Suppression of Bremsstrahlung and Pair Production due to Environmental   Factors
  The environment in which bremsstrahlung and pair creation occurs can strongly affect cross sections for these processes. Because ultra-relativistic electromagnetic interactions involve very small longitudinal momentum transfers, the reactions occur gradually, spread over long distances. During this time, even relatively weak factors can accumulate enough to disrupt the interaction.   This review will discuss a variety of factors which can suppress bremsstrahlung and pair production, as well as related effects involving beamstrahlung and QCD processes. After surveying different theoretical approaches, experimental measurements from the early cosmic ray studies to the recent detailed measurements by the SLAC E-146 collaboration will be discussed. 
Spencer R. Klein