Smoke Alarm types and The Law

Will the SMOKE ALARMS in your home actually detect smoke in time for you to escape a fire? 90% of the smoke alarms in homes are of the type that may not sound the alarm in time.

Not all Fires are the same: There are flaming fires and smoldering fires. Smoldering fires are more deadly because the poison gases accumulate while you are sleeping.

Not all smoke alarms are the same: There are 2 different technologies used in smoke detectors: ionization and photoelectric. A “Dual Sensor” smoke detector contains both technologies.

The Yakima Fire Department highly recommends having at least one PHOTOELECTRIC alarm on every level of your home. A “Dual Sensor” smoke alarms contains both technologies and offers the best protection.



There are three types of smoke alarms, ionization, photoelectric and a combination of the two which is commonly called a “dual” detector. Look for the UL stamp on any smoke alarm.

Dual smoke alarms combine ionization and photoelectric sensor systems to enhance home safety. Recent research has shown that the standard ionization alarms have been unreliable in multiple tests of slow, smoky fires. Ionization alarm perform best in fast flaming fires. A combo unit is considered to provide greatest overall safety in either situation

Ionization smoke alarms monitor “ions” or electrically charged particles. Smoke particles change the electrical balance of the air. The alarm will sound when the change in electrical balance reaches a preset level. (IFSTA Fire and Life Safety Educator Pg 38)

Photoelectric smoke alarms use a beam of light and a light sensor. Smoke particles change the amount of light that reaches the sensor causing the alarm to sound. (IFSTA Fire and Life Safety Educator Pg 39)

Research has shown:

  • Ionization smoke alarms detect flaming fires marginally earlier than photo-electric smoke alarms
  • Photo-electric smoke alarms detect smoldering fires and fires starting in areas remote from smoke alarms significantly earlier than ionization smoke alarms. Fires that begin from a smoldering stage are more deadly
  • Ionization smoke alarms may not operate in time to alert occupants early enough to escape from smoldering fires
  • For both flaming fires and smoldering fires, photo-electric smoke alarms are likely to alert occupants in time to escape safely

If you only have ionization alarms, add at least one photoelectric alarm on each level of your home as soon as possible!
If your ionization alarm is 10 years old, either replace it with a dual-sensor (ionization and photoelectric) alarm at the cost of about $25 or with a photoelectric alarm (about $15).

How to tell the type of smoke alarms you have:
The odds are high that you currently have ionization detectors in your home.

Ionization smoke alarms:

  • Have a lower case letter “i”. Sometimes this “i” can be inside a square box
  • the phrase “contains radioactive material” or the radioactive symbol
  • You will usually get more false alarms when mounted near the kitchen and when you cook

Photoelectric smoke alarms:

  • a capital letter “P”
  • the phrase “photoelectric technology”

Ionization and Photoelectric or “Dual Sensor” smoke alarms:

  • may make reference to a “dual sensor”
  • offers the best protection for detecting fire, but slightly more expensive than other models


Smoke Alarms and Washington State Law

Washington State Requirements for All Homes:

The Building Code has required smoke alarms/detectors in all dwellings since the 1973 edition of the code. Currently, all new dwellings are required to have hard-wired, interconnected smoke detectors with battery back-up on each level of the home as well as in each bedroom.

212-10-015 Application and scope.

(1) The provisions of these rules shall apply to (a) all dwelling units occupied by persons other than the owner after December 31, 1981, and (b) all dwelling units built or manufactured in this state after December 31, 1980.

(2) Notwithstanding the provisions of chapter 19.27 RCW, RCW 43.22.340 through 43.22.434 and 43.22.450 through 43.22.490, the provisions of these rules shall also apply to all buildings or structures, mobile homes and factory built housing used as dwelling units.


212-10-035 Number of smoke detection devices.

(1) At least one smoke detection device shall be installed to protect the sleeping area within each dwelling unit. A sleeping area is defined as the area or areas of the dwelling unit in which the bedrooms (or sleeping rooms) are located. Where bedrooms or rooms ordinarily used for sleeping are separated by other-use areas (such as kitchens or living rooms but not bathrooms or closets), or are located on different stories or floor levels, they shall be considered as separate sleeping areas for the purposes of these rules.

(2) Dwelling units with more than one sleeping area shall require the installation of additional smoke detection devices to protect each sleeping area.


212-10-040 Location of smoke detection devices.

(1) Smoke detection devices shall be installed outside of bedrooms or rooms used for sleeping purposes but in the immediate vicinity of such rooms, centrally located in the corridor or area giving access to the rooms. In dwelling units without separate sleeping rooms, the smoke detection devices shall be centrally located in the main room. Smoke detection devices shall be located on or near the ceiling. NOTE: Smoke detection devices should be installed in those locations recommended by the manufacturer except in those cases where the space above the ceiling is open to the outside and little or no insulation is present over the ceiling. Such cases result in the ceiling being excessively cold in the winter time or excessively hot in the summer time. Where the ceiling is significantly different in temperature from the air space below, smoke has difficulty reaching the ceiling and to a detector which may be placed there. In this situation, placement of the detector on a side wall, with the top four inches to twelve inches from the ceiling is preferred. In dwelling units employing radiant heating in the ceiling, the wall location is the preferred location. Radiant heating in the ceiling can create a hot-air boundary layer along the ceiling surface which can seriously restrict the movement of smoke to a ceiling-mounted detector.

(2) A smoke detection device installed in a stairwell shall be so located as to assure that smoke rising in the stairwell cannot be prevented from reaching the detection device by an intervening door or obstruction.

(3) Smoke detection devices in rooms with ceiling slopes greater than one-foot rise per eight feet horizontally shall be located at the high side of the room.

(4) Smoke detection devices shall not be mounted in front of an air supply duct outlet or between the bedroom and the furnace cold air return.


212-10-050 Maintenance.

It is the responsibility of the occupant of all new or existing dwelling units, owned by other than the occupant, to maintain and test all smoke detection devices installed within the dwelling unit by the owner. Actual costs of maintenance, repair or replacement of smoke detection devices shall be as agreed beforehand by the occupant and owner. However, failure of the owner to abide by the terms of any such agreement does not relieve the occupant of the responsibility to maintain the smoke detection devices in a fully operational condition at all times. Failure to do so can subject the occupant to the penalty provisions of WAC 212-10-055.