Several regulatory agencies, including the Occupational Safety and Health Administration (OSHA), have stipulated that the safe allowable level of residual is 0.1 ppm (parts per million).
Note that this permissible level is for continuous exposure throughout an entire 8 hour day. The temporary effects of such a low exposure would range from headaches to sore throats and irritation in the eyes and nose.
No long term effects have ever been documented from ozone exposure. Here is our Material Safety Data Sheet (MSDS) for reference.