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 εe and mass μ to the Lamb shift of the hydrogen atom is shown to imply universal, i.e. μ-independent, upper bounds on ε: ε < or approx. 10-4 for μ < 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