hmn.. lets see, when Arak is relaxed his purple fire would float around like lil' will-o-wisps, and they are relatively harmless at that point. So if something is thrown at him from the back as a surprise it will go through the purple wisps harmingless and hit Arak directly.
So, unless Arak commands, his purple fire to a defensive mode, purple fire is nothing more than foating fire thingys~
i hope that cleared that up XDD
his purple fire is relatively only for defense and strength raise so no- normally they won't hurt on physical contact, but he can command and control the purple fire so that it has the properties of fire, where it will SCORCH people on physical touch.
yar.