Mosquitoes are highly effective at locating a blood meal, a behavior driven exclusively by the female for the protein needed to develop her eggs. This host-seeking process is a complex, multi-stage operation involving the sequential detection of several different sensory cues. The mosquito tracks a precise sensory gradient, transitioning from long-range chemical detection to short-range visual and thermal targeting. Understanding this attraction mechanism reveals why some individuals are consistently more appealing targets than others.
The Primary Draw: Carbon Dioxide
The initial signal a mosquito uses to detect a potential host is the plume of carbon dioxide (CO2) exhaled with every breath. This gas, a universal sign of a metabolically active organism, serves as the primary long-range attractant. Mosquitoes are equipped with specialized receptors on their maxillary palps, located between their antennae, that allow them to sense minute changes in CO2 concentration.
This sensitivity enables some species to detect a host’s breath from up to 70 meters away, though the effective range varies based on wind conditions. Upon detecting the CO2 plume, the mosquito enters an activated state, flying upwind and casting crosswind to track the concentration gradient.
Chemical Signatures from Skin and Sweat
Once activated by CO2, the mosquito seeks secondary chemical compounds unique to human odor. The skin’s surface is coated in volatile organic compounds (VOCs) produced when resident bacteria metabolize chemicals found in sweat and sebum. This process creates a personalized chemical signature that helps the mosquito confirm it has found a suitable host.
One of the most potent short-range attractants is L-(+)-lactic acid, a substance produced in large quantities by certain skin bacteria and found in sweat. Other attractive compounds include ammonia, carboxylic acids, and octenol. Research indicates that the composition of a person’s skin microbiome directly influences their attractiveness; individuals with a lower diversity but higher abundance of certain bacteria tend to be more appealing.
Compounds like octenol and acetic acid may not be strongly attractive alone, but they significantly enhance the mosquito’s response when combined with lactic acid. This synergistic effect shows that the full blend of human-derived volatiles is more attractive than any single compound.
Visual and Thermal Targets
As the mosquito draws closer, the search transitions from chemical signals to physical cues that guide the final approach and landing. The detection of CO2 activates a strong attraction to visual features, causing the mosquito to look for a physical target. Humans become visible to a mosquito at a distance of approximately 5 to 15 meters, depending on the conditions.
Mosquitoes show a preference for dark, high-contrast objects, particularly colors like black, navy blue, and red. The final, most precise cue for locating a feeding site is thermal detection, effective at a very close range, up to about 70 centimeters. The insect senses the infrared radiation (IR) emitted by the host’s body heat using specialized neurons located at the tips of its antennae. This heat signature, which matches human skin temperature, provides the final confirmation of a viable blood source before the mosquito lands.
The Role of Genetics and Metabolism
A person’s genetics and current metabolic state significantly influence the production of attractants. Genetic factors account for a substantial portion of an individual’s attractiveness, influencing the output of specific VOCs and the composition of the skin microbiome. This biological programming affects the quantity of chemicals like lactic acid produced by the body.
Certain metabolic states naturally increase the release of primary attractants, making a person a more prominent target. Pregnant women, for example, have a higher metabolic rate and exhale a greater volume of carbon dioxide, sometimes up to 21% more than non-pregnant women. They also have a slightly elevated body temperature, which enhances the short-range thermal signal.
The concept of blood type preference is often discussed, with some studies suggesting individuals with Type O blood may be more attractive than those with Type A. Furthermore, about 85% of people are “secretors” who emit a biochemical signal through their skin identifying their blood type, and mosquitoes show a greater attraction to these secretors regardless of the specific blood type.