For Ms Sophie Wilson, inhaling peanut particles could result in a potentially fatal allergic reaction.
So when the 26-year-old boarded a flight to Rio with Lufthansa in 2015 - having alerted them in advance to her allergy - she did not think she would be risking her life.
"I informed them a month before I flew and the airline said the cabin crew would help when I got on board," said Ms Wilson. "On the plane, I asked a flight attendant if an announcement could be made.
"His response was, 'We can't ask 399 other passengers not to eat nuts because of you.'
Airline allergy policies
An allergen-free meal is available by prior request, but this may be subject to availability.
Peanuts are not served but other nuts are. There is no guarantee of a nut-free flight.
Nuts are served on board but, on request, the airline can suspend the sale of nuts on board and make an announcement.
The airline serves nuts as a meal ingredient or accompaniment to drinks. It asks passengers with allergies to take along their own food.
Peanuts are not served on flights but an allergen- free environment cannot be guaranteed.
Those with severe allergies must complete an information form and provide a doctor's letter no less than 48 hours before travel. Passengers with allergies are advised to take along their own food.
The airline removed peanuts from all its flights in the early 2000s and minimised their use in food after passengers voiced their concerns about flying with a severe allergy. Other nuts may be served. Staff are trained to spot anaphylaxis symptoms. Special meals are available.
Passengers can select a nut-free meal that does not contain peanuts and other nuts like almonds and cashew. The request must be made at least 48 hours before departure. But the airline does not provide a nut-free cabin as other passengers may be given food containing nuts.
Peanuts are never knowingly included but meals may contain traces. Other nuts may be served.
"Thirty minutes later, he put a mask on my lap, saying that if I didn't wear it, they would turn the plane around."
Ms Wilson's experience is rare but flying remains risky for people with food and pet allergies.
About two million people in the United Kingdom have a mild to life-threatening food allergy.
Statistics for other countries are not available.
Some airlines train staff to recognise anaphylaxis symptoms, while others provide special meals.
Legislating on airline allergy policies is impossible, due to the number of potential triggers that could be present on board.
It is unclear whether allergens, such as dog hair or peanut particles, which have been filtered through a plane's air system can trigger an airborne reaction.
The Civil Aviation Authority (CAA) has begun research to establish whether there is evidence that this is a risk for allergic passengers.
A CAA representative said: "We believe the findings will be useful to airlines as guidance when developing policies on food allergies... this should help to ensure the development of a more consistent response by airlines."
While there is no blanket policy on how airlines should deal with allergies, the increasing prevalence of the condition, coupled with heightened concern from passengers, means many have independently updated their procedures.
Some airlines, however, said they cannot help.
Student David Dwek flew with South African company Mango Airlines to Cape Town in August last year. Out of a group of 30 students, five had life-threatening nut allergies. He said: "We were told by the air crew that making an announcement was against company policy."
A spokesman for Lufthansa said: "The crew correctly advised Ms Wilson they couldn't guarantee that nobody would eat nuts on board and offered her the chance to leave the flight. They also offered her a mask, which she refused."
Mango Airlines said: "We are still investigating this matter and apologise for the manner in which our guests were responded to by our cabin crew."