My work room is a windowless bathroom (it does have an exhaust fan), so all of my EOs and aromachemicals are in the dark except when I'm working in there. I keep them at a fairly cool room temperature. Some of my EOs I've had for many years, and they're all fine. In fact, some of them get better with age. We don't have to worry about summer heat in Seattle! We're lucky to have summer at all.