A "fireball blast" not only has a burst radius of 20', but also fills 33k cubic feet (33 10' x 10' x10' cubes - paraphrasing S&W Complete, but it follows AD&D if I recall correctly)
Outside, a fireball is fairly easy to account for, but in a dungeon, the opportunity for substantial blowback is considerable.
I remember an article in the Dragon (issue in the late 60's or early 70's of numbering I think) that dealt with the issue, but damned if I can find it.
How do you handle the area of effect of a fireball? BTB, handwave, guestimate or something else entirely?
(The magic-user in the party is now 4th level - I expect fireball will be one of the spells he will be looking to learn ASAP after hitting 5th ;)