I need to change my smoke detectors but I don't know how to choose between ionization and photoelectric smoke alarms. I guess that there are pros and cons to choose one or the other. Should I buy a combination detector that uses both methods?

5 Answers 5


One consideration is where it's going to be installed.

The issue being that some places are more likely to have them go off when they shouldn't.

Ionizing can be affected by steam, so has problems if installed right outside a bathroom. They also don't work too well if mounted near moving air (forced air systems, ceiling fans, etc.)

Photoelectric are set off by lots of dust, so workshops, laundry rooms (lint) and the like are problem areas for them.

They both have problems in kitchens.

Where you can, you should use combination alarms, but you can also use a mix of types to achieve the same effect, while avoiding areas that might cause them to go off when they shouldn't.

  • Good points. You don't want to keep getting false alarms because then people become inclined to disable them altogether.
    – auujay
    Commented Oct 7, 2011 at 16:27
  • i presently have the problem that my smoke alarm set off when i use my bathroom. It is positioned just outside the room. I thougth it was because of its high age but you've just pointed the real problem. Its probably a unit with an ionization detector that is too near my bathroom! Commented Oct 7, 2011 at 17:24

I just posted this on a different thread... but it is useful here also.

Ionization detectors are the most prevalent style installed in residences. This is largely due to a lower cost, but they aren't necessarily the best. They have more false positives (nuisance) alarms and are frequently removed or disabled, which does absolutely no good. We had a fatal cooking fire just last year because of this very phenomenon. Couple this with the fact that they take longer to alarm (up to 20+ minutes) in a smoldering fire, illustrates that you are better served by switching to either a dual sensor unit, or a photoelectric style for your home. Here is the text of an email I send out regularly on this subject.

Do yourself a favor and install photoelectric, it may save your life.

  • Turned the URLs into links for you. Great information, thank you! Commented Jan 19, 2012 at 2:49

I would go for a combination detector, or install one of each type. Each method detects different things:

Optical detect smoke and are better as detecting smouldering fires while ionization detect heat and smaller smoke particles. Ionization are more prone to false alarms than Optical.

The Wikipedia article states:

the National Fire Protection Agency recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke, or use both the ionization and photoelectric / optical processes.

Which is a sentiment I agree with.


I've recently completed research into this topic, in order to buy a set of alarms for my home. I chose Photoelectric detectors for a few reasons:

  1. Ion detectors have a high false-positive rate, leading to disabled detectors. A look around my own home confirms this: the old detectors are all presently disabled due to a nuisance alarm at one point or another.

  2. Combination units are also prone to the "nuisance" issue. If someone in the house is going to take out the battery, then the alarm is no longer useful, even if it would have been better at detecting a smoldering fire. Since it's disabled, it won't detect anything at all.

  3. Ion detectors take 15 to 50 minutes longer to react to a smoldering, smoke-producing, fire, as compared with Photo detectors.


There are two main types of smoke alarms; photoelectric activate when the light beam is broken, and ionization react to heat and dust. The ionization alarms will react to shower steam, so they shouldn't be used near bathrooms. Photoelectric should be used in areas that have steam, and ionization should be used in living rooms, bedrooms, kitchens, near air vents, etc. Some companies label smoke alarms for the appropriate location.

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