Tiny droplets containing the flu virus in numbers high enough to infect you are more prevalent in the air of public places than you might think, a new study suggests.

Half of the air samples in the study, which were taken from high-traffic areas like day care centers and airplanes, yielded concentrations of flu virus high enough to infect someone with the flu, said study researcher Linsey Marr, an associate professor of civil and environmental engineering at Virginia Polytechnic Institute and State University.

And flu-laden particles can stay in the air long after an infected person has left the vicinity, meaning it's possible to be infected just by breathing in the infected air for an hour, she said.

A sick person doesn't "necessarily have to be sneezing continuously" to be contagious, Marr told MyHealthNewsDaily. Coughing, talking and exhaling all project flu particles into the air.

The new study will be published online Feb. 2 in the Journal of the Royal Society Interface.

Suspension in the air

Virginia Tech researchers collected 16 air samples from a health care center, two toddlers' rooms and the babies' area of a day care center, and three flights between Roanoke, Va., and San Francisco over a four-month period.

The researchers found influenza A virus particles in half the air samples. It's possible that no infected people were present in the vicinities of the other half of the samples.

In the infected samples, the average concentration of flu viruses was 15,000 viruses per cubic meter of air and the average size of a flu virus particle was less than 2.5 micrometers. That means the viruses could stay suspended in the air for hours, the study said.

However, not all flu-infected people shed flu particles capable of making others sick, Marr said.

A closer look at flu spread

Health care experts agree that the influenza virus can spread through direct and indirect contact, large respiratory droplets and aerosols left behind from evaporation of the large droplets.

Past research has suggested the influenza virus can survive up to two to three hours in a droplet form, but there has been debate over whether the droplets are able to stay suspended in the air long enough to spur infection . When droplets are big, gravity can pull them down so they don't remain airborne. And some droplets are quickly diluted to low concentrations by surrounding air, Marr said.

Although the study didn't count how many people got infected from breathing in the flu-particle-laden air of these public places, the levels of flu particles in the air are enough to get someone sick, she said.

Much is still unknown about the exact routes of influenza transmission, but this finding shows that the flu virus has the potential to stay in the air, said Marcel Salathe, an assistant professor at the Center for Infectious Disease Dynamics at Pennsylvania State University, who was not involved in the study.

"Every additional piece in the puzzle is welcome," Salathe told MyHealthNewsDaily.

Next, the researchers are developing a model to find the conditions that allow the easiest transmission of airborne flu indoors, Marr said. They are also studying whether variables such as temperature, humidity and ultraviolet radiation contribute to flu outbreaks hitting harder in certain areas, she said.

Pass it on: A cubic meter of air may contain 15,000 flu viruses. They stay in the air for hours and make you sick.

Follow MyHealthNewsDaily staff writer Amanda Chan on Twitter @AmandaLChan.