In an attempt to determine the origin of the light-induced metastable defects in undoped a-Si:H, we have examined the purest material available to date, prepared in a UHV plasma deposition system. In addition, the influence of impurities has been studied by investigating a series of SIMS-characterized films in which the oxygen and nitrogen content has been independently varied by nearly four and five orders of magnitude, respectively. We show that, contrary to the earlier indications, impurities do not influence the number of the light-induced defects in a-Si:H if they are below a critical concentration of ∼1020 cm-3 for O and ∼1019 cm-3 for N. This leads us to conclude that the Staebler-Wronski effect is an intrinsic property of a-Si:H. Also we show that impurities can cause rapid increases in the defect density only when they exceed the critical concentrations. However, in this ``alloying'' regime where significant modifications of a-Si:H network are expected, the generation of metastable defects may involve a different impurity-related mechanism. A ``surface'' or ``interface'' region with a combined thickness of ∼0.6 μm is observed to exhibit a much higher induced spin density than the ``bulk''. Thus, surface band bending is believed to play an important role in the defect creation.