In Japan, many people say that mosquitoes like people with blood type O. A Japanese study found that more mosquitoes landed on type O people. But other studies did not find this difference.

So the idea is not clear. Mosquitoes find people mainly by the carbon dioxide we breathe out, our body heat, and the smell of our skin. Your smell may be more important than your blood type.

Many Japanese people believe that mosquitoes prefer people with blood type O. This idea is actually based on research. In a 2004 study in Japan, a common type of mosquito landed more often on the arms of volunteers with type O blood, especially compared with type A. However, other studies have found no such difference, and a study of malaria mosquitoes in Africa found that sweating mattered more than blood type.

So what really attracts mosquitoes? They first notice the carbon dioxide we breathe out, then sense our body heat, and finally use skin odor to choose. In a 2022 study, the people whom mosquitoes liked most stayed attractive for several years, and their skin released more of certain acids.

The claim that mosquitoes favor people with blood type O is often repeated in Japan, and it does have some scientific basis. In a 2004 Japanese study involving 64 landing trials, Asian tiger mosquitoes landed more often on volunteers with type O blood, with the clearest difference against type A; differences from types B and AB, however, were not statistically clear. The evidence as a whole is inconsistent. A 1970s study of African malaria mosquitoes found no effect of blood type but a strong attraction to sweating individuals, and some studies have even favored other blood groups.

A review of feeding studies has pointed out that methods such as live arms and artificial blood each have strengths and weaknesses, so incomplete evidence can be read in conflicting ways. By contrast, the cues mosquitoes use to locate hosts are well established: exhaled carbon dioxide, body heat and, at close range, skin odor. In a 2022 study using nylon stockings worn on participants' arms, attraction varied enormously between people and remained stable for several years, with the most attractive individuals producing more skin-derived carboxylic acids. Even so, this was an association, and the causal role of individual compounds still needs testing.