Spring is here, and you know what that means: no more marshmallow coats, no more wiping out on icy sidewalks, no more 48-hour Monk marathons on frigid weekends. But for nearly 36 million Americans, throw-open-the-windows season comes with a major buzzkill: allergies. And they're only getting more severe. Allergies to pollen, ragweed, and other common airborne triggers have doubled in the past 20 years — a 5 percent per decade increase since the 1970s — clogging up even those who've always been sniffle-free. Here are the three reasons your tissue box needs replacing more often — and how you can get some relief.