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Rapid Responses: Rapid Responses published:
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Excess heat loss in the hatless
- Simon S Ridout (19 December 2008)
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Excesss heat loss in the hatless
- Richard H George (19 December 2008)
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Head cover in the cold
- Evan L Lloyd (21 December 2008)
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Heat loss from the head in cold weather
- Kenneth J Collins (7 January 2009)
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Hangover cures
- G Romaine Hervey (8 January 2009)
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Poinsettia toxicity - cat is dead
- Sylvia Sabrowski (11 January 2009)
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Is vanilla addictive?
- Hugh Mann (3 March 2009)
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Simon S Ridout, Consultant Occupational Physician Sellafield Ltd, Seascale, Cumbria CA20 1PG
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I caution readers not to go hatless in inclement weather. As countless generations of Yorkshiremen know, to do so could well be fatal (Reference). All with either a basic knowledge of thermal physiology, a degree of (not in)common sense or sufficient inquisitivity to have investigated, know that the words 'because "40 to 45 percent of body heat" is lost through the head' really mean: 'In a hatless but otherwise properly clothed person 40 to 50% of body heat is lost through the head'. Therefore it remains very sensible (and possibly life saving) advice to wear a hat, whether on Ilkley Moor or elsewhere in cold weather. Simon Ridout, Consultant Occupational Physician, Sellafield Ltd, Seascale Cumbria CA20 1PG Reference: http://www.ilkley.org/iguide/baht.htm Competing interests: None declared |
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Richard H George, RYA instructor Retired Consultant Medical Microbiologist
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As a result of the extensive media coverage of this article in the UK over the last few days, a new myth is likely to be borne unto to the authors of this "Seasonal Fayre". The degree of heat loss from the human body via any route is analogous to lagging your house. If you insulate your walls the proportion of heat lost by other routes is relatively greater, although the actual heat loss by these routes remains the same. Whilst research does show that heat loss from the head in naked individuals is only 7 - 10% of the total heat loss, I have yet to observe naked smokers outside our pubs this winter, so this percentage loss is unlikely to be applicable. Similarly few are dressed for arctic survival conditions, so heat loss from their heads may not reach 40 - 45% levels reported in Army survival studies. What is the percentage of heat loss via the head from a normally clothed person. I don't know ? However, the context in which the research findings are applicable is important. For us sailors lagging the roof (our head) as well as wearing windproof, breathable, low thermal conductivity clothing is essential in cold conditions. Snowboarders and skiers similarly seem to consider headgear important and Santa wears a very smart thermal hat. Provided a hatless smoker doesn't die from hypothermia outside a pub this winter the authors advice regarding hats may not matter, but these unfortunate cigarette addicts and many other people can improve their own comfort greatly by hat wearing. Competing interests: None declared |
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Evan L Lloyd, retired 72 Belgrave Road, Edinburgh EH12 6NQ
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I agree that the simple statement that "40-50% of body heat" is lost through the head is inaccurate. The true statement is that at – 4 degC half the heat production of a resting (clad) man may be lost through the head and that at – 15 degC this may rise to 70%. Also in normal circumstances people do not venture into the cold wearing only swimsuits. The head is also different to other parts of the body. 1. The insulating layer of subcutaneous fat over the head is much less than over the rest of the body, and becomes even less in the elderly. 2. Vasoconstriction is much less effective over the head. 3. The head has a covering of hair over a large area which provides some additional protection but, mainly in males, this covering becomes less with age though the extent varies. 4. Covering the head reduces heat loss caused by temperature difference between the skin and the environment. However the effect of wind and wet are much more important (factors often not measured in laboratory studies) and using a hat provides protection from these factors as well. 5. In babies, where the surface area of the head is a much larger proportion of the total body surface than in adults, and the brain is a major heat- producing organ, prevention of heat loss from the head can be critical. In fact the use of a close fitting gamgee-lined hat reduced the rate of fall of core temperature in a naked neonate, and also reduced the oxygen consumption. The same gamgee insulation over the lower abdomen did not have a measureable effect. 6. During personal experience of camping above the snow line with an inferior sleeping bag, I found that I felt too cold and uncomfortable to sleep till I put on a wooly hat. I then slept comfortably. To summarise, covering the head in cold weather can produce a marked effect in subjective comfort and reduction of heat loss. I would hate to think that this myth-busting exercise, while discouraging them from venturing out in swimwear, may lead to people going out in very cold weather (cold, windy and wet) without bothering to protect their heads. References All these points, except number 6, are cited in Lloyd Evan L Hypothermia and Cold Stress. Croom Helm, Kent. 1986. Competing interests: None declared |
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Kenneth J Collins, Clinical physiologist Guildford GU1 2BU
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In exposing the "myth" that 40 -50% of body heat is lost through the head in cold weather (1) the authors conclude that the body should be protected but that it is a matter of individual preference whether or not to cover the head. This appears to be too cavalier a statement to go unchallenged, particularly in the case of elderly people exposed to cold weather conditions. As the main channel of convective and radiant heat loss when the body is well insulated, exposure of the head and face can account for a large proportion of body heat loss. It is, of course, a different matter when the body and head are immersed in water, as in the experiments quoted, when a measured 10% of total surface heat loss occurs from the head. The claim that there is nothing special about the head in heat balance ignores the important influence of facial cooling in air on systemic cardiovascular reflex responses (2) and that body temperature can be selectively influenced by cooling of the head and face. Covering these areas with hat and muffler in cold weather should remain part of the recommendations designed to help reduce winter morbidity and mortality. 1 Vreeman R, Carroll A. Seasonal medical myths that lack convincing evidence. BMJ 2008 ; 337 : a2769 (20-27 December). 2. Collins KJ, Abdel-Rahman TA, Easton JC, Sacco P, Ison J, Dore C. Effects of facial cooling on elderly and young subjects : interaction with breath-holding and lower body negative pressure. Clin Sci 1996 ; 90 : 485 - 492. Competing interests: None declared |
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G Romaine Hervey, Retired Prof. of Physiology, Univ. Leeds Garth House, Beryl Lane, WELLS, Somerset BA5 2XQ
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I was rather disappointed with the section "You can cure a hangover . ." in “Seasonal Medical Myths . .” in the BMJ Christmas issue, vol. 337, p.1443. Known physiology can surely provide some basis for treatment, and there have been articles in the BMJ suggesting that massive blind crossed-over trials are not always essential, particularly where that is the case. In the note quoted water is mentioned once, in the first sentence, coupled with Vegemite (whatever that is). It has been known at least since the days of Shakespeare that excessive consumption of alcohol causes loss of body water [reference surely not necessary: it is to the Porter in Macbeth; I haven't got an edition of Shakespeare that gives line numbers]. From 1942 onwards I was working in the late Professor R. A. McCance's Department of Experimental Medicine for the Royal Navy on the problems of survival at sea. These of course include lack of drinkable water. Prof. McCance was a great practiser and teacher of 'do-it-on-yourself' physiology. In the course of the work I noticed that the symptoms of experimental dehydration corresponded closely with those of a hangover. A few years later I found myself in uniform as the doctor on a small flat-bottomed minesweeper (HMS Truelove: as it happened I was engaged at the time) on Fisheries Protection duty but also to test the new Naval life-raft (the ancestor of all current life-rafts) in Artic waters in early spring. The evening before we were to sail at 0600 into the North Sea from Rosyth, the wardroom officers (about six in number) came to me and demanded preventative treatment for any hangovers. With difficulty, I managed to persuade them that only drinking water would work. "Right, Doc, we'll take you at your word", they responded -- I thought a little threateningly. At about midnight they returned to the wardroom, carrying trays laden with pint glasses of water, which they proceeded to down. I heard no more from them -- which I regard as positive evidence -- and the ship sailed on time into a rough sea with all hands in good shape. I do in all seriousness recommend an adequate dose of water as an effective (and cost-effective) treatment. (I am not in any way disparaging proper comparative trials where possible and appropriate. When we compared motion sickness preventative drugs in the early 1950s, the design of the double-blind crossed-over trials, which I was responsible for, was quoted as an early example of good design.) G. R. Hervey, Emeritus Professor, University of Leeds, and sometime Chairman, Survival at Sea Sub-Committee, Royal Naval Personnel Committee. Competing interests: None declared |
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Sylvia Sabrowski, Special Education 59457 Werl Germany
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I heard about your "news" that Poinsettia was not toxic in German media. It was not mentioned that it means only for human beings. My cat ate some leaves, it is dead now. So Poinsettia is (!) toxic - for pets / small animals. Sorry for my bad school English, but it is important for me to tell how dangerous incomplete information can be. Sylvia Sabrowski Competing interests: None declared |
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Hugh Mann, Physician Eagle Rock, MO 65641 USA
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Despite its benign reputation, vanilla is highly addictive. The active ingredient in vanilla is vanillic acid. Vanillic acid is related to the hormones epinephrine (adrenaline) and norepinephrine (noradrenaline), both of which stimulate the brain. This chemical relationship explains why vanilla is a potent and addictive stimulant. Although people assume that vanilla is harmless, or even beneficial, companies know otherwise. This is why vanilla is added to ice cream, yogurt, cola, chocolate, pastry, perfume, and even cigarettes. It is wise to avoid vanilla and all products that contain vanilla. Competing interests: None declared |
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