2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/194696The leading order vacuum polarization contribution of very light millicharged fermions and scalar (spin-0) particles with charge εe and mass μto the Lamb shift of the hydrogen atom is shown to imply universal, i.e. μ-independent, upper bounds on ε: ε\lsim 10^{-4} for μ\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.10 pages including 3 figures, version to appear in Physical Review D (Rapid Communications)High Energy Physics - Phenomenologytext