Flies buzzing around a person often feel like a personal attack, but this behavior has a purely scientific basis rooted in survival. Flies are not targeting people out of malice; they are following a sensory map that directs them to necessary resources like food, moisture, and potential breeding grounds. The human body acts as a beacon, emitting a complex array of signals that flies interpret as an opportunity to fulfill a biological need.
Chemical Signals Flies Detect
Flies primarily locate resources through olfaction, using specialized receptors on their antennae to detect airborne chemicals from a distance. Exhaled breath is a major component of this chemical plume, as it contains carbon dioxide (CO2). CO2 is a universal long-range signal indicating the presence of a large, warm-blooded animal, and the concentration gradient guides the fly toward its host.
Once closer, flies are drawn in by volatile organic compounds (VOCs) emanating from the skin. These compounds are produced primarily by bacteria living on the skin’s surface, which metabolize sweat and oils. Molecules like lactic acid and various carboxylic acids are byproducts of this microbial activity, signaling a rich source of potential nutrients. Sweat also attracts flies because it provides moisture and a source of salt, which flies seek to replenish their own concentrations.
Visual and Thermal Triggers
Beyond chemical signals, flies rely on non-chemical cues to finalize their approach. The human body generates a distinct thermal signature that serves as a short-range attractant, especially for species seeking warmth or blood meals. Flies have temperature-sensitive organs that detect body heat, confirming a host’s presence.
Movement is another strong visual trigger. Sudden or repeated motion can be interpreted by a fly as a potential food source or a location to investigate. While color preferences vary—some species are attracted to wavelengths like blue and white—the combination of movement and a large, warm mass is often enough to secure a fly’s attention.
Different Flies, Different Motivations
The specific reason a fly is interested in a person is tied to its species and immediate biological needs.
House Flies
House flies (Musca domestica) are common nuisances motivated purely by nutrition. They are scavengers attracted to decaying matter, but they readily land on human skin to consume the thin film of moisture, salt, dead skin cells, and oil. Since they cannot ingest solids, house flies often regurgitate digestive enzymes onto a food source to liquefy it before sucking it up.
Biting Flies
Biting flies, such as horse flies and stable flies, have a more urgent motivation: they require a blood meal for protein to produce eggs. These species are especially sensitive to the CO2 plume and the host’s thermal signature, which they use to locate a target from a considerable distance.
Fruit Flies
Fruit flies (Drosophila) are attracted by the scent of fermentation. This draws them to overripe fruits, spilled alcohol, and sugary residues. They may also be attracted to trace amounts of these substances on a person’s hands or face, or to fermentation byproducts found in some scented personal care products.
Minimizing Your Attractiveness
Reducing your appeal involves directly countering the chemical and thermal signals flies use to find you. Since body odor is a powerful short-range attractant, reducing microbial activity on your skin is an effective strategy. Showering immediately after sweating minimizes the time bacteria have to metabolize sweat and produce attractive volatile organic compounds.
Wearing light-colored clothing can reduce the visual contrast that attracts some fly species. Covering exposed skin also provides a physical barrier against those seeking a salt or blood meal. Eliminating sources of fermentation and decay is important; this means quickly wiping up sugar spills and ensuring trash receptacles are covered and clean. Using fans outdoors or near doorways can also disrupt the CO2 plume, making it harder for flies to locate you.