Hypothermia is a big topic, possibly one of the biggest in expeditionary medicine - if you don't believe me, take it up with Hannibal, who lost 20,000 men to the cold on his 218 BC escapade across the Alps to Rome [1]. This article is not designed to do the whole subject justice, only to answer a question that has forever puzzled me. For the longest time, I’ve carried my little folded-up reflective blanket with me as I’ve gone on various cold-weather trips and thought – how effective is this thing?
The human body loses heat in multiple ways. Under normal circumstances the typically quoted numbers are that radiation accounts for 60% of your heat loss, so is the most significant, followed by evaporation of water at 22%, and conduction/convection sharing the meagre remaining 15% [2]. Where is the remaining 3%, you may ask? Well - you left that in the yellow snow.
But these proportions aren't fixed (or even consistent between reference texts, one could gripe). Different mechanisms assume varying importance depending on the setting. In hot environments obviously the evaporation of sweat takes on greater significance. Wet clothing increases conduction losses to five times that of baseline, and full immersion in water can further increase conduction and convective losses by a whopping twenty-five times [3]. High altitude environments mean low humidity and therefore higher respiratory losses. Higher windspeeds can mean convective losses overtake radiant losses [4].
Still, it seems preventing radiant heat loss is an important tenet in hypothermia prevention. Doing so through the use of lightweight reflective (or "space") blankets has been the subject of a surprising little research, despite their ubiquity in first aid kits. I mean everyone's got one [5,6]. It's probably the most common piece of "first aid" equipment in a hiker’s backpack. So why not do a deep dive into what they are and whether they're effective.
Those gold and silver reflective blankets are made of metallised polyethylene terephthalate ("Mylar"), which – believe it or not – was initially invented for use as an insulation material in spacecraft, hence the name space blankets. NASA spacesuits still use five layers of the very same material in their construction. It even got us to the moon [7] - here's the Apollo 11 Lunar Module with it's descent stage encased in a mixture of Mylar and Kapton, a similar polymer:
So - do they work on humans? Well that's a difficult question to answer. Firstly, a literature review is a painful experience. Some studies refer to them as "space blankets", others as "metallised" sheets (or "metallized" if you're of that persuasion), as well as "foil blankets", "reflective blankets", or any other hideous combination of the above. Secondly, it's very hard to randomise individuals at risk of hypothermia into "space blanket" or "no space blanket" meaning the body of evidence is largely relegated to thermal models and surrogate outcomes.
A few studies investigated what proportion of body heat is detectable through one of these blankets - and found a single blanket blocked the transmission of 82.1% of a body's infrared emissions [8]. This makes a lot of sense, because we know it's harder to see people through them on thermal imaging – the Taliban did that exactly that in order to avoid FLIR detection. This is good for reducing radiant heat loss, but makes for a rather embarrassing Achilles heel when trying to undertake nighttime search and rescue [9].
In terms of patient-centred outcomes (albeit in-hospital ones), one small study found them to be marginally effective at preventing heat loss in the operating theatre [10], the most salient results of which can be seen below:
The researchers then went on to look and see if the same effects were present in labouring women undergoing epidural analgesia, and promptly found that they were not [11].
A Swedish study tested the thermal insulation properties of a variety of materials and found their version of a metallic foil blanket to have a thermal resistance of 0.7 m^2K/W, half that of five woollen blankets (1.3m^2K/W), though the foil was only 0.1mm thick and weighed 60g. The blankets on the other hand were 30mm thick and weighed 10kg [12]. My (potentially misguided) interpretation of this study is reading this to mean that 1 space blanket is therefore equivalent to just over 2.5 woollen blankets but at 1/150th of the thickness and 1/8th of the weight.
In a manikin study, foil blankets were found to slow heat loss, though not as effectively as many other tested materials [13].
Yet another manikin study was designed to replicate a maritime environment, this time exposing their model to temperatures of -5°C and windspeeds of 4m/s (about 8 knots), alongside a "moisture simulation" - presumably designed to replicate sea spray. They found that the addition of a reflective blankets to other layers did reduce the decremental effects of moisture on thermal insulation values by 22-29% [14].
But, in an actual study on actual human volunteers, Light et al found absolutely no difference when it came to the change observed in mean skin or core temperature, whether metallised sheeting was used or not [15]. This was an older study, relegated to the annals of a pre-internet era, and so I was unable to find the text in its entirety in order to fully assess its methodology, but I can tell you the size of the study: n=5. Not exactly the resounding answer I was hoping for.
In summary:
It's unclear whether the space blanket has more or less utility than we give it credit for – I suspect that depends on how much credit you were giving it in the first place. I was certainly surprised to discover its history and multipurpose nature when researching this material. There’s still a paucity of evidence to demonstrate it improves patient-centred outcomes, and some literature reviews have only just fallen short of scathing in their conclusions [16]. I’m yet to be entirely convinced myself, but I do think it offers yet another tool for the toolbox, and certainly doesn’t have significant drawbacks from a warmth-to-weight ratio perspective. The mistake would be carrying one in place of other sensible garments or shelters.
Heat loss should be looked at the same way as any other pathophysiology: there are individual components to which therapy can be targeted. We don't treat every exacerbation of COPD with bronchodilators alone, because we recognise that bronchospasm is only one element of the disease process; bronchial inflammation requires corticosteroids, infection requires antibiotics. Heat loss is no different, using a layered system of insulation and staying dry prevents conductive losses, staying out of the wind and wearing windproof outer shells prevents convective losses, and perhaps the humble space blanket is our answer to preventing radiant losses.
If you want to read 101 other ways in which a space blanket might be useful in the wilderness - look no further than the rampant objectophilia of Wallner et al [17] who, ascending to new heights of unchecked enthusiasm, would have you believe you can do basically anything with one.
Credit:
DR1
References:
[1] Guly H. History of accidental hypothermia. Resuscitation. 2011;82(1):122-5.
[2] Koop LK, Tadi P. Physiology, heat loss. StatPearls. [Updated Jul 17, 2023]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK541107/
[3] Walls RM, Hockberger RS, Gausche-Hill M, et al. Rosen's Emergency Medicine. 9th Ed. Elsevier. 2018.
[4] Shitzer A. Assessment of the effects of environmental radiation on wind chill equivalent temperatures. Eur J Appl Physiol. 2008;104(2):215-20.
[5] Freeman S, Deakin CD, Nelson MJ, Bootland D. Managing accidental hypothermia: a UK-wide survey of prehospital and search and rescue providers. Emerg Med J. 2018;35(11):652-6.
[6] Lier M, Jebens C, Lorey-Tews A, et al. What is the best way to keep the patient warm during technical rescue? Results from two prospective randomised controlled studied with health volunteers. BMC Emerg Med. 2023;23(1):83.
[7] Apollo experience report - thermal protection from engine-plume environments. National Aeronautics and Space Administration (NASA). 1972. Available at: https://ntrs.nasa.gov/api/citations/19720018272/downloads/19720018272.pdf
[8] Kranebitter H, Wallner B, Klinger A, et al. Rescue blankets-transmission and reflectivity of electromagnetic radiation. Coatings. 2020;10(4):375.
[9] Isser M, Salchner H, Groemer G, Kofler A. Rescue blankets hamper thermal imaging in search and rescue missions. SN Appl. Sci. 2020;2(9): DOI:10.1007/s42452-020-03252-6
[10] Buggy D, Hughes N. Pre-emptive use of the space blanket reduces shivering after general anaesthesia. Br J Anaesth. 1994;72:393-6.
[11] Buggy D, Gardiner J. The space blanket and shivering during extradural analgesia in labour. Acta Anaesthesiol Scand. 1995;39(4):551-3.
[12] Henriksson O, Lundgren P, Kuklane K, et al. Protection against cold in prehospital care: evaporative heat loss reduction by wet clothing removal or the addition of a vapor barrier—a thermal manikin study. Prehosp Disaster Med. 2012;27(1):53-8.
[13] Zasa M, Flowers N, Zideman D, Hodgetts TJ, Harris T. A torso model comparison of temperature preservation devices for use in the prehospital environment. Emerg J Med. 2016;33(6):418-22.
[14] Jussila K, Rissanen S, Parkkola K, Hannu A. Evaluating cold, wind, and moisture protection of different coverings for prehospital maritime transportation - a thermal manikin and human study. Prehosp Disaster Med. 2014;29(6):580-8.
[15] Light IM, Norman JN. The thermal properties of a survival bag incorporating metallised plastic sheeting. Aviat Space Environ Med. 1980;51(4):367-70.
[16] Kosiński S, Podsiadło P, Darocha T. Prehospital use of ultrathin reflective foils. Wilderness Environ Med. 2022;33(1):134-9.
[17] Wallner B, Salchner H, Isser M, et al. Rescue blankets as multifunctional rescue equipment in alpine and wilderness emergencies - a narrative review and clinical implications. Int J Environ Res Public Health. 2022;19(19):12721.