  Current constraints on energy injection in the form of energetic particles
before the epoch of recombination using CMB spectral distortions assume that
all energy goes into $y$ and $\mu$-type distortions. We revisit these
constraints with exact calculations of the spectral distortions by evolving the
electromagnetic cascades. The actual spectral distortion differs in shape and
amplitude from the $y$-type distortion and depends on the energy and nature of
injected particles. The constraints on the energy injection processes such as
dark matter decay can be relaxed by as much as a factor of 5.
