A single sneeze infests a room with deadly bugs and the contamination can last for hours.
Microscopic droplets sneezed or coughed out float around the air in large enough concentrations to spread disease, researchers say.
A sneeze typically contains as many as 40,000 droplets, some of which leave the body at more than 160 kmph.
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Breathing in airborne specks of virus found in a typical office, plane or train could infect a person after just one hour, the Journal of the Royal Society Interface reports.
It also highlights why so many holidaymakers pick up coughs, colds and sniffles at the start of their trip, following a flight, according to the Daily Mail.
Flu passes from person to person through direct physical contact, or when someone sneezes or coughs.
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U.S. researchers collected samples of air from the waiting room of a healthcare clinic, three rooms in a nursery and three cross-country flights. Half the samples contained small droplets containing the flu virus.
Scientists found that a typical cubic metre of air contained an average of 16,000 particles of flu virus. Most were less than 2.5 thousandths of a millimetre across, which remain suspended in the air for hours on end.
“Given these concentrations, the amount of viruses a person would inhale over one hour would be adequate to induce infection,” said Linsey Marr, who led the study at Virginia Tech.
“The virus-laden aerosols are small enough that the smallest ones can remain suspended for days,” she added.