The effect of telluric absorption on the intensity of He I 10830

The observed intensity of He I lambda-10830 in planetary nebulae is roughly half of that expected by standard level population calculations. Several recent studies have suggested that the physical processes populating excited states of helium are not now understood. In this paper, absorption of lambda-10830 by water vapor in the Earth's atmosphere is examined as a means of explaining this discrepancy. We find that this effect can reduce the intensity of He I lambda-10830 by as much as 20%-25% in the well-studied planetary nebula NGC 7027, depending on the time of year. This effect accounts for approximately 50% of the discrepancy discussed by previous workers. The ramifications of these results are discussed, and radial-velocity "danger zones", in which the line is most affected, are given to aid in the assessment of this effect in other objects. We also provide tables to determine the observing seasons when the line is least affected by telluric absorption.