  The observation of astrophysical neutrinos requires a detailed understanding
of the atmospheric neutrino background. Since neutrinos are produced in meson
decays together with a charged lepton, important constraints on this background
can be obtained from the measurement of the atmospheric muon flux. Muons,
however, can also be produced as mu+ mu- pairs by purely electromagnetic
processes. We use the Z-moment method to study and compare the contributions to
the atmospheric muon and neutrino fluxes from different sources (pi/K decay,
charmed and unflavored hadron decay, and photon conversion into a muon pair).
We pay special attention to the contribution from unflavored mesons (eta, eta',
rho0, omega and phi). These mesons are abundant in air showers, their lifetimes
are much shorter than those of charged pions or kaons, and they have decay
branching ratios of order 10^-4 into final states containing a muon pair. We
show that they may be the dominant source of muons at E_mu >10^3 TeV.
