At last year’s conference, we examined a series of aluminum films deposited in the presence of varying amounts of water vapor. The resulting structure and chemistry of those films were explored using transmission electron microscopy. Those results beg two questions: 1) how much contamination do I have, and 2) how much contamination can my product tolerate? The answer to the first question is straightforward and will be described. The second question is application specific and more challenging but will be responded to with a specific example. The results will help us understand why we can get good coatings using a high speed, industrial roll-to-roll metallizer at pressures in the 10-4 Torr region (and higher), while we need much lower pressures in our lab-scale bell-jar evaporators. It’s all about the impingement rates of the metal atoms and the water molecules.