  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 \lsim 10^{-4} for \mu \lsim
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.
