From the current cosmological observations of CMB and nuclear abundances we show , with an analytic procedure , that the total effective number of extra neutrino species \Delta N _ { \nu } ^ { tot } < 0.3 .
We also describe the possible signatures of non standard effects that could be revealed in future CMB observations .
This cosmological information is then applied to neutrino mixing models .
Taking into account the recent results from the SNO and SuperKamiokande experiments , disfavouring pure active to sterile neutrino oscillations , we show that all 4 neutrino mixing models , both of 2+2 and 3+1 type , lead to a full thermalization of the sterile neutrino flavor .
Moreover such a sterile neutrino production excludes the possibility of an electron neutrino asymmetry generation and we conclude that \Delta N _ { \nu } ^ { tot } \simeq 1 , in disagreement with the cosmological bound .
This result is valid under the assumption that the initial neutrino asymmetries are small .
We suggest the existence of a second sterile neutrino flavor , with mixing properties such to generate a large electron neutrino asymmetry , as a possible way out .