Fears that the confined space of commercial airliners and recycled air in aircraft cabins help spread flu and other respiratory diseases among passengers are largely unfounded, a new study says.
  
"Commercial airlines are a suitable environment for the spread of pathogens carried by passengers and crew," according to the research, which appears tomorrow in the British medical weekly The Lancet.
  
But, it adds, "The environmental control system used in commercial aircraft seems to restrict the spread of airborne pathogens, and the perceived risk is greater than the actual risk."
  
The study appears against a backdrop of a surge in air travel, fears about the uncontrolled spread of new respiratory viruses such as SARS and bird flu, concerns about bioterror agents such as smallpox and widening scepticism among the travelling public about cabin ventilation.
  
But, say the authors, the best available knowledge suggests modern airliners are no greater a source of contagion than other confined spaces, such as offices.
  
Pressurised airliners draw their cabin air through the engines. The air is heated, compressed and cooled, and then passed into the cabin to be circulated by the ventilation system.
  
Most commercial aircraft in service recirculate 50% of the passenger cabin air, partly to save on fuel costs.
  
However, the potential for spreading viruses or microbes by this recycling is reduced by several factors, say the researchers, led by Mark Gendreau of the Department of Emergency Medicine at the Lahey Clinic Medical Centre in Burlington, Massachusetts.
  
The recirculated air usually passes through high-efficiency particulate filters which remove 99.97% of dust, vapours, bacteria and fungi, and effectively capture viral particles, because these are usually spread in droplets, expelled by someone who coughs or sneezes.
  
In addition, the design of ventilation systems limits the spread of airborne particles though the passenger cabin, according to Gendreau's team.
  
The air is changed far more often than a typical office building and is circulated in sections.
  
The airflow is from side to side, entering from overhead and leaving the cabin via the floor, which means that there is very little front-to-back flow - in other words, there is only a relatively minor risk that someone who sneezes at the front of the cabin may infect someone at the back.
  
Putting this to further test, Gendreau's team looked at past cases where investigators have probed suspected transmission of tuberculosis, Severe Acute Respiratory Syndrome (SARS) and influenza during a commercial flight.
  
In the case of TB - the most-studied of these pathogens - the only known major case occurred over five months in 1992, when an infected flight attendant passed on the bacteria to 212 fellow crew members and 59 frequent-flyer passengers.
  
Passengers who are infectious with TB are likely to be contagious for those seated within two rows, and on long flights, of more than eight hours, other probes found.
  
As for SARS, the pneumonia-like disease is believed to have been transmitted to 37 people on five flights during the 2003 scare. Most of the transmissions occurred within five rows of the SARS-infected passenger.
  
A worrying and still unexplained exception to this was on a Hong Kong to Beijing Air China flight on March 2, which showed a far wider transmission, up to seven rows in front and five rows behind.
  
In the case of flu, the infection risk was also mainly limited to two rows of a passenger with the virus, inquiries found.
  
Larger-scale on-board infections of flu have occurred when ventilation systems have failed to work properly or been turned off while the plane is on the ground with passengers on-board.
  
The study sounds a loud note of caution, saying the issue of on-board infection was still under-researched, and calls for tighter and universal regulations about ventilation hygiene.
  
At present, there is no regulation requiring carriers to use the high-efficiency particulate filters.
  
In addition, only 85% of commercial airliners that carry more than 100 passengers in the current US fleet and recirculate cabin air are equipped with them.
  
The authors also noted that, even if in-flight transmission was relatively low, jet travel could still quickly propagate a disease globally.
  
More than one billion passengers travel by year annually, and 50 million of these travel to the developing world, which is the usual source of new diseases.