Research Summary of Smoke Detectors & Hearing Loss. 

In a Hurry? Quick Snapshot Summary 

  • Standard signal emitted by residential smoke detectors falls between 3000 - 4000 Hz1
  • Nearly 50% of adults with hearing loss do not wake up to a 3100 Hz tone presented at intensity levels meeting current Federal Fire Codes
  • The most effective signal in waking adults with hearing loss is a 520 Hz square wave
  • A bed shaker at pillow level is essentially as effective as a 520 Hz square wave stimulus over a period of 180 seconds 
  • The least effective signal in waking adults is a strobe light
  • Combining a 520 Hz square wave auditory stimulus at bedside with a bed shaker at pillow level is the most ideal signal for waking adults with hearing loss 
  •  If a 520 Hz square wave is not available, any system designed to generate a loud auditory alarm in conjunction with a bed shaker at pillow level in response to an activated smoke detector is anticipated to be significantly more effective than current residential smoke detectors alone
  • CALL TO ACTION: 1) educate patients about latest research on how hearing loss may interfere with responding to an active smoke detector during sleep at a time when hearing instruments are not worn (see SMOKE DETECTOR FACTS FOR PATIENTS) and 2) recommend viable BEDSIDE SMOKE DETECTOR SOLUTIONS including HomeAware, Central Alert, or AlertMaster, that generate combined auditory (ideally 520Hz square wave) and vibrotactile stimulation (via bed shaker placed ideally at pillow level)

 

Introduction 

Over the past decade, a relatively significant amount of research has been published on the effectiveness of smoke detectors in alerting sleeping individuals with and without hearing loss.  These studies were designed to compare the efficacy of signal type (i.e. auditory versus vibrotactile versus visual) in waking sleeping individuals with varying degrees of hearing loss. The following is a summary of some of the research conducted by Dorothy Bruck and her colleagues of the Centre for Environmentral Safety and Risk Engineering (CESARE) at Victoria University (Australia) as compiled by Bankaitis.2  While other studies are available, the investigations from CESARE consistently and carefully controlled for variables such as method of stimulus presentation and sleep stages of subjects. 

How well do adults with normal hearing respond to smoke detectors?

Based on the number of studies published over the past two decades, adults with normal hearing wake up fairly quickly to an activated residential smoke detector.3,4  As illustrated by the green bar in the bar graph below, 100% of adults with normal hearing woke to a standard smoke detector signal alarm within 32 seconds of alarm initiation. While this research summary is focusing on adults with varying degrees of hearing loss, the same study also looked at how well normal hearing children (11-16 years of age) responded to an active smoke detector while sleeping. Whereas 100% of normal hearing adults awoke within 32 seconds of alarm activation, as indicated by the purple bar in the same figure, only 17% of children awoke to a smoke detector within that same 32 second time period. 

Bruck-normalhearing-cumulative-1

Since smoke detectors must continually emit the T3 pattern for at least 180 seconds, Bruck (2010) documented arousal patterns in response to a smoke detector over a period of 180 seconds to determine how many additional normal hearing individuals woke up to an active smoke detector beyond the initial 32 second time interval (figure above right). The first green dot indicates that 100% of adults successfully woke up to a smoke detector within the first 32 seconds of activation; since all normal hearing adults were awake within that time period, the green dots at subsequent time intervals (33-60 seconds, 61-90 seconds), 91-120 seconds, 121-150 seconds, 151-180 seconds) are also plotted at 100%.  With regard the children, the first purple triangle reflects the 17% of normal hearing children who successfully woke up in response to an active smoke detector during the 32 second time period from the bar graph.  As illustrated by the other triangles plotted as a function of cumulative time, be the end of the 180 second time interval, an additional 6% of children (total of 23%) successfully woke up in response to an active smoke detector. In other words, 77% of children with normal hearing did not respond to an active smoke detector after three full minutes (see Fire Facts on timeline of evacuating a residential fire once smoke detector activated).

 

How well do adults with mild to moderately-severe hearing loss respond to smoke detectors?

Bruck and Thomas5 investigated the effectiveness of a variety of auditory, vibrotactile, and visual signals in waking up 38 adults with bilateral, mild to moderately severe sensorineural hearing loss falling within the highlighted area in the audiogram below.  The green bars in the graph below show the percentage of subjects who woke up within the first 30 seconds of exposure to a particular alarm delivered at benchmark which represents intensities consistent with NFPA 72 minimum requirements. Six types of alarms were used in the study including a strobe, bed shaker at torso level, bed shaker at pillow level, and three different auditory signals. Two low frequency square waves signals (400 Hz and 520 Hz) were used based on previously published research. The third auditory signal was a 3100 Hz pure tone which was included as it represents the signal signal emitted by most commercially available smoke detectors. Eighty seven percent (87%) of adults with mild to moderately severe sensorineural hearing loss woke up to a 400 Hz square wave signal and (92%) of the same subjects woke up to a 520 Hz square wave signal presented at benchmark within the first 30 seconds of signal activation.  In contrast, only 56% of adults woke up to the 3100 Hz pure tone. When the same signals were presented at maximum intensity levels (higher than NFPA 72 intensity requirements) for a full 180 second alarm cycle as reflected in the red bars, 100% of adults woke up to a 520 Hz square wave, 95% to the 400 Hz square wave and 84% to the 3100 Hz alarm.

audiblesignals-2009

Two vibrotactile-only stimuli were assessed individually for each subject in the same study whereby one position involved a bed shaker positioned at torso level and the other at pillow level. As reflected by the blue bars in the graph below, a vibrotactile-only stimulus in either position essentially yielded similar findings with 80% and 84% of adults with hearing loss waking up within the first 30 seconds of alarm activation respectively.  The percentage of adults who woke up to the same vibrotactile stimuli at maximum allowable levels over a period of 180 seconds is depicted by the orange/brown bars. A total of 89% woke up in response to the torso level bed shaker whereas 97% woke up to a bed shaker at pillow-level. 

vibrotactile-2009

A visual-only stimulus in the form of a strobe light resulted in 27% of adults with hearing loss to wake up within the first 30 seconds. Exposure to a visual stimulus for the full 180 seconds with the addition of two more strobe lights improved the wake up rate but only to 57%. 

visual-2009

What is the best signal? 

Whether comparing benchmark level performance or maximum alarm level performance, the 520 Hz square wave represents the most effective signal in waking adults with mild to moderately severe hearing loss.  The least effective signal was a visual only strobe light. The efficacy of the 3100 Hz pure tone in this study is of particular interest since research indicates that nearly 50% of adults with hearing loss did not wake up to the 3100 Hz pure tone signal at benchmark. Since the standard signal emitted by most commercially available residential smoke detectors falls between 3000 - 4000 Hz, the ability for patients with even a mild to moderately severe hearing loss to wake up to a smoke detector in a timely fashion is of significant concern. Combining a 520 Hz square wave auditory stimulus at bedside with a bed shaker at pillow level represents the most ideal signal for waking adults with hearing loss a smoke detector.  In the event a product that emits a 520 Hz square wave is not available, any system designed to generate a loud auditory alarm at bedside in conjunction with a bed shaker at pillow level in response to an activated smoke detector is anticipated to be more effective than a traditional smoke detector alone.   

Call To Action:  

  • Educate patients with hearing loss on increased risk of failing to be aroused from sleep in response to an active smoke detector. See SMOKE DETECTOR FACTS FOR PATIENTS as a reference


References:

  1. Lee, A. (2005). The audibility of smoke alarms in residential homes.  CPSC-ES-0503. 
  2. Bankaitis, A.U. (2010).  Alarming facts about smoke detectors.  Live webinar presented on AudiologyOnline.com, February 3, 2010.
  3. Bruck, D., Ball, M., Thomas, I., & Rouillard, V. (2008). How does pitch and pattern of a signal affect auditory arousal thresholds? Journal of Sleep Research, June 18(2):196-203.
  4. Bruck, D. (1998).  Arousal from sleep with a smoke detector in children and adults (Technical report FCRC-TR 98-04). Sydney, Australia: Fire Reform Center. 
  5. Bruck, D. & Thomas, R. (2009). Smoke alarms for sleeping adults who are hard-of-hearing: comparison of auditory, visual, and tactile signals. Ear & Hearing, 30(1),73-80.