Rescue Blankets in Direct Exposure to Lightning Strikes—An Experimental Study

Abstract

Lightning strikes pose a significant risk during outdoor activities. The connection between conventionally used rescue blankets in alpine emergencies and the risk of lightning injury is unclear. This experimental study investigated whether rescue blankets made of aluminum-coated polyethylene terephthalate increase the likelihood of lightning injuries. High-voltage experiments of up to 2.5 MV were conducted in a controlled laboratory setting, exposing manikins to realistic lightning discharges. In a balanced test environment, two conventionally used brands were investigated. Upward leaders frequently formed on the edges along the fold lines of the foils and were significantly longer in crumpled rescue blankets (p = 0.004). When a lightning strike occurred, the thin metallic layer evaporated at the contact point without igniting the blanket or damaging the underlying plastic film. The blankets diverted surface currents and prevented current flow to the manikins, indicating potentially protective effects. The findings of this experimental study suggest that upward leaders rise from the edge areas of rescue blankets, although there is no increased risk for a direct strike. Rescue blankets may even provide partial protection against exposure to electrical charges.

1. Introduction

While staying in a park, a family of eight was injured by lightning that hit a nearby tree during a thunderstorm in July 2024 (Delmenhorst, Germany). A 5-year-old boy and a 14-year-old girl suffered cardiac arrest and had to be resuscitated. All eight family members had to be taken to hospital by emergency medical services, although the girl did not survive her injuries [1]. This tragic event raises numerous questions. When surprised by a thunderstorm during outdoor activities in a group, it is recommended that people keep a safe distance from trees and from each other. However, the suggestion to keep distance from each other is hardly feasible for a family with small children. Hypothetically, sitting together on a rescue blanket at a distance from the trees could at least prevent ground potential injuries. So far, the properties of rescue blankets during a thunderstorm are not known [2]. In particular, we cannot tell whether the metallic surface of a rescue blanket can induce the formation of upward leaders [3]. The aim of this experimental study was to evaluate whether the use of a rescue blanket can alter the risk of a lightning strike.

Factors Influencing the Risk of a Lightning Strike on a Rescue Blanket

The risk of being struck by lightning varies considerably and is related to locations, activities, and individual behavior [4]. Metal on a person’s body will conduct electricity but does not attract lightning [5,6]. Lightning can harm people in a variety of ways, including a direct strike, a side flash, upward leaders from the ground, and step-potential or blast-type injury [7,8]. People are injured by the effects of electrical, thermal, and mechanical energy [9]. The vascular system and the nervous system are most sensitive to electrical energy [4,10,11]. Amazingly, despite voltages exceeding a million volts and currents up to hundreds of thousands of amperes, the majority of lightning strikes can be survived due to the extremely short duration of contact with the electrical current [12]. Lightning is caused by static electricity from friction between ascending ice crystals and water drops in thunderclouds [13,14]. The primary charge structure of an isolated, mature thundercloud consists of many tens of Coulombs of positive charge in its upper portions and a more or less equal negative charge in its lower levels [15]. Discharges between clouds account for the majority of all lightning discharges. Of the cloud-to-ground lightning flashes, about 90% are initiated by a negatively charged, downward-propagating leader [15]. Stepped leaders near the ground induce positively charged upward leaders from the ground, eventually determining the primary lightning current path between cloud and ground [15]. The role of rescue blankets in the electromagnetic field is not clear. On the one hand, the metallic layer of the rescue blankets may increase the formation of upward leaders. On the other hand, when acting like a Faraday cage, a rescue blanket may minimize the current path over the body surface and mitigate the risk of lightning injury.

2. Materials and Methods

This experimental study was conducted by a study group at the Nikola Tesla Laboratory at Graz University of Technology in Austria. Study design and data management followed the SQUIRE 2.0 Revised Standards for Quality Improvement Reporting Excellence to improve the quality, safety, and value of healthcare [16].

2.1. Study Design

An impulse voltage generator (Marx Stoßgenerator, Meßwandler-Bau GmbH, Linz, Austria, 1972) was used to simulate atmospheric discharges up to ±2500 kV (maximum 165 kJ) from a metal rod hanging in a fixed position in the middle of the lab, simulating the head of the artificial leader. This rod is called the central electrode in the setup, and it generates the starting leader of the artificial flash. Within a selected distance of approx. 2.5 m between the central electrode and the test object, the path of the artificial discharge was defined (Figure 1a). The impulse voltage generator was used to generate the standard waveform (1.2/50 μs) for lightning impulse tests according to the International Electrotechnical Commission (IEC) (Figure 1b). The applied impulse peak voltage of the cascade with 13 stages was 1.89 MV; the applied peak current was approx. 4.6 kA and depended on the flashover distance and the current path in the laboratory.

The basic experiment was conducted with a rescue blanket hung on a wooden frame (distance (d₁): 2.1 m; length (l₄): 0.78 m) in the vicinity of a lightning rod (length (l₃): 0.97 m). The distance (d₂) between the lightning rod and the rescue blanket was 0.25 m; the length above ground (l₂) to the lower end of the rod at the central electrode was 2.5 m; and the length of the rod (l₁) was approximately 3.5 m (Figure 1a).

The electric field (E) from the lightning voltage (U) in the distance between the central electrode and the grounded rescue blanket was estimated using the following formula:

$$\mathrm{E}={\displaystyle \frac{\mathrm{U}}{\mathrm{d}\mathrm{l}}}$$

To capture the lightning effects in the laboratory, a digital 35 mm single-lens reflex camera (CANON EOS 760D, Canon Inc., Tokyo, Japan) was used. For the applied exposure time, the shutter speed was set to 8 s and the aperture to 16 (ISO 100, 3984 × 2656 pixel); the camera was triggered by hand. In order to evaluate the lengths of the discharges, a measuring rod made of non-conductive material was placed in the plane of the main discharge (taking into account the distance from the camera).

Two different kinds of manikins were used—either a model torso with a resistance of 1 kOhm (Franklin Doll, Graz University of Technology, Graz, Austria) or a non-conductive fiberglass manikin (ID: 1707, LV CORP SAS, Boulogne, France). The Franklin Doll, with a height of 1.1 m, resembled a squatting person.

Rescue blankets are commonly used for hypothermia prevention in prehospital emergency medicine and outdoor sports. They are made of a polyethylene terephthalate sheet with either gold or silver (1% aluminum-coated) color and a thickness of 10 μm. They are low-weight and low-bulk medical devices; Category 1 (Directive 93/42/EEC). The metallized surface reflects electromagnetic waves; the foils are watertight and windproof [2]. We investigated two different brands of rescue blanket. One brand with a silver/gold surface is currently used by emergency medical services (EMS), mountain rescue, and helicopter EMS in Tyrol (LEINA-WERKE GmbH, D-51570 Windeck, Germany) [2]. Another brand with a silver/silver surface is more frequently used in Anglo-American countries (Steroplast Healthcare Ltd., Manchester, M22 4TE, UK). The length of both brands of blankets was 2.1 m. The area of the silver/silver foil was 27,300 cm², and the area of the silver/gold foil was 38,850 cm². The area differed between the two brands by 30% due to the smaller width of the silver/silver foil (1.3 m) as compared to the silver/gold foil (1.85 m). When unfolded and fanned-out, the recue blankets presented with a net-like pattern of rectangles that were formed from the longitudinal and transverse fold lines. During the laboratory investigations, the rescue blankets were grounded by contact with the floor. The percentage of damaged area (dA) in a rescue blanket after a lightning strike was calculated as the quotient of the estimated damaged area (eA) in cm² divided by the total area (tA) of the foil multiplied by 100, as outlined below:

$$\mathrm{d}\mathrm{A}={\displaystyle \frac{\mathrm{e}\mathrm{A}}{\mathrm{t}\mathrm{A}}}\times 100$$

The environmental conditions of air pressure, temperature, and relative humidity were recorded during the investigations.

The following scenarios were evaluated:

• Effects of lightning discharges on rescue blankets;
• Effects of lightning discharges on a manikin under a rescue blanket.

2.2. Preparation of a Balanced Test Environment Prior to the Experiment

After having established a balanced test environment in a preliminary test, the additional effects of rescue blankets on the Franklin Doll and manikin were assessed. The test conditions were deemed to be balanced when about half of the discharges hit the ground and half of the discharges hit the grounded protection rod at 3.0 m in distance from the central electrode (Figure 1a). In this constellation, 60% of discharges occurred into the ground and 40% into the protection rod. Further approximation was achieved by increasing the voltage of the impulse generator.

2.3. Hypotheses and Outcome

The null hypothesis (H₀) stated that the frequency of the development of upward leaders and direct discharges does not increase with the use of a rescue blanket near the grounded protection rod. Alternative hypothesis I (A₁) stated that the frequency of the development of upward leaders is greater on a manikin that is covered with a rescue blanket than on a manikin that is not covered with a rescue blanket. The primary endpoint of this study in the lab was the assessment of the formation of upward leaders on rescue blankets. The secondary endpoint of this study was the assessment of downward discharges on a manikin under a rescue blanket.

Descriptive statistics were applied to determine measures of central tendency (mean) and measures of dispersion (range, minimum, and maximum). Data were tested for normal distribution using the Shapiro–Wilk test. The non-parametric Mann–Whitney U test was used to estimate the differences in rank for comparison between the two groups [18]. A p-value of <0.05 was deemed significant.

3. Results

Ambient conditions during testing were air pressure 970 hPA, temperature 23.8 °C, and relative humidity 68.4%. Balanced conditions were established by adjusting the height of the central electrode and adjusting the lightning voltage to 1.8 MV and the resulting electrical field strength to 1.04 MV/m [1.8 MV/(2.5 m − 0.775 m) = 1.04 MV/m].

Process Measures and Outcome

The basic experiment investigated the effects of lightning discharges on a fanned-out rescue blanket spread over a wooden frame (Figure 1). A total of twenty tests were performed, ten tests with each brand—of which five tests were conducted with the metal surface facing up and five tests with the plastic surface facing up. The number of detected upward leaders was between two and nine per test. In accordance with the smaller area of the silver/silver foil, the total number of upward leaders was smaller than for the silver/gold foil (56 vs. 67) and differed in trend between the two brands when the plastic surface was facing up (29 vs. 20; p = 0.095). The average length of upward leaders was 10.5 cm (range: 2 cm–46 cm) for both surfaces of the silver/gold foil and 8.6 cm (range: 2 cm–28 cm) for both surfaces of the silver/silver foil (p = 0.165). In fanned-out silver/gold foils, the average length of upward leaders was greater in trend with the plastic surface facing up than with the metal surface facing up (mean: 12.1 cm vs. 8.8 cm; p = 0.056) (

Table 1. Number of upward leaders and length of upward leaders in series of five tests per section on both surfaces of the two fanned-out foils—silver/gold and silver/silver.

Section Identification Number (IN) of Upward Leaders Per Test [Length (cm)]
IN-1	IN-2	IN-3	IN-4	IN-5	IN-6	IN-7	IN-8	IN-9
Silver/gold—metal surface facing up (n = 38)
4	6	6	14	10	6	6	7	4
6	8	10	6	6	6	10	4	4
4	6	30	12	8	4
3	3	3	8	12	32	12	8
6	2	16	18	10	6
Silver/gold—plastic surface facing up (n = 29)
4	18	12	14	4	12
12	6	6	10	12	6	6
10	16	20	24	6
6	28	6	10	12	4
3	46	6	18	4
Silver/silver—metal surface facing up (n = 36)
6	24	12	12	8	4	12
3	20	4	10	8	9	6
8	6	10	24	4	4	2
5	16	5	12	6	6	3
4	7	28	8	10	16	6	6
Silver/silver—plastic surface facing up (n = 20)
8	8	12	8	6
6	4	4	8
3	16	8	8	8	16
8	6	10
4	12

Abbreviations: IN, identification number.

).

In order to provoke a downward discharge to the rescue blanket, the central electrode had to be lowered to 1.9 m in height to achieve the imbalanced conditions. The resulting heat from the lightning strike melted 283.5 cm² of the foil (approx. 0.84%) at the contact points, although it did not make a hole in the foil. The peak value of the current was determined in previous tests and varied between 4.5 kA and 4.9 kA depending on the length of the discharge channel and the current return path.

After the fanned-out foils have been pushed together to about half of their surface (width: approximately 1 m), the average number of upward leaders remained the same but their height increased significantly. In the crumpled silver/gold foils with metal surface facing up, the average length of upward leaders increased from a mean of 8.8 cm to 14.0 cm and was higher than for fanned-out foils (p = 0.004) (

Table 2. Number of upward leaders and length of upward leaders in series of five tests per section on the metal surface of the two foils—silver/gold and silver/silver—crumpled to half the area.

Section Identification Number (IN) of Upward Leaders Per Test [Length (cm)]
IN-1	IN-2	IN-3	IN-4	IN-5	IN-6	IN-7	IN-8	IN-9
Silver/gold—metal surface facing up (n = 31)
10	12	3	3	14	16	18	14
4	14	24	4	8
20	22	22	12	16	18	8
20	6	8	16
6	16	18	1	32	18	20
Silver/silver—metal surface facing up (n = 27)
4	10	6	20	10	6	4
10	18	4	26	10	8
8	6	10	8	4
4	4	24	20	4
6	6	36	2

Abbreviations: IN, identification number.

).

In the first setting, the effects of lightning discharges on a manikin under a rescue blanket were investigated. In this experiment, a plastic manikin with grounded metal gear (carabiner) attached to a climbing harness was wrapped in a gold/silver rescue blanket with the plastic surface facing up (Figure 2). In addition to the main discharge in the middle third—which developed into a broad flash foot at the transition to the ground—we observed that upward leaders formed preferentially along the longitudinal and transverse fold lines of the blanket. A total of seven upward leaders were identified along the fold lines, ranging in length from 16 to 76 cm (average: 34 cm), with three located in the upper third, three in the middle third, and one in the lower third of the blanket.

After lowering the central electrode, a single discharge occurred into the rescue blanket without a flashover to the metal gear. This resulted in evaporation on the surface of the grounded metal coating of the foil and melting of 240.3 cm² (approx. 0.62%) of the foil surface.

In the next test, the manikin was covered only halfway along its entire length in order to facilitate the observation of emerging upward leaders. We observed four upward leaders ranging from 14 cm to 28 cm in length (average: 22.5 cm). After a discharge into the rescue blanket with flashover to the grounded metal gear occurred, the metalized coating melted over an area of about 125.7 cm² (approx. 0.32%). The aluminum layer evaporated without damaging the underlying plastic foil (Figure 3). The type of damage depended on the current intensity and the time of exposure, and was random for each lightning impact. The peak value of the current was determined in previous tests and varied between 4.5 kA and 4.9 kA depending on the length of the discharge channel and the current return path.

In the second setting, we used the Franklin Doll that is characterized by low electrical resistance and high conductivity through the metal torso. In this setting, we covered the doll with a rescue blanket at a distance of 3.3 m to a long lightning protection rod (length: 3 m) and two short lightning rods (length: 0.25 m) at a distance of approximately 1.5 m. The distance to the central electrode was 2.8 m. The plastic surface of the rescue blanket was facing up. The Franklin Doll simulated the posture of a squatting person with a height of 1.1 m. The body of the Franklin Doll corresponded to a 1 kOhm high-voltage resistor. To avoid damage to the resistor, in parallel, a spark gap (2 iron rods) at a distance of 3 cm was fixed. This spark gap indicated a surface discharge on the human skin. When the doll was hit in the left arm by a discharge next to an upward leader, evaporation of the metal layer occurred in the contact area. Since no electrical flashover was detected at the doll’s spark gap (simulating surface discharge on the human skin), the current was grounded by the foil (Figure 4).

In the third setting, the manikin wearing metal gear on a climbing harness was lying on its back under a gold/silver rescue blanket hung over a wooden rod (cross-sectional area: 16 cm²). The plastic surface was facing up and the manikin and the metal gear on the harness were grounded with a copper wire prior to the test. In this setting, we raised one side of the tent to allow observation of upward leader formation from the metal gear. When a discharge hit the rescue blanket, about 148.4 cm² (approx. 0.38%) of the foil surface melted (Figure 5). In addition to the split bolt in the middle third—which merged into a common flash foot on the rescue blanket—we observed seven upward leaders on the edges along the fold lines of the foils, ranging from 8 cm to 46 cm in length (average: 23 cm): three in the upper third, two in the middle third, and two in the lower third. There was a single upward leader rising from the harness, although no flashover occurred from the rescue blanket on the manikin or the harness.

4. Discussion

Our experimental investigations revealed that upward leaders are frequently observed on rescue blanket surfaces in an electrical field, particularly on edges along the fold lines of the coated foils. With crumpling of the foils, the number of upward leaders did not increase, although we observed that the leaders became longer. We attribute this finding to the increased inhomogeneity of the electric field on the edges along the fold lines of the foils [19]. Whereas even small changes in the folding of the rescue blanket create totally different discharge patterns, the overall appearance as well as the number and length of the upward leaders provided information regarding the discharge activities on rescue blanket surfaces. We did not observe vertical streamers rising from the manikins. In humans acting as a conduit for charges, the formation of vertical streamers was reported to potentially harm [7]. Although several upward leaders emerged on the surface of the foils, a direct discharge to the rescue blanket did not occur unless the central electrode was lowered. This suggests that, by itself, a foil does not increase the probability of a lightning strike.

In the first setting, the discharges on a manikin wrapped in a rescue blanket, resulting in evaporations of the metal layer and occurred in less than one percent of the surface area of the foil. Presumably, the conductive layer surrounding the manikin can block the effects of external electrical fields and shield the manikin from electromagnetic interference [19]. So far, we can assume that being wrapped in a rescue blanket acts like a Faraday cage, although this is limited by the extremely thin metal coating of the rescue blanket [20].

When a torso like the Franklin Doll was covered with a rescue blanket in the second setting, evaporation occurred only near the contact point of lightning strike while the current was conducted via the coated foil. This indicates that, depending on the grounding situation, the foil by itself does not increase the probability of a lightning strike. Moreover, it may even diminish the exposure time from diverted surface currents and prevent current flow to the manikins.

In the third setting, no flashover from the rescue blanket on the manikin occurred despite upward leaders from the metal gear. In this setting, the rescue blanket even displayed a protective effect for the manikin under it as the entire lightning strike impact was discharged into the ground. When the manikin was only half covered with the rescue blanket, a flashover to the grounded metal gear occurred from the inhomogeneities in the electrical field along the edge of the foil. We assume that with a completely covered manikin, no flashover occurs. However, there is still the side-flash probability when sitting on a rescue foil close to the object struck by lightning [5]. Furthermore, a rescue blanket cannot protect against the heat from a lightning strike impact that causes evaporation on the metal surface at the contact point of the strike. The extremely thin metal layer of the rescue blankets used in this study caused the metal coating in the direct contact areas to evaporate. Less than one percent of the surface area melted from the heat caused by the energy density following a discharge to the foil but the underlying plastic layer always remained intact. This indicates a protective effect from the electrical field for objects under the foil. It must be taken into account that the current amplitude used in our experimental study was approx. 5 kA, which corresponds to a 50% median value of natural lightning in Austria [13]. Although the rescue blanket is considered highly flammable, no direct impact was seen to cause the foil to catch fire during our investigations.

Numerous limitations have to be considered. The significance of our findings only applies to a limited extent to real-life events. First of all, the physical properties in the electrical field differ greatly between the manikin used in our study and a human being. This may explain in part the absence of vertical streamer formations. The electrical resistance of a manikin is far greater than that of a human body. We did not evaluate the additional effect of moisture and rain on the surface of the foil, which could reduce current exposure and associated mechanical and thermal injury [21]. We cannot tell whether grounding of rescue blankets by contact with the floor is comparable to the grounding situations that would arise in the field during a thunderstorm. The likelihood of a side flash depends on the resistivity of the blankets. In the case of lightning current flowing in thin metal conductors, there is a possibility of arcing to a nearby human [5]. Furthermore, we cannot know the extent to which the higher conductivity of humans (e.g., sweaty skin surface) can influence the results. Finally, lightning in the alpine regions during a thunderstorm may have a wider range of amplitude and energy capacities than the artificial discharges simulated in our experimental study. Further research is needed to explore this complex relationship in detail.

4.1. Relevance to Rationale and Specific Aims

This study is relevant for people who cannot flee a thunderstorm and seek to keep the risk of lightning as low as possible. The main findings of this experimental investigation indicate that rescue blankets do not increase the risk of lightning strike, despite the fact that the conductive surface of the blanket can produce upward leaders. Moreover, rescue blankets may afford a potential protective effect by dissipating current density and reducing exposure time in the case of a strike. As long as the defects are restricted to the metal layer, no current will pass through the intact plastic foil. Presumably, this protective effect also applies to protection against step voltages on the earth.

4.2. Implications

4.2.1. Association Between Intervention and Outcome

This experimental study observed the probability of downward discharges into a manikin covered with a rescue blanket. Rescue blankets do not increase the risk of lightning strike, despite the fact that the conductive surface of the blanket can produce upward leaders. Moreover, rescue blankets may provide a potential protective effect by dissipating current density and reducing exposure time in the case of a strike. So far, we can assume that when wrapped in a rescue blanket, the foil acts like a kind of Faraday cage. Despite the fact that rescue blankets are highly flammable, we did not observe ignition of the foils after direct lightning strike in this study.

4.2.2. Impact on People and Rescue System

Rescue blankets can be safely used during a thunderstorm to protect from rain and hypothermia. Although guidelines recommend that metal gear such as climbing sticks and ice axes should be safely set on the ground [10,22,23], the findings of our study reveal that, unless grounded, metal gear does not attract lightning. The metalized coating of the rescue blanket acts like a thin electrical conductor that can disperse the power of the lightning and even lessen the chance of damage or injury.

5. Conclusions

The findings of this experimental study suggest that both the metallic surface and the plastic surface of a rescue blanket can induce the formation of upward leaders. Upward leaders frequently formed on the edges along the fold lines of the foils and were significantly longer in crumpled rescue blankets. No flashover occurred from the rescue blanket on the manikin. According to the findings of this experimental study, rescue blankets do not increase the risk for direct strikes and may even provide partial protection against electrical exposure.