Not all smoke detectors are created equal

>>> we're back as promised with an nbc news investigation about smoke detectors that may surprise a lot of people including those of us who follow the rules about where to install them, how often to check the batteries. the problem is, as you're about to see, even when the deadliest fires erupt, they may not deliver the warning needed to save lives. tonight's report from our national investigative correspondent jeff rossen is an urgent matter that gets off to a very disturbing start.

>> 911.

>> i have a fire in my home.

>> reporter: a desperate mother waking up to a house full of smoke, trying to save her kids.

>> my first thought is the four people that i have upstairs. trying to make sure they're not scared to death, that they're safe.

>> reporter: the kids didn't make it. cause of death -- smoke inhalation . so why didn't they have more warning? after all the house had working smoke detectors .

>> then when it was time, they never went off.

>> reporter: amanda says she had the most common type of smoke detector used in 90% of homes. inexpensive, easy to find alarms that rely on ionization technology. they work well to detect fires with fast flames, but experts say they may not save i in smoldering smoky fires that can strike while you sleep. don russell is a scientist at texas a&m . when i buy a smoke detector , i assume they're going to sound when there's smoke.

>> that's a reasonable assumption, but it's wrong.

>> reporter: we had him set up a test, playing thee ionization detectors in a room, then setting a couch on fire. toxic smoke is building, but it takes 36 minutes for the first detector to go off. but there's another technology out there that gives you better warning in these fires. it's called a photo electric detector. so dr. russell set up another test. this time with a photo electric next to those three ionization detectors. 17 minutes in, with barely any smoke in the room, the photo electric sounds the alarm. the ionizations? they're still silent for another 21 minutes. even with smoke everywhere.

>> if i would have relied on ionization, then my family probably wouldn't make it out. but with the photoelectric they'd have plenty of time to get out.

>> reporter: the leading smoke detector companies do make photoelectric alarms but still sell most of their products without it.

>> it's probably a business decision.

>> reporter: they cost less money to make than the photoelectric?

>> that is a correct statement.

>> reporter: the companies told us all their detectors provide adequate escape time and meet safety standards but critics say the government should force higher standards. so we went to the agency overseeing the companies. why not mandate photoelectric?

>> because both technologies are working in saving lives.

>> reporter: we know of several cases where the smoke alarm , people say, just did not go off.

>> in those cases, they need to practice a fire escape plan to make sure they can get out.

>> reporter: but if the smoke detector didn't go off and the house is full of smoke by the time it does, what does an escape plan do?

>> it helps them escape better when the smoke alarm eventually goes off.

>> reporter: but eventually isn't good enough for the mom that lost nearly everything.

>> i would think that if i had known i might have a family of seven instead of a family of three.

>> reporter: just to be clear, no one is saying throw out your smoke alarm tonight. fire officials say the best advice is to have both technologies in your home. you can buy a dual detector that has both in one. thing is, they're harder to find in the stores and will cost you a little more money, too.

>> jeff rossen , thank you for your reporting on this.