We discuss collective effects like percolation, quark gluon plasma, or stri
ng fusion and their effect on the longitudinal development of high energy s
howers in Air. It is shown that Iron-Air showers could develop more slowly
than expected and produce the observed change in the slope of X-max at the
highest energies. (C) 2000 Elsevier Science B.V. All rights reserved. PACS:
13.85.Tp; 13.85. - t; 96.40.De.