The acetone, and most of the another substances added in it (as acetone rarely is 100% pure) are volatile organic solvents, which will evaporate.
The not-so-volatile ones are the water-like liquid which remains in the dish, and are washed away. Even if something remain mixed with the hash oil, you must remember the amount of this substance would be very small (as the amount of hash smoked is usually very small too).
And remember, also, that when we smoke the weed, we inhale all the smoke of burnt weed, which contains lots of harmful chemicals, tars, etc, and in a far greater quantity than anything present in a small piece of hash oil. So, all in all, i think smoking hash oil is still healthier than smoking weed. :thumbsup: