Yes, it is safe to inflate balloons by mouth. However, there are a few things to keep in mind when doing so. First, be sure to blow up the balloon slowly. If you blow it up too fast, the air pressure inside the balloon can become too high and cause the balloon to burst. Second, don't overinflate the balloon. Once the balloon is about 3/4 full, stop blowing and tie off the end of the balloon. Finally, be careful not to inhale any of the helium gas that escapes from the balloon while you're inflating it. Inhaling helium can cause your voice to sound high-pitched for a short time afterward.