  Millicharged particles (MCPs) arise in many well-motivated extensions of the
Standard Model and are a popular subject for experimental searches. We
investigate attenuation of the MCP flux produced at accelerator experiments due
to their interactions in the media. Considering, as an example, the dedicated
MCP search at SLACmQ, we demonstrate that this effect can significantly affect
the final sensitivity to the MCP parameter space leaving its essential part
still unexplored. Applying our analysis to the SLACmQ experiment [53], we
correct their exclusion bounds in close accordance with Ref. [54]. We also show
that this newly reopened area with the MCP masses in the range $10^{-4}$ eV -
$1$ GeV and charges $\gtrsim 10^{-5} e$ can be effectively probed by the
NA64$e$ experiment at the CERN SPS. Light MCPs are mostly produced by virtual
photon in electron scattering off nucleus. The main source of heavy MCP is
decays of vector mesons, produced by the electrons on nuclei.
