300 feet (100 meters) away. Because CO2 is present in the atmosphere (plants take in CO2 and give off oxygen), mosquitoes respond to higher-than-normal concentrations, especially when the CO2 is mixed with host-odor. They follow your scent upwind, and can see you at a distance of about 30 feet (10 meters).