WASHINGTON - Mosquitoes abound in the hot season.

But a lucky few people seem immune to the bites of the pesky insects, while others cannot seem to avoid them.

New research explains the insects' apparent selectivity. According to an article in Smithsonian magazine, an estimated 20 per cent of people are "especially delicious" to mosquitoes. They are bitten more often than others.

Blood type, metabolism, exercise, shirt colour and even drinking beer could play a role, scientists believe.

Here's a look at some of the factors considered:

• Blood type

According to one study, mosquitoes landed on people with Type O blood nearly twice as often as those with Type A. People with Type B blood fell somewhere in the middle of this itchy spectrum.

• Carbon dioxide

The smell of carbon dioxide emitted in the breath is another way mosquitoes locate their target, even if he is 50m away. As a result, people who simply exhale more of the gas over time have been shown to attract more mosquitoes. Larger people tend to exhale more of the gas and scientists believe this could explain why adults tend to get bitten more often than children, on the whole, according to the report.

• Exercise and metabolism

Mosquitoes also find victims at closer range by smelling the lactic acid, uric acid, ammonia and other substances expelled via their sweat.

Strenuous exercise increases the build-up of lactic acid and heat in the body.

• Skin bacteria

A 2011 study found that certain types of bacteria on skin make the individual more appealing to mosquitoes, the Smithsonian magazine report said.

Surprisingly, though, having lots of bacteria but spread among a greater diversity of different species of bacteria seemed to make skin less attractive.

• Beer

Another study found that just a single 350ml bottle of beer can make an individual more attractive to the insects.

• Pregnancy

In several different studies, pregnant women have been found to attract roughly twice as many mosquito bites as others, likely a result of the unfortunate confluence of two factors: They exhale about 21 per cent more carbon dioxide and are on average about 0.7 deg C warmer than others.

Some of the other factors considered were clothing colour and genetics.

Besides scent, mosquitoes also use vision to locate humans, so wearing colours such as black, dark blue or red tend to make an individual stand out, according to medical entomologist James Day, at the University of Florida.

Alongside, underlying genetic factors could account for 85 per cent of the variability between people in their appeal to mosquitoes.

And some researchers have started looking at the reasons why a minority of people seem to rarely attract mosquitoes in the hopes of creating the next generation of insect repellents, the report said.