The safest way for a rescuer to perform CPR is to use a resuscitation mask during ventilations.
Mouth to mouth or mouth to nose breathing will always be an essential component of CPR, but it brings with it a whole new set of risks, especially given the unknown status of a casualty in a public area.
There will always be a risk of infection providing ventilations to a casualty, and using a pocket mask provides the rescuer with total protection. Not only protection, but obtaining an airtight seal between the rescuer and casualty is so much easier with the soft, flexible seal surrounding the mask.