According to Dr. Matthew Greenhawt, a Research Direct in allergies and paediatrics, it is impossible for nuts to circulated through a plane's air con system: allergicliving.com/2014/08/21/anaphylaxis-in-the-air-two-recent-airline-incidents/
"The short answer is that it is highly unlikely for a passenger to inhale nut protein from someone consuming nuts a few rows in front of him/her. There is no evidence that has been able to show that such dust circulates. Five studies in the past 10 years have addressed this concept and found the following (favorable) results:
a) Close range (12 inches) exposure to inhaling peanut butter resulted in no reaction in severely reactive peanut allergic subjects.
b) Peanut dust could not be detected in the air from stomping on peanuts on the floor or from opening an airline-style bag. If a scientific measuring tool is placed an inch or two above peanuts being de-shelled, dust can be briefly detected in very low quantities (300 micrograms), but only while the item is being de-shelled. As soon as the shelling stops, dust is no longer detectable. This implies that the dust settles, rather than circulates.
c) Peanut butter and peanut dust are both easily cleaned from hands and surfaces using a variety of commercially available cleaners.
Smelling pre-roasted nuts being re-warmed on board is similar to the smell of peanut butter (which was proven to not cause reactions). This may not make concerned passengers more comfortable, but most experts agree that there are no active proteins involved in breathing in the aroma of re-heated, pre-roasted nuts.
Apart from reactions where one eats a contaminated food, airline reactions most likely occur from allergen that may accumulate on surfaces. This we know happens in many different environments. On the plane, without being aware, one could potentially touch a surface that hadn’t been wiped down first, and theoretically ingest some level of allergen. In fact, from my own studies and studies I’ve reviewed, I’m suspicious that this surface contact may be more of a problem than the air being inhaled. The good news is that pre-cleaning your personal seating area surfaces can reduce the risk of an unintended, unnoticed ingestion.
Is re-circulated air on an airplane likely to contain enough airborne allergens to cause reactions throughout the plane?
Dr. Greenhawt: It is very unlikely that someone would inhale a dose of an airborne allergen. Airflow dynamics for commercial jets actually mandate that there is frequent air-exchange and HEPA filtration of that air. Depending on the type of aircraft, its age, and its make, there are varying levels of air exchange between the cabin and the sky, meaning that there is minimal “re-circulation” of the same air during a flight. A nice explanation for this can be found here.
Have you ever heard of a situation similar to the Ryanair case (i.e. a suspected severe airborne reaction on an airplane)?
Dr. Greenhawt: I have had the opportunity to conduct two large studies of airline reactions, and reactions by inhalation were reported in both studies. Stories of similar types of reported reactions make the news from time to time, are reported to advocacy groups, or can be read about on the Internet. However, it is strongly felt that it is very hard to say with any certainty that the allergen in these situations was inhaled. Again, there is no evidence to show that peanut or tree nut circulates in the air, as opposed to it quickly settling on surfaces. In such situations, it is likely that there is some unnoticed ingestion of settled dust on a surface through casual hand-to-mouth contact. This type of ingestion may be overlooked when potentially focusing on someone nearby who may be eating a nut-containing item, which may mistakenly be presumed by the passenger to be the trigger.
In my 2012 study, we actually showed a decreased risk of reporting an in-flight reaction associated with wiping down one’s seating area. We feel this is a simple, proactive, and effective strategy that removes residue, thus decreasing the chance of inadvertent contact leading to unnoticed ingestion. However, we also realize that many people still assume or report their child has had an airborne reaction, in spite of the available evidence that this is highly unlikely to occur."