Just one sneeze can infect a room with flu virus for hours

Researchers at Virginia Tech in the US found that the microscopic infected droplets emitted in a cough or sneeze float around the air in large enough concentrations to spread disease.

Breathing in airborne specks of virus found in a typical office, hospital, plane or train could infect a person after just one hour, the Daily Mail reported.

Flu passes from person to person through direct physical contact, or when someone sneezes or coughs. Most studies have concentrated on large droplets, known as aerosols, that carry the virus in the air but which quickly fall to the floor and nearby surfaces.

Few studies have looked at the threat from the smaller droplets, which can remain airborne for hours or even days.

In the latest study, the 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.

The scientists found that a typical cubic metre of air contained an average of 16,000 particles of flu virus. Most were less than 2.5thousandths 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 Dr Linsey Marr, who led the study.

"The virus-laden aerosols are small enough that the smallest ones can remain suspended for days," she said.

"In this case, they are removed by other mechanisms such as ventilation, usually within one hour, from a building.

"An aircraft or office air-conditioning system does help remove the viruses by removing old air and bringing in fresh air."

A typical cough shoots out jets of air several feet long, along with around 3,000 droplets of saliva at speeds of up to 50mph. Sneezes typically contain as many as 40,000 droplets, some which leave the body at more than 100mph.

The new findings, published in the Journal of the Royal Society Interface, give a ring of truth to the old advice that you should never visit a doctor`s waiting room unless you want to get sick, the researchers said.