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Tremendous effort has been invested in the laboratory to ensure side air bag (SAB) deployments minimize injury metrics in pediatric anthropometric test devices (ATDs). Little is known, however, about the experience of children exposed to this technology in real world crashes. Therefore, the objective of this study was to determine the prevalence of SAB exposure in children and provide estimates of injury risk among those exposed. This study utilized data from the Partners for Child Passenger Safety study, a large-scale child-focused crash surveillance system, to identify a probability sample of 348 child occupants, age 0–15 years, weighted to represent 6,600 children, in vehicles of model year 1998 and newer, equipped with SABs, in side impact crashes from three large U.S. regions between 1/1/05 and 12/31/06. In the study sample, 27 children per 1000 children in crashes were exposed to a deployed side airbag. Over 75% of these children were seated in the rear seat and 83% were exposed to a head curtain SAB. 65% of those exposed were less than 9 years of age. Of those exposed, 10.6% sustained an AIS2+ injury; all injuries were of the AIS 2 level and limited to the head or upper extremity. This paper provides the first population-based estimates of the exposure of children to SABs. Initial experience suggests that the risk of injury is fairly low with only one in ten sustaining injury – none of which were serious or life threatening. These findings offer assurance that efforts by regulators and the automotive industry to minimize negative consequences from SABs to vulnerable occupants appear to be effective and cause no change in the current recommendation of safe seating for children next to SABs.
Side air bags (SABs) were introduced in the mid 1990’s as a safety strategy to reduce serious injuries and fatalities occurring in side impact crashes. The first SAB was the torso SAB. It was designed to cover the thoracic region although design varied relative to total coverage area. Initial crash tests involving vehicles equipped with this technology revealed that the head was still at risk for serious injury in side impact crashes. (Dalmotas et al. 2001; Prasad et al. 2001) The desire for head protection led to air bags with coverage area extending to the head, so–called combination torso-head SABs. In order to maximize the protection of the head for adult front and rear seat occupants of a variety of statures and seating postures, the roof-rail or head curtain air bag was developed and has become the preferred head protection system for side impact crashes. These systems, frequently accompanied by a separate torso SAB, provide more extensive coverage of the upper vehicle side interior, often extending the entire length of the vehicle including the rear rows. SABs have become common safety technology in the vehicle; 79% of model year 2006 vehicles have some type of SAB either as standard or optional equipment. (McCartt and Kyrychenko in press)
Crash tests from several organizations including the National Highway Traffic Safety Administration (NHTSA), the Insurance Institute for Highway Safety (IIHS), and Transport Canada have pointed to the benefit of SABs, especially those that are directed toward protecting the adult head. (Dalmotas et al. 2001; Prasad et al. 2001) As this is new technology, only a few studies have used field data to evaluate the effectiveness of SAB technology for adult drivers and passengers. Several groups from multiple countries have conducted analyses of a number of individual cases of adult drivers and passengers exposed to deploying SABs to provide an initial assessment of SAB field performance. (Otte and Hufner 2007, Kirk and Morris 2003, Yoganandan et al. 2005) In general, these studies found that SAB deployment was not the cause of injury to the occupants and head protection SABs led to reduced likelihood of head injury.
Other researchers used more population-based datasets to estimate SAB effectiveness. McGwin et al reviewed data from the National Automotive Sampling System (NASS) General Estimates System (GES) and found no benefit in injury reduction for SABs. (McGwin et al. 2003) This analysis however had substantial limitations including using a poor indicator of injury, the assumption that all vehicles with optional side air bags were equipped, and not distinguishing among the types of side air bags. These researchers attempted to improve upon their earlier studies and analyzed NASS Crash Data System (CDS) to provide a better assessment of injury and found a non-significant benefit of SABs in reducing injuries to the head and thorax. (McGwin et al. 2004) This study suffered from many of the same limitations however, most importantly the misclassification of optional SABs as standard equipment.
Braver and Kyrychenko used data from the Fatal Analysis Reporting System to estimate fatality benefits for SABs for adult drivers. (Braver and Kyrychenko 2004) Their analysis, with a more accurate assessment of side air bag availability and deployment, found an 11% reduction in fatalities for torso SABs and a 45% reduction in fatalities for head SABs. In addition, they attempted to account for the confounding associated with the other potential safety improvements and demographic effects in vehicles equipped with SABs that would reduce the overall fatality risk. McCartt and Kyrychenko built on this previous work and recalculated the fatality benefit for SABs using an enhanced adjustment method. (McCartt and Kyrychenko in press) In their study, for passenger cars, torso SABs reduced fatality risk by 26% and head SABs by 37%. Similar benefits were seen for SUVs. NHTSA recently conducted an analysis of side impact protection with a focus on SAB technology. (Kahane 2007) They determined that SABs resulted in a reduction of struck side fatality risk of 18% in multi-vehicle crashes and substantial improvement in a thoracic injury metric, the Thoracic Trauma Index (TTI), in laboratory assessments. Benefits were greater for head SABs than those with torso SABs alone.
All of these previous studies have been based on analysis of protection for adults. A concern about adverse effects of SABs on children, in part fuelled by the unintended consequence of pediatric injuries and fatalities associated with the initial introduction of frontal air bags (Winston and Reed 1996), dictated an effort to ensure SAB deployments maximize benefits for adult occupants while causing no negative outcome for children. Child safety with SAB deployment has exclusively been studied in the laboratory through efforts to minimize injury metrics in pediatric anthropometric test devices (ATDs). In 1999, NHTSA issued a warning regarding the potential for serious injury to children associated with SAB deployment and called on the industry to develop voluntary standards so that future SAB systems did not pose serious injury risks to vulnerable vehicle occupants. (Prasad et al. 2001) As a result, the Side Airbag Out-of-Position Injury Technical Working Group (TWG) chaired by IIHS developed a standard set of test procedures, including the specification of out-of-position (OOP) occupant positions and injury criteria, for assessing side airbag inflation injury risk to children. This work was informed by many organizations that conducted both static and dynamic sled and crash tests with pediatric ATDs exposed to deploying SABs. (Pintar et al. 1999; Tylko and Dalmotas 2000; Prasad et al. 2001; Tylko and Dalmotas 2001) In part promoted by the evolution of SAB designs described above, the door-mounted torso SAB became less prevalent due to the poor outcome in the OOP conditions for this design. (Yoganandan et al. 2005)
While the effort to ensure the protection of children exposed to SAB deployments represented a tremendous dedication to the issue by many stakeholders, it relied on laboratory assessment using pediatric ATDs with known biofidelity limitations. The ultimate goal, however, is protection of children in the real world. Little is known, however, about the experience of children exposed to SAB technology in actual crashes. In NHTSA’s Special Crash Investigation Division, a focus has been placed on identifying crashes with SAB deployment in which injuries were attributed to the air bag. As of March 2007, only 8 cases of children less than 16 years of age have been identified as being exposed to a deploying SAB and of these, none sustained any serious injuries. (National Highway Traffic Safety Administration 2007) The recent NHTSA report examined the issue of children and SABs and reported on the lack of cases of side impacts with SAB deployment that raise concern about a negative interaction between the child and SABs. (Kahane 2007) Specifically, they were looking for fatal injuries involving crashes of low speeds or severity that would not ordinarily result in fatalities, or any reports of injuries that could be related to deploying air bags. None of the cases they reviewed raised any questions about the performance of the SAB. These previous studies are encouraging, however, they represent an analysis of a number of individual cases and are not population-based assessments of the experience of children exposed to SAB deployment. The objective of this study was to use a large child crash surveillance system to provide initial estimates of the number of children at risk of exposure to SAB deployments and provide estimates of injury risk among those exposed.
Data for the current study were drawn from the Partners for Child Passenger Safety (PCPS) program, collected from January 1, 2005 to December 31, 2006. A description of the study methods has been published previously. (Durbin et al. 2001) PCPS consists of a large scale, child-specific crash surveillance system: insurance claims from State Farm Insurance Co. (Bloomington, IL) function as the source of subjects, with telephone survey and on-site crash investigations serving as the primary sources of data. Vehicles qualifying for inclusion were State Farm-insured, model year 1990 or newer, and involved in a crash with at least one child occupant 15 years of age or younger. Qualifying crashes were limited to those that occurred in 15 states and the District of Columbia, representing three large regions of the United States (East: NY, PA, DE, MD, VA, WV, NC, DC; Midwest: OH, MI, IN, IL; West: TX, CA, NV, AZ). After policyholders consented to participate in the study, limited data were transferred electronically to researchers at CHOP and the University of Pennsylvania. Data in this initial transfer included contact information for the insured, the ages and genders of all child occupants, and a coded variable describing the level of medical treatment received by all child occupants as reported by the policyholder (no treatment, physician’s office or emergency department only, admitted to the hospital or death).
A stratified cluster sample was designed in order to select vehicles (the unit of sampling) for the conduct of a telephone survey with the driver. Vehicles containing children who received medical treatment following the crash were over-sampled so that the majority of injured children would be selected while maintaining the representativeness of the overall population. If a vehicle was sampled, all child occupants in that vehicle were included in the survey. Drivers of sampled vehicles were contacted by phone and, if a passenger had received medical treatment, screened via an abbreviated survey to verify the presence of at least one child occupant with an injury. All vehicles with at least one child who screened positive for injury and a 10% random sample of vehicles in which all child occupants who were reported to have received medical treatment but screened negative for injury were selected for a full interview; a 2.5% sample of crashes where no medical treatment was received were also selected. The median length of time between the date of the crash and the completion of the interview was six days, with 95% of interviews completed within 47 days of the crash. The study protocol was reviewed and approved by the Institutional Review Boards of both The Children’s Hospital of Philadelphia and The University of Pennsylvania School of Medicine.
For a subset of cases in which child occupants were admitted to the hospital or killed, in-depth crash investigations were performed. Cases were screened via telephone to confirm the details of the crash and a full-scale on-site crash investigation was conducted using custom child-specific data collection forms. For the purposes of this analysis, these cases were used to examine the validity of information obtained from the telephone survey. The eligible study population consisted of 239,915 children riding in 168,167 State-Farm-insured vehicles newer than 1990 reporting a crash claim between January 1, 2005 and December 31, 2006.
Restraint status and seat position of children was determined from the telephone survey. Among the 161 children for whom paired information on restraint use was available from both the telephone survey and crash investigations, agreement was 88% between the driver report and the crash investigator (kappa statistic=0.74). Among the 170 children for whom paired information on seating position (front versus rear) was available from both the telephone survey and crash investigations, agreement was 99% between the driver report and the crash investigator (kappa statistic=0.99).
Direction of first impact was derived from a series of questions regarding the vehicle parts that were involved in the first collision. Side impact crashes were defined as crashes in which the vehicle parts involved in the first collision were on the lateral planes of the vehicle. Crash severity was categorized by driver report of intrusion into the occupant compartment of the vehicle.
Survey questions regarding injuries to children were designed to provide responses that were classified by body region and severity based on the Abbreviated Injury Scale (AIS) score (AAAM 1998), and have been previously validated for their ability to distinguish AIS 2 or greater from less severe injuries. (Durbin et al. 1999) For the purposes of this study, children were classified as seriously injured if a parent/driver reported a clinically significant injury: any injury with an AIS score of 2 or greater (concussions and more serious brain injuries, all internal organ injuries, spinal cord injuries, and extremity fractures). Injuries included all those injuries sustained by the child occupant, not just those injuries that could be attributed to the air bag deployment.
The availability of a SAB was determined from decoding the Vehicle Identification Number (VIN). The VIN in combination with the Vehicle Features database maintained by the Highway Loss Data Institute (www.hldi.org) provided information on whether or not the vehicle was equipped with a SAB, including whether they were standard or optional. To be conservative, only vehicles in which the SAB was standard were considered equipped with SABs. Deployment of the SAB was determined from survey responses.
The study sample for this analysis was limited to vehicles of model year 1998 and newer as this represented a time period in which SABs began to appear in vehicles. Subjects of interest were children seated in all outboard seat positions in these vehicles that were equipped with SABs and were involved in a side impact crash.
Analyses assessing risk of injury were limited to side impact crashes with a child occupant in an outboard seat position who was exposed to a deployed SAB. Children exposed to SABs were included in the analysis regardless of whether they were on the struck side or non-struck side of the crash. Because sampling was based on the likelihood of an injury, subjects least likely to be injured were underrepresented in the study sample in a manner potentially associated with the predictors of interest. (Korn and Grubard 1995) To account for this potential bias, sampling weights equal to the inverse of the probability of selection were used in the computation of the logistic regression model parameters. To compute p-values and 95% confidence intervals to account for the stratification of subjects by medical treatment, clustering of subjects by vehicle, and the disproportional probability of selection, Taylor Series linearization estimates of the logistic regression parameter variance were calculated using SAS-callable SUDAAN®: Software for the Statistical Analysis of Correlated Data, Version 9.0 (Research Triangle Institute, Research Triangle Park, NC, 2005).
Ideally, in order to place the injury rate to those exposed to a deploying SAB in context, a comparison group of children in outboard seat positions in side impact crashes in comparable vehicles not equipped with SABs should be used. Because of the well-described relationship between crash severity and injury risk, identifying crashes of similar severity to the SAB deployments is important. The telephone survey methodology of PCPS does not lend itself to quantitative determination of change in velocity or maximum intrusion – two accepted methods to assess crash severity. In the absence of these measures, two other crash-severity metrics can be used – the presence of intrusion or the fact that the vehicle needed to be towed from the scene. Thus, we compared the injury risk to children exposed to a deploying SAB to two comparison groups of children seated in the struck side seat positions in vehicles of model year 1998 or newer, not equipped with SABs, involved in side impact crashes where 1) the vehicle was towed and 2) intrusion in the occupant compartment was reported. The Pearson chi-square test was performed to assess differences in the distribution of categorical variables among the two restraint equipment types.
The study sample for these analyses included children age ≥15 years seated in outboard seating positions in vehicles of model year 1998 and newer, equipped with SABs and involved in side impact crashes (n=231 vehicles containing 348 children, weighted to represent n=4,742 vehicles containing 6,600 children). No children in pickup trucks were observed, so the remainder of the analyses was limited to those children in passenger cars, SUVs, and minivans only. Of these, a total of 180 children (31 unweighted) were identified as exposed to a deploying SAB, representing 27 children per 1000 side impact crashes meeting the inclusion criteria described above. When restricting the study sample to those children seated on the struck side of the crash only (n=129 vehicles containing 186 children, weighted to represent n=2,545 vehicles containing 3,468 children), this percentage approximately doubles to 52 children per 1000 crashes. Vehicle, crash and child characteristics of the 180 children exposed to a deploying SAB are described in Table 1. Of note, none of the children exposed to SABs was unrestrained.
Of the 180 children exposed to a deploying SAB in a side impact crash, 10.6% of them sustained an AIS 2+ injury. The body regions of injury for these children were as follows: Head 42.1% or Upper Extremity 57.9%. Of the head injuries, all were either concussions or altered consciousness.
Two comparison groups were chosen: those children seated on the struck side of the crash in model year 1998+ vehicles not equipped with side air bags 1) in tow-away crashes and 2) in crashes with intrusion. The vehicle, crash, and child characteristics of these comparison groups are shown in Table 2. None of the variables were significantly different than the sample group of children exposed to deploying SABs except for vehicle model year distribution. The injury risk to those children in comparison group 1 was 5.7% and comparison group 2 was 8.0%. None of these injury risks were statistically different from the injury risk to children exposed to deploying SABs (SAB group vs. comparison group 1: OR=2.27, 95% CI (0.60–8.60); SAB group vs. comparison group 2: OR=1.55, 95% CI (0.38–6.26)). Of note, there were approximately 5% unrestrained children in each of the comparison groups. Although this was not an exclusion criterion for the SAB exposed group, none of the children exposed to SAB were unrestrained. To consider the effect of this on the data, we excluded the unrestrained children from each of the comparison groups and recalculated the injury risk. This resulted in little change (comparison group 1: 5.2% and comparison group 2: 7.9%). Children from both comparison groups sustained injuries to all body regions. (Figure 1)
This study provides the first population-based child-specific field assessment of side air bags. In the sample of model year 1998 and newer vehicles equipped with SABs and involved in a side impact crash, 27 children per 1000 side impact crashes were exposed to a deploying SAB. When the sample was limited to those children on the struck side of the crash, this percentage approximately doubled to 52 children per 1000 crashes. Approximately 83% of these children were exposed to a head curtain air bag. This percentage is somewhat over represented compared to adults (~60% in McCartt and Kyrychenko, 2006), as almost 80% of the children exposed to a SAB were in the rear rows of the vehicle, where torso SABs are uncommon.
These data represent a single time window (2005–2006) and the rates of children exposed to deploying SABs may increase over time as SABs, particularly multiple-row head curtain air bags, become standard equipment in new vehicles. In this study, 88% of those children exposed to a deploying SAB were in passenger cars and 34% were in model year 2004 and newer. Based on the recent PCPS Fact and Trend Report, in 2005, passenger cars represent only 47% of the vehicles overall in the PCPS study. (Children’s Hospital of Philadelphia 2006) As more SUVs and minivans come equipped with standard SABs, more families choose the optional SAB in these vehicles and those vehicles currently equipped with SABs penetrate the fleet, we expect the exposure numbers to increase. It will be important to reassess these estimates in the future as availability and SAB deployment design decisions potentially evolve.
Approximately 1 in 10 children exposed to a deploying SAB sustained an AIS 2+ injury. None of the injuries sustained by children exposed to deploying SABs were serious or life threatening; they were limited to upper extremity fractures and concussions with brief loss of consciousness. These children spanned the pediatric age range with 65% being less than 9 years of age. Children less than 9 years of age are likely of a stature significantly less than the smallest test dummy currently used in side impact regulation - the 5th percentage female dummy. This finding may suggest that efforts to improve side impact protection through SAB implementation for adult occupants are being transferred to children as well.
The SAB TWG procedures are directed toward minimizing injury metrics for the head, chest, and the neck. (Prasad et al. 2001) This early look at injuries sustained by children exposed to SABs suggests that this effort has been effective. There were no neck or chest injuries and the head injuries sustained were not at an AIS3+ level towards which the TWG efforts are focused.
Two comparison groups were used to provide context for injury rate of children exposed to deploying SABs: those in struck-side crashes in vehicles of model year 1998 and newer, not equipped with SABs, that were 1) towed from the scene or 2) with intrusion into the occupant compartment. Due to the challenge in accurately assessing crash severity by telephone survey methodology, vehicle towaway status and intrusion were used as proxies of crashes of similar severity to the SAB deployment crashes. These choices may not result in ideal comparison groups but in absence of a better crash severity metric, represent samples of crashes that exclude the most minor crashes and can be used for comparison. For both groups, the injury rate was not statistically different than that of children exposed to deploying SABs. Of interest, however, these children sustained injuries to a more diverse set of body regions, including the abdomen, thorax, and neck/spine that represent injuries of potentially greater severity than the relatively lower severity injuries (upper extremity fractures and concussions) sustained by those exposed to the SAB.
It is of interest to compare the SAB injury risk to that of children exposed to frontal air bags. We have previously quantified the injury risk to children exposed to frontal air bags as 14.9% and 9.9%, for first-generation and second-generation frontal air bags, respectively. (Arbogast et al. 2005) The frontal air bag study used a slightly less severe definition of injury however, as it included facial lacerations, an AIS 1 injury, in the injury definition. This likely suggests that the injury rate, when using similar injury definitions, to children exposed to SABs is higher than that of those exposed to 2nd generation frontal AB, however this would be expected as the overall injury rate in side impacts is higher than in frontal impacts.
The current recommendation by NHTSA and many vehicle manufacturers is that it is acceptable for children to be seated next to a SAB. Specifically, NHTSA states that they “have not seen any indication of risks to children from current roof-mounted head SABs” and “recommend that children not lean or rest against chest-only or head/chest combination SABs.” (National Highway Traffic Safety Administration 2007) Based on this data and NHTSA’s recent report on SABs (Kahane 2007), we do not see need to alter those recommendations. These results offer assurance to policymakers and restraint designers that their proactive efforts to assure that deployments of SABs are not associated with unintended consequences for children have been successful.
As mentioned above, however, this analysis represents a snapshot in time as it looks at usage patterns, current SAB designs, and SAB penetration into the fleet during 2005 and 2006. SAB designs continue to evolve in an effort to optimize their effectiveness in serious crashes including rollovers while minimizing their risk of adverse injuries in more minor crashes. Continual monitoring is necessary to assure that the positive message derived from this data analysis does not change as these designs and practices change. This should include both epidemiological studies such as the analysis contained herein as well as in-depth investigations of side impact crashes of children exposed to deployed SABs. These two methodologies conducted in concert will provide both the context of population-based injury risk as well as detailed information about possible injury mechanisms associated with this technology.
This research is conducted on crashes involving State Farm Insurance Co. policyholders only. State Farm is the largest insurer of automobiles in the United States, with over 38 million vehicles covered; therefore, its policyholders are likely representative of the insured public in this country. The surveillance system is limited to children occupying model year 1990 and newer vehicles insured in 15 states and the District of Columbia. Our study sample represents the entire spectrum of crashes reported to an insurance company including property damage only, as well as bodily injury crashes. While our sample included a significant number of vehicles with intrusion into the occupant compartment, it is possible that we do not have a representative sample of the most severe crashes.
Nearly all of the data for this study were obtained via telephone interview with the driver/parent of the child and is, therefore, subject to potential misclassification. Of note, none of the children exposed to SABs was unrestrained. On-going comparison of driver-reported child restraint use and seating position to evidence from crash investigations has demonstrated a high degree of agreement. In addition, our results on age-specific restraint use and seating position are similar to those of other recently reported population-based studies of child occupants. (Edwards and Sullivan 1997; Wittenberg et al. 1999) Therefore, it is unlikely that errors in reporting restraint use or seating position would substantially alter the results of this study.
This paper provides the first population-based estimates of the exposure of children to SABs. In model year 1998+ vehicles equipped with side air bags and in a side impact crash, 27 children per 1000 children in crashes were exposed to a deploying SAB. For those on the struck side of the crash, this number increases to 52 children per 1000 children in crashes. Initial experience suggests that the risk of injury is fairly low with only 10.6% sustaining AIS 2+ injury - all were either upper extremity fractures or mild traumatic brain injuries. An understanding of the magnitude and characteristics of the population of the children exposed to deploying SABs provides guidance to the vehicle and restraint designer as the SAB technology continues to evolve. These findings offer assurance that efforts by regulators and the automotive industry to minimize negative consequences from SABs to vulnerable occupants appear to be effective and cause no change in the current recommendation of safe seating for children in the rear next to SABs.
The authors would like to acknowledge the commitment and financial support of State Farm Mutual Automobile Insurance Company for the creation and ongoing maintenance of the Partners for Child Passenger Safety (PCPS) program, the source of data for this study. The authors also thank the many State Farm policyholders who consented to participate in PCPS. The results presented in this report are the interpretation solely of the author(s) and are not necessarily the views of State Farm.