  High energy protons produced by various sources of cosmic rays, {\it e.g.}
supernova remnants, pulsar wind nebulae, active galactic nuclei and gamma-ray
bursts participate in $p\gamma$ and $pp$ interactions. Although $pp$
interactions may be the dominant mechanism in our Galaxy, it is unclear how
important $p\gamma$ process is. We show that the upper bound on the fraction of
protons participating in $p\gamma$ interactions inside all Galactic
astrophysical sources of cosmic rays is 10%.
