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Evaluation of Type I Hard Hats in Reducing Head Injury at Construction Sites

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Glossary

ANSI z89.1: American National Standard Institute outlining specific specifications hard hats are required to meet [1].

ATD: Anthropometric Test Device, a high precision test instrument designed to simulate human motion. Hybrid III 50th percentile male is the most common ATD used, although other models do exist [2].

AIS: Abbreviated Injury Scale, is used to classify and code injuries on a scale of 1 (minor) to 6 (untreatable).

HIC15/36: Head injury criterion, is the most widely used injury metric, developed to assess the likelihood of skull fracture. A 15 ms or 36 ms time duration can be used for this calculation[3].  

Introduction

Work-related accidents are frequently observed among workers who perform physically demanding work[4]. Construction work, in particular, is considered one of the most physically demanding and high-risk careers with a high risk of fatalities[5]. Building machinery and materials, particularly flying objects, account for 40% of construction occupational injuries[1] and at least 100 fatal injuries per year in the United States (US) between 2003 and 2012[6]. Brain injuries are particularly common injuries occurring on construction sites. Among the 1.7 million traumatic brain injuries (TBI) incurred each year in the US, 18% of those have occurred from falls on construction sites[7]. Similar trends are witnessed across the world, with 11-61% of all head injuries being construction work related TBIs in Sweden and Germany[8].

Hard hats are critical personal protective equipment (PPE) used to protect workers' heads from falling objects, impacts, and other hazards in industrial and construction environments[1]. In Canada, it is the responsibility of the workplace to “assess the risk of head injury and provide the appropriate protection”[9]. The risk of head injury is what determines which type of hard hat must be used [9]. Most hard hats use a suspension system to hold the hard shell off the head. Deformation of the suspension system components and hard hat shell attenuates energy from impact[6]. The effectiveness at which hard hats perform this task can determine the likelihood of injury in an accident. Hard hats are the primary protective mechanisms against TBIs, which are mostly a result of abrupt head acceleration changes[10]. It is recommended to inspect hard hats before every use for the presence of cracks, dents or cuts. If there is damage to the hard hat, or it has been struck by an object, the hat should be replaced immediately[11].  

Figure 1. Type I hard hat used in construction and industrial environments

Numerous types of hard hats exist (i.e. Type I and Type II) are used in various settings. Type I hard hats typically consist of a rigid outer shell, commonly made from high-density polyethylene or polycarbonate, and an internal 4 or 6 point suspension system designed to absorb and distribute impact forces away from the wearer’s head. Type I hard hats are generally used as the baseline for other hard hat development[7] and have been the industry standard for decades [12]. Type II hard hats are similar to Type I, although they  utilize an expanded polystyrene foam on the inside of the shell that is used in conjunction with the harness suspension[7]. Although there are numerous types of hard hats, for the purpose of this literature review, the focus will be primarily on Type I hard hats.

Currently, the main standards used for hard hat testing come from American National Standard Institute (ANSI). The ANSI Z89.1-2014 standard states that hard hats must limit force transmission to 4450N [6]. This test uses an impactor with a mass of 3.6 kg to be dropped from a height that will yield an impact velocity of 5.50 m/s[6].  Penetration from the top of the head[7] as well as acceleration requirements are also outlined[6].  

There are numerous studies that examine injury reduction from the use of helmets for recreational sports and motorcycles however, there is limited research on hard hats[4][6]. Due to the high prevalence of construction accidents[5], research on hard hats and safety equipment is of particular importance. This literature review critically examines the methods used for hard hat testing, and summarizes the effectiveness and the limitations of Type I hard hats in reducing head injuries.

Type I hard hat testing methods

Type I hard hats are primarily subjected to drop tests to analyze impacts to the crown region and resultant damages. Understanding the correct applications, range of protection, and potential resulting injuries of head impacts with a Type I hard hat, is vital to improving safety and standardizing hard hat designs. While research study methodologies vary in regards to impactors, test subjects, and manipulated parameters, the overall goal for researchers is to understand crown injury risks when the head is impacted and how the Type I hard hat mitigates this. Results of the test methods are compared against ANSI standard requirements.

Common head impacts observed at construction sites are caused by workers falling and objects falling onto the head. These are often replicated through rigid object block drops (concrete, steel, wood), lead pellets, or impactor strike during hard hat testing [4][1][6][7][10][13]. Test subjects in the studies include 50th percentile male Hybrid III anthropomorphic test devices (ATDs), structured headforms, finite element simulated head models, and V-Sim [4][1][7][13]. The specific test subjects were chosen based on resources and the overall research objective .

Drop tests for Type I hard hats can be performed with varying impactor contact surface areas and densities to analyze the hard hat's effectiveness [7][4]. Notable drop tests compare the Type I hard hat undergoing point impacts, such as concrete block corners and single lead pellets, versus a higher contact point impact such as concrete block face [4][7][10][13] The object deformation properties can have significant impacts on the result of the test. Thus, less rigid impactors with higher deformation can also be included in certain testing protocols; such as the bag of lead pellets used in Suderman et al.’s[6] study analyzing the effect of hard hats on head impacts[6]. Contrastingly, studies similar to Santos et al.’s[10] research on hard hat design and the influences of head acceleration attenuation, test the point impacts of lead pellets, on the risk of head injury.

Drop tests are performed with varying parameters including the drop height of the object, object weight, protected versus unprotected head impacts, rigid versus deformable objects, and angle of impact [4][1][7]. Drops were performed at heights ranging from 0.6m - 2.74m in the head form and ATD tests, while larger drop height tests ranging from 5m - 25m were conducted in Horak et al’s [4] finite element simulated model [4][1][7][10]. The drop heights were chosen to simulate the range of object velocities commonly observed on construction sites.

The head injury criterion (HIC) values, angular and linear acceleration of the head, and kinetic energy were calculated for each test to analyze the risk of injury based on the impact [1][10]. Hard hat specific parameters such as deformation, stress, and strain were accounted for in Ptak et al’s [1] study analyzing the energy-absorbing limitations of hard hats[1]. Either HIC15 or HIC36 are calculated from the tests depending on the research being conducted. Resulting values were compared against industry ANSI standard Z89.1 and European standard 12482, PN-EN 397 + A1 standard [1][13]. A summary of various study parameters and strengths are shown in Table 1 below.

Analyzing the results of the drop tests holistically allowed researchers to determine the likelihood and severity of injury from the specific impact test; and how the Type I hard hat mitigated the severity of injury [4][7].

Table 1. Summary of hard hat testing study methods including the impactor type, weight, drop height, output parameters and regulatory standards compared.
Study authors Type of impactor ATD Drop height(s) (m) Impactor weight (kg) Output parameters Regulatory Standard Strengths
Suderman et al.[6] Concrete block corners, Lead pellets Hybrid III male 50th percentile 0.91, 1.83, 2.74 9.1 HIC, Linear Acceleration, mTBI Risk ANSI Z89.1, European standard 12482 Realistic variety of impactors.
Santos et al.[10] Concrete block Hybrid III male 50th percentile 1.83 3.6 HIC, Linear Acceleration, Concussive Injury ANSI/ ISEA Z89.1-2014 Realistic impact scenarios were replicated.
Horak et al.[4] Concrete block (simulation) Finite Element Head Model 5 to 25 0.3, 0.8 HIC, Angular & Linear Acceleration, Energy Absorption EN ISO 397 Good variety of drop heights.
Ptak et al.[1] Construction prop Hybrid III male 50th percentile 1.75 13 HIC, hard hat Deformation, Energy Absorption PN-EN 397 + A1 Detailed exploration of different impactor scenarios.

Biomechanical injury analysis

The overall findings from the biomechanical research papers is that using hard-hats appreciably reduces head injuries such as concussion and traumatic brain injuries (TBI)  when compared to unprotected conditions (Table 1) [1][6][10]. Studies primarily use HIC or linear acceleration to quantify head responses.

Suderman et al.’s [6] study investigating head response from vertical impacts with concrete block (at drop height 1.83 m) found that Type I hard hats reduced linear acceleration and HIC by 70-95% compared to unprotected conditions. This in turn reduced the likelihood of skull fractures and AIS4+ brain injuries from greater than 99% to less than 1%[6]. The likelihood of mild traumatic brain injuries (mTBI) estimated using HIC was also reduced from 99% for unprotected impacts to 1% with hard hats[6]. Similarly, research by Santos et al.[10] looking at head acceleration during impact on the crown from a 1.83 m drop height, found that hard hats reduced HIC by 79-99% and peak linear acceleration by 78-95% (from 331.0 g to 27.9 g using 3.6 kg steel impactor), as compared to unprotected conditions [10]. These results were compared to a linear acceleration-based binomial logistic injury risk curve and it was found that unprotected impact correlates with a high probability of concussive injury (>99.9%), unlike hard-hat protected impacts which are relatively low risk (<6.2%). Concussive injuries were also reported in a finite element simulation study using smaller impactors (0.3 kg and 0.6 kg) from a 10 m drop height [4]. The study found that small objects cause concussion with at least a 50% probability when impact occurs on workers without protective gear, from a height of 10 m or more[4].

Despite the reduction of head acceleration and injuries from hard hat use, studies looking at extreme situations doubt the effectiveness of hard hats for very heavy falling objects[1]. A study by Ptak et al.[1] investigated a real case of head injury from a high impact falling object (13 kg, 1.7 m drop height) through accident reconstruction[1]. They found that although applying a hard hat to the head model reduced the HIC by 8-10% and absorbed significant energy (245% of the standard requirement), the high impact was ultimately fatal irrespective of the worker not having worn a protective hard hat[1]. From these studies, we infer that the effectiveness of hard hats in reducing the likelihood of head injury is dependent on the energy of the impact.

Table 2. Summary of hard hat impact testing results from studies
Study Author Impactor type Drop height(s) (m) Impactor weight (kg) Reduction in head injury criteria (HIC) with hard hat Reduction in head linear acceleration with hard hat Injury reduction
Suderman et al.[6] Concrete block corners, Lead pellets 0.91, 1.83, 2.74 9.1 70-95% reduction 70-95% reduction Reduction from >99% to <1% for skull fractures and AIS4+ brain injuries
Santos et al.[10] Concrete block 1.83 3.6 79-99% reduction 78-95% reduction Reduction from >99.9% to <6.2% for concussive injuries
Ptak et al.[1] Construction prop 1.75 13 8-10% reduction N/A Fatal outcome
Horak et al.[4] Concrete block (Simulated) 10 0.3, 0.8 N/A N/A >50% probability of concussion without protection

Discussion

Limitations of hard hat research methods

In the hard hat research papers we reviewed, no standardized uncertainty approaches were employed to capture the wide range of impact angles, object shapes, and environmental factors influencing real-world head injuries, despite such methods being common in vehicle accident reconstructions[1].

Testing protocols often rely on new, undamaged hard hats, which may overestimate actual performance as real-world hard hats accumulate wear, experience smaller impacts, and endure environmental exposure[6]. Additionally, reliance on ATDs with rigid neck constraints cannot perfectly replicate active muscular control, posture changes, or inter-individual anatomical differences, further limiting how well these evaluations mirror actual on-site conditions[6]. Field-based or longitudinal studies tracking real-world hard hat performance remain scarce, leaving a gap in understanding how repeated, low-height collisions and environmental stressors influence injury outcomes. Overall, while current methods help establish baseline performance, they often underestimate the complexity of on-site hazards, highlighting the need for more rigorous and comprehensive testing, including standardized uncertainty approaches, and realistic wear-and-tear conditions.

Limitations of Type I hard hats

The gap that we have identified in the research field as a whole is that Type I hard hat research only focuses on crown impacts, as required by the ANSI standard. However, there are additional common impacts, such as lateral, that account for 52-62% of impacts[7]. Thus, the focus on crown impacts limits our scope of understanding on the variety and severity of head injuries possible on construction sites. The continued emphasis on Type I hard hat research in combination with Type I hard hats being the most commonly used in construction sites, raises concerns that workers and researchers may be unaware of more protective types of hard hats[7].

This limitation prompted our brief exploration of what types of hard hats exist and what research is currently being done on them to address the shortfalls of Type I hard hats. Type II hard hats, which differ from Type I due to their expanded polystyrene (EPS) liner, are designed to protect against lateral impacts and are tested in accordance with ANSI standards. There is limited research evaluating the differences in impact characteristics for Type I vs Type II hard hats, and whether one should be more adopted than the other. We found two papers of particular interest comparing Type I and II hard hats - both of which were conducted at the Legacy Research Institute and had the same first author. In the first study, Bottlang et al.[7] assess the effectiveness of hard hats for different directions of impact, by simulating impacts to the front, crown, lateral, and rear of the head according to Type II hard hat ANSI standards, for both Type I and Type II hard hats. The authors found that between Type I and Type II hard hats, there was no statistically significant difference in linear head acceleration of the head form due to crown impacts[7]. However, the authors did find that the Type II hard hat containing the EPS liner had a significantly lower front, lateral, and rear linear acceleration in the Z axis when compared to the Type I hard hat[7].

In the second paper examining the differences between Type I and Type II hard hats, Bottlang et al.[13] found that for lateral impacts, the Type I hard hats (n=3) tested exceeded the 150g threshold set by ANSI standard, while Type II hard hats (n=4) remained below the 150g limit. According to Yu et al.[14], 50-70% of work-related TBIs occur from falls, resulting in non-crown impacts. This statistic, combined with Type I hard hats being the industry standard and their limited attenuation in lateral impact, highlights the need for a larger body of research on the effectiveness of various types of hard hats, to better inform workers about which style they should be using[7][14][12].

Research controversies

Although ANSI Z89.1 provides a foundational approach to assessing Type I hard hats, it still falls short of replicating the wide spectrum of impacts that occur in actual construction environments such as falls, trips, or falling objects from height larger than 0.6 m. Of construction related TBIs, 68% are related to falls and 12% are related to falling objects[7]. Therefore, the regulatory standard’s sole focus on crown (top) impacts for Type I hard hats does not sufficiently address the wider spectrum of head injuries likely to occur on a construction site[1].  

The ANSI standard Z89.1 - 2014 effectiveness in testing for impacts representative of real-world scenarios is questioned due to the low drop height, impact mass, and lack of consideration for head injury due to rotational motion[7]. Researchers argue this is not enough energy for proper hard hat testing as this height and energy are not representative of real-world impact scenarios[7]. In fact, when testing hard hats at a height of 1.96 m, which is used for testing of bicycle helmets by the United States Consumer Product Safety Commission (US CPSC) and American Society for Testing and Materials (ASTM), researchers found that this resulted in a significantly increased probability of AIS 2 brain injury and concussion compared to the standard 0.6 m test height[7]. This raises concerns about the testing rigor hard hats are subjected to before being sold commercially.

Additionally, rotational forces applied to the head cause shearing forces within the brain tissue, leading to deformation[15]. These rotational forces are known to cause TBIs and concussion[15]. However, the ANSI standard Z89.1 - 2014 does not address any rotational velocity or rotational acceleration of the head.

Conclusion

Overall, current studies indicate that Type I hard hats provide appreciable protection against crown impacts by substantially reducing linear acceleration and HIC values, in turn lowering the likelihood of severe head injuries such as skull fractures and TBIs [4][6][10]. However, it is also important to recognize that Type I hard hats are predominantly designed and tested for crown impacts, despite lateral impacts being common mechanisms, accounting for 52-62% of head injuries in real-world construction scenarios[7]. Existing standards under ANSI Z89.1 for Type I hard hats focus on vertical impacts and lack rotational motion and lateral impact considerations. This further highlights the gap in replicating on-site conditions and does not address a significant portion of head injuries that arise from non-crown impacts[13]. With a high prevalence of work-related TBIs linked to lateral impacts, there is a clear need for additional research into other types of hard hats and refinement of testing protocols to more accurately incorporate a variety of real-world hazards.

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 Ptak, Mariusz; Dymek, Mateusz; Wdowicz, Daniel; Szumiejko, Adrianna; Kwiatkowski, Artur (July 2024). "Energy-absorbing limitations of hard hat safety helmets in mitigating trauma from falling objects". Archives of Civil and Mechanical Engineering – via Springer Nature Link.
  2. ↑ Mertz, Harold J. (1993). Accidental Injury: Biomechanics and Prevention. New York: Springer New York, NY. p. 66. ISBN 978-1-4757-2264-2.
  3. ↑ Troy, Karen L.; Tetreault, Kimberly; Goodworth, Adam D; Ji, Songbai; Popovic, Marko B (2019). Biomechatronics. pp. 451–494. ISBN 978-0-12-812939-5.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 Horak, Zdenek; Tichy, Petr; Vilimek, Miloslav (October 2024). "Evaluating protective helmet efficacy in work-related accident: A forensic biomechanical analysis of concussion risk from falling objects". Legal medicine – via National Library of Medicine.
  5. ↑ 5.0 5.1 Konda, Srinivas; Tiesman, Hope M.; Reichard, Audrey A. (2016). "Fatal traumatic brain injuries in the construction industry, 2003−2010". American Journal of Industrial Medicine. 59: 212–220 – via Wiley.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 Suderman, Bethany L.; Hoover, Ryan W.; Ching, Randal P.; Scher, Irving S. (December 2014). "The effect of hard hats on head and neck response to vertical impacts from large construction objects". Accident Analysis & Prevention. 73: 116–124 – via Elsevier Science Direct.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18 7.19 7.20 7.21 Bottlang, Micheal; DiGiacomo, Gina; Tsai, Stanley; Madey, Steven (July 2022). "Effect of helmet design on impact performance of industrial safety helmets". Heliyon – via National Library of Medicine.
  8. ↑ Brolin, Karin; Lanner, Daniel; Halldin, Peter (2021). "Work-related traumatic brain injury in the construction industry in Sweden and Germany". Safety Science. 136 – via Elsevier Science Direct.
  9. ↑ 9.0 9.1 "CCOHS: Headwear — Selecting Protective Headwear". Canadian Centre for Occupational Health and Safety. 2024.
  10. ↑ 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.10 10.11 Dos Santos, Arthur Alves (December 2021). "The Influence of Hard Hat Design Features on Head Acceleration Attenuation". Journal of applied biomechanics – via National Library of Medicine.
  11. ↑ Equipment: Chapter 12 Head Protection. Infrastructure Health & Safety Association.
  12. ↑ 12.0 12.1 Barnes, Ryan (May 2023). "Safety Starts at the Top—and with Better Head Protection". EHS Today.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 Bottlang, Micheal (December 2023). "Climbing style safety helmets do not improve impact protection over type II hard hats". Safety Science – via Elsevier Science Direct.
  14. ↑ 14.0 14.1 Yu, Xiancheng (February 2023). "Head Impact Location, Speed and Angle from Falls and Trips in the Workplace". Annals of biomedical engineering – via National Library of Medicine.
  15. ↑ 15.0 15.1 Meaney, David F. (January 2011). "Biomechanics of Concussion". Clinics in Sports Medicine.
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