Harmful bacteria and less opportunity to wash your hands make flying a significant health risk, according to experts.
Professor Sally Bloomfield of the London School of Tropical Medicine and Hygiene, told Which? Travel: 'You're in an environment where you're more likely to catch something - in a closed space, surrounded by hundreds of people. On a large plane there's bound to be at least somebody who's ill.'
According to a Which? Travel survey, Ryanair is the dirtiest flight operator in the UK, with one in four passengers rating the airline's cleanliness as 'poor'.
Our travel health advice is based on research from our expert travel investigations team and surveys of thousands of Which? members. Find out more aboutWhich? Travel.
Video: How to stay safe while travelling
We boarded a Ryanair flight to find out why its passengers are unimpressed with the airline's level of cleanliness.
Health risks at your seat
One study of airline cleanliness found that almost half of all plane surfaces swabbed contained potentially harmful levels of bacteria, yeast or mould.
Research conducted last year by Marketplace, a Canadian TV show, on domestic North American flights found headrests contaminated with staphylococcus and E. coli.
Seat-back pockets, which hold the safety briefing cards, were also found to contain coliform bacteria - found in human faeces.
E.coli on your armrest?
According to the NHS, the majority of viruses that are responsible for colds will survive on hard surfaces for up to an hour, but there are some that can survive six hours or more.
The American Society for Microbiology carried out research that found MRSA lasted for a week on the seat-back pocket. A strain of E. coli survived for four days on the armrest.
According to experts, the passengers sitting near you may present the largest risk of infection.
Scientists from Centers for Disease Control and Prevention in the US investigated an outbreak of norovirus on a 2008 flight from Boston to Los Angeles. They found that passengers who then suffered vomiting and diarrhoea were most commonly those sitting next to someone who was already infected - or those seated in aisle seats with many passengers passing and touching their seat.
Dirty Ryanair flights
This evidence emphasises the importance of airlines properly cleaning their planes between flights.
Most airlines were highly rated for cleanliness in this year's airlines survey of almost 8,000 passengers, with 81% of passengers saying it was good or excellent.
Ryanair passengers, however, were not so happy. Fewer than half said the same thing.
A quarter of Ryanair passengers went as far as to say cleanliness on flights was poor.
Keeping planes clean
Meanwhile, two-thirds of Easyjet passengers rated the cleanliness as 'good', 'very good' or 'excellent'.
This went up to three in four passengers for those who flew British Airways.
Almost all the passengers who flew with Air New Zealand (97%), Singapore Airlines (96%), Emirates (95%), Qatar Airways (95%), Cathay Pacific (94%) and Swiss (94%) told Which? they found the cleanliness on board to be good.
10 tips for keeping yourself healthy when flying
- Wash your hands before eating: On a flight where you'll have to get out of your seat and queue up at a busy toilet it's easy to forget this, but it's the single most effective way to avoid getting ill.
- Avoid touching your eyes, mouth or nose: This might be easier said than done, but that's how bugs are most likely to get from your armrest into your system.
- Carry a hand-sanitiser gel: This means that you don't have to worry too much about washing your hands. Even if the seatbelt light is on and you're stuck in your seat you can still make sure any bugs are eradicated before you start to eat. However, try not to become obsessive as alcohol-based sanitisers can irritate the skin if over-used.
- Use the overhead fans: Some people switch them off because they don't like the thought of the recycled air. But Professor Sally Bloomfield of the London School of Tropical Medicine and Hygiene recommends them. You'll be breathing in air directly from above rather than that of the people coughing or sneezing around you.
- Try and sit in a window seat: Aisle seats have been shown to have a higher concentration of bugs because so many people touch them, either walking past, or getting in and out of the row.
- Wipe down the tray table before eating: You wouldn't eat your dinner off a dirty table and it's not a good idea to do it in a plane either.
- Use a nasal spray: Try sprays such as Boots Cold & Flu, Coldzyme, Prevalin or Vicks First Defence that block airborne viruses before they get a chance to infect you. While the jury is still out on their effectiveness, Professor Bloomfield suggests that they can act as a prophylactic if used during the flight.
- Remember to stay hydrated: We recommend filling a water bottle at an airport fountain before boarding. A well-maintained fountain at a major airport should be as safe as the water coming out of your tap.
- Use a handkerchief or tissue: If you have a cold, make sure to cough or sneeze into a handkerchief or tissue. It might be too late for you, but the biggest responsibility for not spreading illnesses rests with those who've already caught them.
- Don't worry: Although a few hours of stress in itself isn't likely to lead you to catch a cold, it might stop you falling asleep. Eating and sleeping well is, on the plane as at home, another good way to keep yourself healthy.