As they are a brachycephalic breed, French Bulldogs are banned by several commercial airlines due to the numbers that have died while in the air. This is because dogs with snub noses find it difficult to breathe when they are hot and stressed. The temperature in a cargo space in an aircraft can rise as high as 30 °C (86 °F) when waiting on the runway. French Bulldogs have a tendency towards eye issues. Cherry eye, or an everted third eyelid, has been known to occur, although it is more common in Bulldogs. Glaucoma, retinal fold dysplasia, corneal ulcers and juvenile cataracts are also conditions which have been known to afflict French Bulldogs. Screening of prospective breeding candidates through the Canine Eye Registration Foundation (CERF) can help eliminate instances of these diseases in offspring. The skin folds under the eyes of the French bulldog should be cleaned regularly and kept dry. Tear stains are common on lighter-colored dogs.