  We present a new one-dimensional calculation of low and intermediate energy
atmospheric muon and neutrino fluxes, using up-to-date data on primary cosmic
rays and hadronic interactions. We study several sources of uncertainties
relevant to our calculations. A comparison with the muon fluxes and charge
ratios measured in several modern balloon-borne experiments suggests that the
atmospheric neutrino flux is essentially lower than one used for the standard
analyses of the sub-GeV and multi-GeV neutrino induced events in underground
detectors.
