We present a detailed analysis on the extent of the errors in the intensity measurements of low signal-to-noise narrow emission lines. Our first goal is to determine a model for the probability distribution function (p.d.f.) associated with the measured intensities of a line characterized by its signal-to-noise ratio. Our final purpose is to provide an error domain: — for the measured signal-to-noise ratio of an observed line; — for the ratio of two lines in terms of their signal-to-noise ratios, and eventually (with the knowledge of the noise energy) to get errors on the corresponding intensities and intensity ratios.