Purchasable nitrogen without traces of oxygen in it is a fantastic thing, and presence of 1% O2 or below shouldn't be a big issue. Breathing an oxygen-depleted gas mixture with only 6% O2 or less produces rapid unconsciousness and death in several minutes. Breathing a mixture with 6 - 8% O2 is also commonly fatal, but inducing unconsciousness and dying may happen within significantly longer time intervals.
An imperfect delivery of nitrogen to the respiratory system may lead to mixing the asphyxiant with air, so the actual concentrations of O2 in the breathed mixture may be greater than the concentration of O2 in the nitrogen cylinder, but keeping them under the 6% threshold should be quite easy as long as the initial concentration of O2 is somewhere in the range 0 - 1%.