The field emission (FE) properties of carbon films can be understood in terms of local field enhancement beta (x,y), which can be determined with x,y-scanning FE. beta (x,y) double right arrow the spatial distribution of emitting sites, which can be counted as f(beta) alpha exp(-k beta). f(beta) is connected with the presence of sharp protruding objects, whiskers or nanotubes on the surface. Investigations of the current-time (I-t) characteristics of field emission from single-walled carbon nanotubes (SWNT) do not show any significant dependence on ambient partial pressures of hydrogen or water up to 10(-5) mbar. Oxygen however, causes a substantial reduction of the FE current. Field emission microscopy (FEM) during short-time nanotube annealing (similar to 1000 K) reveals dim five-fold as well as six-fold feinstructures, which are believed to be nanotube cap states. The nanotube cap states have a short lifetime due to impinging atoms/ions that are adsorbed due to the high local electric field at the cap (similar to 3000 V/mum) and create resonant tunnelling states. The anode material is believed to be the main source of adsorbed species and not the ambient gas phase.