Nearly 5 million homes don’t have a smoke alarm installed. And in many homes that do, there’s another big problem: As in that house in St. Louis, the smoke alarms don’t work.

In fact, three out of every five people killed in a house fire didn’t have a working smoke alarm. The answers to these three questions could save your life:

• Do you have working smoke alarms with functioning batteries?
• 2-Are they in the right room?
• How old are they?

In Orlando, Florida, TODAY national investigative correspondent Jeff Rossen accompanied the local fire department as they went door to door, checking homes for safety risks and installing new alarms for free where needed. They encountered many problems, including alarms without batteries, kitchens with no smoke detectors within 10 feet, and even an alarm whose smoke sensor no longer worked, even though it beeped when tested.

That detector was manufactured in 2002, it turned out. “We like to change them out every 10 years,” the Orlando fire chief told Rossen. “Dust starts blocking the sensor; it can no longer see smoke.”

An easy way to test the sensor is the candle test, he explained. Light a candle, then blow it out and place your alarm right over the smoke. It should go off.

Another tip: If you’re worried about forgetting to change the batteries in your smoke detectors, you can buy lithium batteries at any big box store for $5 to $10. They work for 10 years.