We have investigated material initiated air breakdown using a long 10.6‐&mgr;m pulse (tens of microseconds) and attributed the breakdown initiation to a thermal mechanism. In particular, breakdown initiated on glass fibers after irradiation times up to 35 &mgr;sec with power densities of 106−107W/cm2is reported. Measured parameters included breakdown threshold, time to breakdown, plasma expansion dynamics, and transmission of 10.6‐&mgr;m radiation through the plasma.