Summertime humidity isn't just uncomfortable for you, it's a breeding ground for spores. Niles remembers inspecting an unoccupied home with the A/C turned off and discovering a subfloor covered with mold. "In the summertime it doesn't take much time for mold to grow and become visible—72 hours if the conditions are right.
Set the central air for 77 in the summer to keep the house dry and mold-free while you are away. Tom also suggests setting up dehumidifiers over sinks, so they can run constantly and drip into the drain. If you have double-hung windows with storm windows, keep air circulating by opening the top sash while leaving the screen down over the bottom sash.