We discuss the bounds on the cosmological lepton number from Big Bang Nucleosynthesis ( BBN ) , in light of recent evidences for a large value of the neutrino mixing angle \theta _ { 13 } , \sin ^ { 2 } \theta _ { 13 } \gtrsim 0.01 at 2- \sigma .
The largest asymmetries for electron and \mu , \tau neutrinos compatible with ^ { 4 } He and ^ { 2 } H primordial yields are computed versus the neutrino mass hierarchy and mixing angles .
The flavour oscillation dynamics is traced till the beginning of BBN and neutrino distributions after decoupling are numerically computed .
The latter contains in general , non thermal distortion due to the onset of flavour oscillations driven by solar squared mass difference in the temperature range where neutrino scatterings become inefficient to enforce thermodynamical equilibrium .
Depending on the value of \theta _ { 13 } , this translates into a larger value for the effective number of neutrinos , N _ { eff } .
Upper bounds on this parameter are discussed for both neutrino mass hierarchies .
Values for N _ { eff } which are large enough to be detectable by the Planck experiment are found only for the ( presently disfavoured ) range \sin ^ { 2 } \theta _ { 13 } \leq 0.01 .