There is never a wrong time to think about the effects of moisture and humidity on rare books. Just as too much sun can damage your rare books, so can too much moisture. And we're not just talking about direct moisture, such as liquid spills. The relative humidity of the air is also a concern. Excess humidity (usually relative humidity above 75%) can encourage the growth of fungi and mildew, which can lead to foxing. If foxing occurs, what are the best ways to reverse it? Better yet, how can you prevent foxing in your rare books?