  I discuss an upper bound on the boost and the energy of elementary particles.
The limit is derived utilizing the core principle of relativistic quantum
mechanics stating that there is a lower bound for localization of an elementary
quantum system and the assumption that when the localization scale reaches the
Planck length, elementary particles are removed from the S-matrix observables.
The limits for the boost and energy, M_{Planck}/m and
M_{Planck}c^{2}\approx\,8.6 * 10^{27} eV, are defined in terms of fundamental
constants and the mass of elementary particle and does not involve any dynamic
scale. These bounds imply that the cosmic ray flux of any flavor may stretch up
to energies of order 10^{18} GeV and will cut off around this value.
