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 $\epsilon$: $\epsilon \lesssim {10}^{-4}$ for $\mu \lesssim 1\mathrm{keV}$ 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.