Low relative humidity is also a concern. If high RH can cause a book to expand, low RH can case a book to shrink and warp. Since a book is made of several different component parts that will respond to RH fluctuations at different rates, keeping books and artifacts at a stable RH is crucial.
These facts can be a little abstract and when sharing them with colleagues as I advocate for the care of the Library's collections, it can be difficult to convince people that a book that has survived hundreds of years without environmental control is still susceptible to low RH. So I was particularly horrified and delighted on a recent cross-country plane flight when I noticed that the book I had brought with me, Maurice Craig's "Dublin 1660-1860" had started to respond to the low RH (10-12% at best) almost two hours into my flight. Note how the covers have warped away from the body of the text block (pages). The book had been completely flat when I packed it that morning. I had been reading it during the flight and only noticed the warping when the flight attendant gave me a cup and I put the book down on the tray table. I took the picture to make sure I wasn't mis-remembering the condition of the book and resolved to compare it with the condition of the book once I had returned home and the book had a chance to re-adjust to my air-conditioned home in Charlottesville.
The low relative humidity on the plane is extreme, but it isn't uncommon. The winter time RH for the Northeastern United States is often in the mid to low teens and collections that are housed in areas that don't have supplemental humidity control are at risk. Most libraries address the need by keeping their fragile books in boxes. I was going to return this book, but maybe I will renew it through the winter to see whether we need to make a box.