Nope its perfectly safe as long as you don't over do it (gawd that rule could apply to so many things in life!!!), I used to use this method to power my Humbrol Modellers Airbrush many years ago before I finally aquired a small compressor. Although I'm not sure how easy it will be to obtain a car inner tube these days as so many car tyre are tubless - you best bet might be to go for a motorcycle inner tube. Thats assuming they haven't gone done the tubless path as well, its been a long time since my biking days.