PSI outside the system is 0 / atmospheric. Psi only affects the velocity of the water which is part of the equation for energy. There is usually a factor determined empirically which relates the drag force on the 'object' in question to its velocity and in air its usually a square or higher order so it will slow down very quickly, and there's got to be a lot of other stuff unique to the spray into air situation. So in summary effective psi is not the right question, whatever the right measure is it drops off very quickly - one inch, cut your hand off - three inches, exfoliating - 12 inches, wash your dog.