Short of having an asthmatic attack at altitude
And if that DOES happen ? For example, induced by the great stress of a mid-air emergency ? The name asthma covers a wide range of levels of disability. Admittedly, some can be pretty trivial, and lived with easily. Let it not be forgotten, though, that at the other end of the spectrum asthma can be life-threatening.
Bad news if you are at the controls of an aeroplane in flight.