Overspray is a problem.... but there are things to do to minimize it.
What are you airbrushing with? What is your equipment and what air pressure are you using?
Too high a PSI and you will be putting more color into the air than on the cake - plus color will be literally bouncing off of the cake.
Technique also makes a difference.
Do you spray only at the cake? Or are you airbrushing back and forth past the cake? If so, you're airbrushing air...
What is your room ventilation?
If you have only fans, you're only stirring the color into the air.
If you have circulation, the color can be pulled up into the air system and travel along the air ducts to another area.
Shilds can help... but they bounce the overspray back to you.
Best bet is a filtered fan system.
Either DIY or a pro system.... if you're doing a lot of airbrush - or large projects - it is a good investment.
Here's a video I did for Grex on the subject: http://www.youtube.com/watch?v=mmm2PrkETe8