Well, it's nothing new in theory — and a main reason my pooch always rides in the cabin with me — but the proof is in the stats. According to data released Friday by the Department of Transportation , 122 dog deaths have been reported since the DOT required airlines to make that information public (beginning in May 2005). Of those numbers, the highest single number of the 108 purebred deaths belongs to English Bulldogs  at 25 followed by Pugs  at 11.
While many airlines refuse to transport any brachycephalic breeds  (aka smooshies) below the plane during temperature extremes, it's the pups' short snouts that attribute to extra difficulty breathing and extra wheezing .
Dan Bandy, chairman of the Bulldog Club of America's health committee, explains this particular risk in being transported in the cargo holds of airliners :
The way all dogs cool themselves is basically through respiration, either just panting or the action of breathing in or out, is a method of heat exchange for them. A dog that has a long snout or a long muzzle has more surface area within its nasal cavity for that heat exchange to take place. So breeds like labradors or collies or those types of dogs with the long muzzles have a more efficient cooling system.
Adding in the Frenchies (six), American Staffordshire Terriers (four), Boxers (two), Pekingese (two), and other short snouts, that means that the smooshies make up nearly half of the total number  — scary stuff!