Age-Dependent Responsiveness to Smoke Alarm Signals Among Children

Residential fires are an important cause of injury and death in the United States and children 5 to 12 years old have a higher residential fire fatality rate than teenagers and adults up to age 35 years.¹ Smoke alarms represent a core prevention strategy; however, when residential alarms were initially developed, the focus was on detection technologies for smoke or heat, with little attention directed toward effectiveness of the alerting signal, including during sleep.² Waking sleeping individuals and alerting them to a residential fire emergency is important because the rate of apartment fire-related mortality is 3 times higher during the sleep period of 1:00 am to 7:00 am than other times.³ Although the high-frequency tone alarms found in most households awaken adults, they are not adequately effective in waking children.^4–12  However, research by Smith, et al and Bruck, et al have demonstrated that children do respond to alarm signals other than the high-frequency tone. These alarm signals include a male voice, female voice, low-frequency tone, and a combination of a female voice and low-frequency tone.^{4,8–11,13,14}  In addition, these alternative alarm signals effectively awaken sleeping adults and older adults.^15,16 

Previous research among children has focused primarily on the signal characteristics of audible alarms, and although it has been established that children are more difficult to awaken from sleep than adults,¹² no attempt has been made to characterize how alarm responsiveness changes with age during the childhood years. The objective of this study is to evaluate the age-dependent responsiveness to various smoke alarm signals among children 5 to 12 years of age. This understanding can help guide prevention recommendations and efforts to reduce residential fire-related injuries and deaths among children.

The study population consisted of 540 children 5 to 12 years old, who were enrolled in 3 previous studies on smoke alarms for children.^8–10  Subjects for these studies were recruited via institution-wide emails in a large academic children’s hospital and study announcements via the hospital’s Facebook account. Children were eligible to participate if (1) they spoke English, and they did not (2) have a clinical diagnosis and were not taking a medication that might affect sleep, arousal, or their ability to perform the escape procedure, (3) have an acute illness at the time of the study, or (4) have a hearing impairment. Children received a pure tone hearing screening test on the first study night using a Maico MA25 portable audiometer; to participate in the study, children had to successfully respond to all tested frequencies of 500, 1000, 2000, and 4000 Hertz (Hz) at ≤30 decibel (dB) in both ears. This research was approved by the institutional review board of the authors’ institution. Written informed consent was obtained from children’s parents, and assent was obtained from children ≥9 years old.

Smoke alarm responsiveness was evaluated during the deepest stage of sleep. Sleep consists of 3 nonrapid eye movement stages (N1, N2, and N3) followed by a rapid eye movement (REM) stage that cycle throughout the night. N3 is the sleep stage relevant to these studies and is defined as high voltage (>75 microvolts peak-to-peak amplitude), slow wave (0.5–2 Hz) electroencephalography (EEG) activity accounting for ≥20% of a 30-second EEG or polysomnography (PSG) epoch, measured over the frontal regions.¹⁷ N3 sleep was formerly separated into stage 3 sleep (S3S) and stage 4 sleep (S4S) under an older nomenclature according to Rechtschaffen and Kales,¹⁸ in which N3 began with S3S and then progressed into S4S, which is characterized by the slow wave, high voltage (δ wave) activity accounting for >50% of a 30-second EEG and PSG epoch.

The 3 studies included in this study used identical methods (described previously^8–10 ) with a randomized, nonblinded, repeated measures design, in which, children were each exposed during S4S of separate sleep cycles to 4 smoke alarm signals. Each study used a different combination of 4 signals, which included: (1) human voice (mother’s voice, female stranger’s voice, or male stranger’s voice), (2) a hybrid voice-tone, consisting of a female stranger’s voice plus low-frequency tone, (3) low-frequency tone, and (4) high-frequency tone. The voice message used in the human voice and hybrid voice-tone alarms was “Fire! Fire! Wake up! Get out of bed! Leave the room!”

Although the low-frequency tone alarm was adopted in 2014 as the US standard for commercial sleeping areas, such as in hotels and motels, it is not the standard for residential sleeping areas unless being provided voluntarily for individuals with hearing loss.¹⁹ A high-frequency (approximately 3200 Hz) tone alarm is the alarm type currently found in most homes. The low-frequency (500 Hz square wave) alarm in this study was a Simplex 1996, 4100 Fire Alarm and is the same alarm previously used in studies by Proulx and Laroche,²⁰ Bruck, et al,²¹ and Smith, et al.^9,15 Alarm signals were amplified through small, smoke alarm-size speakers in the study bedrooms to provide consistent signals at 85 dB when measured at the pillow. This is louder than required by the National Fire Alarm and Signaling Code (NFPA 72) and the UL 217 Standard for Smoke Alarms, which is 85 dB at 10 feet.^20,22 The study was conducted in a sleep research center to standardize conditions; sleep rooms were comfortably furnished to resemble a typical residential setting.

Scalp and facial electrodes were attached to each child by a PSG technician to monitor stage of sleep. The EEG montage consisted of F3, F4, C3, C4, O1, O2, M1, and M2 electrodes. After bedroom lights were turned off, continuous EEG, electro-oculography, and chin electromyography via telemetry with synchronized low-light video monitoring were conducted.

Each child was allowed to progress into S4S and remained there for 5 minutes before an alarm was triggered. S4S is the sleep stage with the highest auditory arousal threshold (AAT), which is the intensity level in decibels of an auditory stimulus required to awaken an individual from sleep.¹² “Time-to-awaken” is the interval from the triggering of the alarm to the initiation of at least a 3-second arousal associated with movement and subsequent awake EEG. Subjects were taught an escape procedure on the night of the study, which was to get out of bed when awakened by an alarm, walk to the bedroom door, and exit. The interval from when the alarm was triggered until the child opened the bedroom door is the “time-to-escape.” If an alarm failed to awaken or prompt the subject to escape after 5 minutes, the child was awakened or escorted from the sleep room by research staff and the parent.

This procedure was conducted during the first and second sleep cycles on 2 separate study nights at least 6 days apart, resulting in each child being exposed to 4 different alarm signals (2 different signals each night). Testing on consecutive nights was not done to avoid possible confounding effects of sleep deprivation and altered sleep architecture. A senior certified PSG technician determined the “time-to-awaken” from EEG-video recordings. The sleep stage during which an alarm was triggered and the “time-to-awaken” interval were later reviewed and verified by 1 of the authors, who is a physician board-certified in sleep medicine, while blinded to the alarm used. No discrepancies were identified during this review.

For this study, we combined data from 3 of our previous child studies.^8–10  This was possible because the protocols and measurements were identical across studies. There was no overlap of subjects among studies; however, some children in this study received more than 1 alarm signal. Because children demonstrated a similar response to alarms using mother’s voice, female stranger’s voice, and male stranger’s voice (Table 1), these 3 alarm types were grouped into a “human voice” category during statistical analyses.

All statistical analyses were performed using SAS 9.4 (SAS Institute Inc., Cary, NC). The Kaplan-Meier estimator was used to estimate the probability functions for time-to-awaken and time-to-escape, which were censored after 5 minutes. Linear regression was used to assess the association between child’s age on a continuous scale and the proportion awakened and the proportion escaped. Cox regression models were used to assess the effect of age on time-to-awaken and time-to-escape in response to each of the 4 alarm signals. We used time-dependent variables in the Cox regression models if the proportional hazards assumption was violated. Statistical significance was determined at P < .05. Although age was treated as continuous for modeling, it was grouped into 4 categories (5–6, 7–8, 9–10, and 11–12 years old) for easy visualization of data during descriptive analysis.

There were 540 subjects; the median age was 9 years old, and 51.7% were male. Among participants exposed to a human voice alarm, 84.0% awoke and 83.0% escaped. Responses were similar among the subcategories of mother’s voice, female stranger’s voice, and male stranger’s voice alarms (Table 1). Among participants exposed to the hybrid voice-tone alarm, 88.8% awoke and 88.8% escaped, and among children exposed to the low-frequency tone alarm, 88.1% awoke and 85.8% escaped. In contrast, among subjects exposed to the high-frequency tone alarm, 53.2% awoke and 51.5% escaped (Table 2).

Child age showed a statistically significant (P < .05) effect on the proportion who awakened for each of the 4 alarm types. When comparing 5-year-olds to 12-year-olds, the proportion who awakened increased significantly (P < .05) by 62.2%, 90.0%, 28.9%, and 177.9% in response to human voice, hybrid voice-tone, low-frequency tone, and high-frequency tone alarms, respectively (Fig 1). The proportion of subjects who awakened increased significantly (P < .05) at a rate of 5.3%, 5.1%, 3.1%, and 7.6% for each 1-year increase in age from 5 years old to 12 years old in response to human voice, hybrid voice-tone, low-frequency tone, and high-frequency tone alarms, respectively (Table 3). Child age did not interact with the type of alarm in determining the proportion who awoke (P > .05). The proportion of subjects who awoke did not differ by sex for any of the alarm types (P > .05).

Similarly, child age showed a statistically significant (P < .05) effect on the proportion who escaped for each of the 4 alarm types. When comparing 5-year-olds to 12-year-olds, the proportion who escaped increased significantly (P < .001) by 66.2%, 90.0%, 36.5%, and 167.9% in response to human voice, hybrid voice-tone, low-frequency tone, and high-frequency tone alarms, respectively (Fig 2). The proportion of subjects who escaped increased significantly (P < .05) at a rate of 5.5%, 5.1%, 4.2%, and 7.4% for each 1-year increase in age from 5 years old to 12 years old in response to human voice, hybrid voice-tone, low-frequency tone, and high-frequency tone alarms, respectively (Table 3). Child age did not interact with the type of alarm in determining the proportion who escaped (P > .05). The proportion of subjects who escaped did not differ by sex for any of the alarm types (P > .05).

Median time-to-awaken and median time-to-escape decreased with increase in child age and this trend was observed for all 4 alarm types (Tables 1 and 2). Overall, the Kaplan-Meier cumulative probability functions for time-to-awaken (Fig 3) and time-to-escape (Fig 4) were significantly different for the 4 age groups in each of the 4 alarm type categories (Gray’s test, P < .001). The effect of child age on median time-to-awaken and median time-to-escape for the different alarm types is shown in Figs 5 and 6. The probability function of time-to-awaken was significantly affected by child age for the high-frequency (HR: 1.16, 95% CI: 1.05–1.28) and low-frequency (HR: 1.21, 95% CI: 1.08–1.37) tone alarms, and the effect of child age was time-dependent for the human voice (HR: 1.07, 95% CI: 1.03–1.10) and hybrid voice-tone (HR: 1.16, 95% CI: 1.04–1.19) alarms (Table 4). Similarly, child age showed a statistically significant effect on the probability function of time-to-escape for the low-frequency (HR: 1.42, 95% CI: 1.10–1.85) and human voice (HR: 1.33, 95% CI: 1.19–1.49) alarms, whereas the effect was statistically insignificant for the high-frequency (HR: 1.22, 95% CI: 0.99–1.49) and hybrid voice-tone (HR: 1.10, 95% CI: 0.85–1.41) alarms (Table 4).

If 80% is used as the threshold for success, none of the single-year age groups awakened or escaped at this threshold within 3 minutes to the high-frequency tone alarm. Among 12-year-old children, which is the age group with the highest response rate in this study, only 56.3% escaped within 1 minute and 67.6% escaped within 2 minutes to the high-frequency tone alarm (Fig 7). For the low-frequency tone alarm and using 80% escape success as a threshold, children younger than 12 years old did not meet this cutoff within 1 minute and children younger than 9 years old did not meet this cutoff within 2 minutes. If 90% escape success was used as a cutoff, none of the children met this cutoff within 1 minute and children younger than 12 years old did not meet this cutoff within 2 minutes for the low-frequency tone alarm. For the human voice alarms and using 80% escape success as a threshold, children younger than 9 years old did not meet this cutoff within 1 or 2 minutes. If 90% escape success was used as a cutoff, children younger than 12 years old did not meet this cutoff within 1 minute and children younger than 9 years old did not meet this cutoff within 2 minutes for the human voice alarms. Finally, for the hybrid voice-tone alarm and using 80% escape success as a threshold, children younger than 7 years old did not meet this cutoff within 1 minute and children younger than 6 years old did not meet this cutoff within 2 minutes. If 90% escape success was used as a cutoff, children younger than 9 years old did not meet this cutoff within 1 or 2 minutes for the hybrid voice-tone alarm; however, children 9 years and older had an escape success rate of >96% at 1 and 2 minutes (Fig 7).

The proportion of children who awakened demonstrated a statistically significant increase with increasing child age for each of the 4 alarm types. The proportion of subjects who awakened increased by 3.1% to 7.6% for each year of age between 5 and 12 years old for the 4 alarm types. Although the rate of increase was greatest for the high-frequency tone alarm, its performance was still inferior to that of the other alarm types at 12 years of age. The proportion of children who successfully performed the escape procedure also demonstrated a statistically significant increase with increasing child age for each of the 4 alarm types. The proportion of subjects who escaped increased by 4.2% to 7.4% for each year of age between 5 and 12 years old for the 4 alarm types. The proportions of subjects who awoke and who escaped did not differ statistically by sex for any of the alarm types.

The age-related changes in smoke alarm waking effectiveness in our studies are consistent with previous findings by Busby, et al,⁶ who showed that AATs were greater for 5 to 7-year-olds than 8 to 12-year-olds during different stages of sleep using a 3-second 1500 Hz “pure” tone stimulus. However, our study is the first to demonstrate changes in responsiveness among children to different types of alarm signals by single-year-of-age and sex. The observed age-related changes in smoke alarm waking effectiveness may be associated with age-related changes in slow oscillations (SOs) of the brain during N3 sleep. SOs are synchronized EEG waves that predominate during N3 sleep with a frequency from 0.5 Hz to 1.0 Hz and are associated with increased AATs.¹² SOs are higher amplitude in younger children and there is a shift in the concentration of slow wave activity from the centro-parietal region in young children to more frontal locations by adolescence.²³ These changes may alter the responsiveness to auditory stimuli by different brain regions and neural pathways during sleep and influence arousal from sleep.

High-frequency tone alarms are the predominant type of smoke alarm currently in US homes. When evaluating responsiveness to this type of alarm, children younger than 8 years old had a median time-to-awaken greater than 5 minutes and children younger than 9 years old had a median time-to-escape greater than 5 minutes. Per study protocol, 5 minutes after alarm initiation, children were manually awakened if still sleeping or escorted from the sleep room if awake but still in the room; therefore, a median time of 301 seconds (the value used in study analyses for times greater than 5 minutes) underestimates what the duration may have been without physical intervention. Median response times to the high-frequency tone alarm did not approach those of the other alarm types until children were 11 to 12 years old (Figs 5 and 6). Compared with 2.0 and 12.0 seconds, respectively, among 20 to 49-year-olds from another study using identical study methods, the median time-to-awaken and time-to-escape in response to a high-frequency tone alarm were 60.5 and 147.5 seconds, respectively, among 9 to 10-year-olds and 9.5 and 47.5 seconds, respectively, among 11 to 12-year-olds (Table 2).¹⁵ 

In a previous study, the low-frequency tone was marginally better at awakening children but had a slightly longer time-to-escape than the female stranger’s voice alarm.⁹ Because the content of the voice alarm message may provide valuable urgency and instructional information regarding life-saving escape behaviors to a child during the sleep inertia-associated confusion experienced upon awakening,^24,25 we hypothesized that there may be advantages to combining these signals into a hybrid voice-tone alarm. The comparative performance of these different alarm types can be seen in the cumulative probability function curves by age group and type of alarm for time-to-awaken and time-to-escape (Figs 3 and 4). When examining Figs 3 and 4, 1 measure of alarm effectiveness is the area under the curve, which demonstrates the marginal superiority of the hybrid voice-tone alarm across the age groups for both time-to-awaken and time-to-escape.

Fire safety professionals warn that individuals have as little as 1 to 2 minutes to escape after a smoke alarm sounds.²⁶ Using this as the criterion for smoke alarm success, results in Fig 7 show the superiority of the hybrid voice-tone alarm, which awakens and prompts escape among greater proportions of children and at younger ages than the other alarm types. It also demonstrates the stark lack of effectiveness of the high-frequency tone alarm, which is the type found in most homes in the US. When considering the devastating outcomes that can occur if an individual does not promptly escape a residential fire, a smoke alarm should have a high success rate in awakening and prompting escape of sleeping occupants. The hybrid voice-tone alarm achieved success rates of >96% among children 9 years and older, which was superior to the performance of the other alarms. Therefore, these data indicate that the high-frequency tone alarm is inadequate for awakening and prompting escape among children 12 years old and younger; this may be true for older children as well, but the maximum age of children in this study was 12 years. These data also demonstrate that the hybrid voice-tone signal is the most effective alarm for children 9 years and older. It is important to remember that a fire escape plan that includes adult rescue of children is strongly recommended regardless of a child’s age because layers of protection (ie, multiple back-up protection strategies) are key to effective injury prevention.²⁷ 

This study demonstrates that not only does the response to alarm stimuli change with child age, but that the response is influenced by the type of alarm stimulus. A study by Portas and colleagues may offer insight into why there is a differential response based on alarm type.²⁸ They monitored seven subjects 23 to 34 years old using simultaneous EEG and functional MRI across the sleep-wake cycle and captured responses to a repeated pure tone 1400 Hz sine wave stimulus and a repeated presentation of the subject’s own first name, both delivered at 80dB and 500 milliseconds in duration. During non-REM sleep (all non-REM sleep stages were collapsed into 1 category during analysis), the left amygdala and left prefrontal cortex were more activated by the first-name voice stimulus than the tone. This indicates that different auditory stimuli affect some regions of the brain differently, which may explain the differential effectiveness of the various alarm stimuli to awaken children in our studies. As a child’s brain matures over time, it is possible that the differential response of these brain regions to various alarm stimuli becomes more homogeneous or that the contribution of these brain regions to arousal from sleep changes. Additional research is needed on the changes during childhood that occur in the neural pathways and brain regions involved in monitoring auditory stimuli during sleep and the effect of these changes on arousal from sleep. Regardless of the mechanisms involved, this is a reminder that child development can profoundly affect the effectiveness of injury prevention strategies; what works for adults or even teenagers, such as motor vehicle airbags,²⁹ may not work for younger children.

Portas et al suggested that the left amygdala and left prefrontal cortex were more activated by the stimulus with special affective significance (individual’s first name) compared with the tone stimulus. This interpretation is supported by research demonstrating a heightened human response to one’s own first name during wakefulness and sleep, starting as early as infancy.^28,30–39  Additionally, the fetus and newborn demonstrate a selective preference for their mother’s voice, which is thought to be attributable to prenatal exposure to her voice.^40–43  In a study of 4-month-old infants, Purhonen and colleagues demonstrated a different change in the amplitude of auditory event-related potentials among infants exposed to mother’s voice compared with those exposed to the voice of a female stranger, suggesting a differential cerebral processing of mother’s voice.⁴⁴ Based on these studies, we initially hypothesized that children would respond better to smoke alarm signals during S4S that included the child’s first name or mother’s voice; however, this was not the case.^8,9 Children 5 to 12 years old responded similarly to alarm stimuli with or without use of the child’s first name and to a human voice, regardless whether the voice was that of the mother or a stranger, male or female.^8–10  The saliency of these stimuli appear to be generally equivalent during deep sleep among children and clearly out-performed the high-frequency tone alarm. Interestingly, the low-frequency tone alarm (alone or as a hybrid alarm in combination with a female voice) performed similarly to the voice alarms and better than the high-frequency alarm. The reason that the low-frequency tone alarm is effective in awakening children is unknown but may be related to its fundamental frequency of 520 Hz being more closely aligned with that of human speech (males 85–155 Hz; females 165–255 Hz) than the high-frequency tone alarm (>3000 Hz).⁴⁵ Another theory proposes that stimuli with increased complexity of the frequency spectrum, such as the low-frequency square wave tone alarm, are more effective than “pure” tones, such as the high-frequency tone alarm.⁴⁶ 

This study on smoke alarms can also inform treatment of enuresis, which is another issue that affects this same age group during sleep. Nocturnal enuresis is common during childhood, affecting up to 20% of 5-year-olds, and alarm therapy is the first-line treatment of this condition.^47–49  Smoke alarm signals that effectively awaken children may be useful as signals for enuresis alarms.

This study had some limitations. The alarm intensity was 85 dB at the pillow, which is louder than required by the National Fire Alarm and Signaling Code (NFPA 72) and the UL 217 Standard for Smoke Alarms. In addition, study participants knew that they were going to be awakened by alarm signals and rehearsed the escape procedure immediately before falling asleep; such “priming” may have affected results and generalizability of findings.

This study provides important new information to help guide prevention recommendations and efforts to reduce residential fire-related injuries and deaths among children. It also may help inform development of more effective enuresis alarms. Age substantially influences the effectiveness of audible smoke alarms during childhood, whereas child’s sex does not. Young children are resistant to arousal during slow wave sleep, especially in response to the high-frequency tone alarm found in most homes. Among 12-year-old children, which is the age group with the highest response rate in this study, only 56.3% escaped within 1 minute and 67.6% escaped within 2 minutes to the high-frequency tone alarm. However, the hybrid voice-low-frequency tone alarm is >96% effective at awakening and prompting escape within 1 minute among children 9 years and older. A fire escape plan that includes adult rescue of children is strongly recommended.

Abbreviations

AAT

auditory arousal threshold

CI

confidence interval

dB

decibel

EEG

electroencephalography

HR

hazard ratio

Hz

hertz

IQR

interquartile range

PSG

polysomnography

REM

rapid eye movement

SO

slow oscillation

S3S

stage 3 sleep

S4S

stage 4 sleep