From their website:
Do the kilometer “Color Zones” affect runner vision? Or breathing?
The Color Run™ explosions are basically food grade cornstarch and are 100% natural and SAFE. As with any substance, you want to keep it out of your eyes and our “certified” color throwers make sure to aim low as you pass by. Some “Color Runners” opt to wear glasses or goggles for their eyes and use a bandana or dust mask for their mouths. As shown on the “about” page the middle lane is the “low color lane,” which offers a “lighter” color experience. In summary: We work pretty hard to make it so our precious “Color Runners” get as much or as little color on them as they like.
Is cornstarch anything like flour when it comes to being flamable if the powder is floating in the air?