Mosquitoes attracted by carbon dioxide?

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Stephen Tashi
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Summary:

Are the localized concentrations of CO2 around people sufficient to attract mosquitoes?
It's commonly said that mosquitoes are attracted by carbon dioxide - so I'll guess that there's empirical evidence for that assertion. But is the mechanism for this known? What is the localized concentration of carbon dioxide around a person respiring at a normal rate? Do we know that it's the higher carbon dioxide concentration that attracts mosquitoes - or could the detection be based on the lower concentration of some other gas that is being displaced by the CO2?
 
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Ygggdrasil
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You are correct that while CO2 plays a role in mosquito's abilities to sense people, other gasses play roles in ways that complement the role of CO2. For example, see this paper:

Dekker Geier & Carde. Carbon dioxide instantly sensitizes female yellow fever mosquitoes to human skin odours. J. Exp. Biol 208: 2963 (2005)
https://jeb.biologists.org/content/208/15/2963
Abstract:
Female mosquitoes are noted for their ability to use odours to locate a host for a blood meal. Two sensory organs contribute to their sense of smell: the maxillary palps, which measure the level of CO2, and the antennae, which detect other host-released odours. To establish the relative importance and interactions of CO2 and other body emissions in freely flying mosquitoes, we presented female yellow fever mosquitoes Aedes aegypti L. with broad plumes of human skin odour and CO2 at natural concentrations and dilutions thereof in a wind tunnel. 3-D video-recorded flight tracks were reconstructed. Activation, flight velocity, upwind turning and source finding waned quickly as skin odours were diluted, whereas in the presence of CO2 these parameters remained unchanged over more than a 100-fold dilution from exhaled concentrations. Although mosquitoes were behaviourally less sensitive to skin odours than to CO2, their sensitivity to skin odours increased transiently by at least fivefold immediately following a brief encounter with a filament of CO2. This sensitization was reflected in flight velocity, track angle, turning rate upon entering and exiting the broad odour plume and, ultimately, in the source-finding rate. In Ae. aegypti, CO2 thus functions as a `releaser' for a higher sensitivity and responsiveness to skin odours. The initially low responsiveness of mosquitoes to skin odours, their high sensitivity to CO2, and the sensitization of the olfactory circuitry by CO2 are ecologically relevant, because rapidly fluctuating CO2 levels reliably signal a potential host. Possible mechanisms of the instantaneous sensitization are considered.
 
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