Passengers should stick to puzzles or books during a flight rather than making big decisions, the first UK professor of aerospace medicine says.
Mental tasks are impaired at high altitude, so dealing with key emails is best avoided, says David Gradwell.
Passengers should follow health advice to avoid side-effects such as jet lag and dehydration, he says.
"Flying is safe," he told a news conference. "Millions of people do it every year without any issues at all."
Prof Gradwell, of King's College, London, said most people could tolerate cabin pressurisation quite well.
But he said learning was a little impaired at a higher altitude, as was performance of recently learned tasks.
"It's perhaps not where you want to make the most important decision of your life," he said.
"Those of you who are looking forward to having emails in flight might want to think twice about sending that email to your bank manager."
Prof Gradwell said more research was needed to improve the advice given to air passengers.
There was still no conclusive evidence on the wearing of compression stockings to prevent blood clots, he said, and whether these should be of a medical grade rather than bought over the counter.
The professor said few people should be excluded from flying because of potential medical problems.
They include those with active tuberculosis, people who have been coughing up blood, those with severe anaemia or individuals experiencing a recent acute psychotic event.
A research group at King's College will assess advice given to passengers before travel and look at whether GPs are providing appropriate guidance.
Prof Gradwell said most aircraft were now better equipped to deal with medical emergencies, carrying devices such as defibrillators.
Cabin crews were trained to deal with a passenger being taken ill and could also take advice from trained staff on the ground, he added.