Smoke alarms (probably also carbon monoxide alarms — does anyone know?) contain a small amount of Americium 241, a radioactive material. It literally emits gamma rays (which sounds kind of awesome but I can only assume is not.) They should not be chucked willy-nilly into the garbage.
While there is no indication on their First Alert’s Web site, I’ve assessed through my own sleuthing that First Alert does take non-working alarms for recycling. Check out this helpful post for details.
If your alarm has an Aurora, IL address on it, chances are it’s made by First Alert or one of its subsidiaries. If you’ve got a Canadian one made by American Sensors — why a Canadian firm would call itself that remains a mystery — give ’em a call at 1-800-387-4219 for information on recycling.
For info on other smoke detector manufacturers, check out this list from Morris County Municipal Utilities Authority.
While you’re at it, check your alarm once in a while to see if it’s working. They tend to stop being effective after about ten years or so. Hold it over some smoke to see if anything happens. Hey, I’m worried about you!