Lamb shift contribution of very light charged particles

The leading-order vacuum polarization contribution of very light millicharged fermions and scalar (spin-0) particles with charge {epsilon}e and mass {mu} to the Lamb shift of the hydrogen atom is shown to imply universal, i.e. {mu}-independent, upper bounds on {epsilon}: {epsilon} < or approx. 10{sup -4} for {mu} < or approx. 1 keV in the case of fermions, and for scalars this bound is increased by a factor of 2. This is in contrast to expectations based on the commonly used approximation to the Uehling potential relevant only for conventionally large fermion (and scalar) masses.