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If you must go outdoors into the sun, shade yourself with an umbrella or a wide-brimmed hat with lots of ventilation (to allow the sweat on your head to evaporate).” (Health Canada, Extreme Heat and Your Health, 2009)

The Issue
The head, one of the most highly exposed areas of the body to solar radiation during outdoor activities, is of particular thermoregulatory importance as a major source of convective heat loss and evaporation of sweat.1 Consequently, wearing a hat outdoors may reduce the level of thermal stress by limiting the amount of solar radiation to the head, but covering the head may also impede convective and evaporative heat loss during extreme heat. The human brain is the control centre for physiological processes and therefore brain function is vulnerable to slight fluctuations in internal body temperature, which may be elevated by exposure to extreme heat.2
To date, insufficient evidence is available to properly assess the most effective style of hat to be worn by the general public when involved in outdoor activities in hot environments. Many studies have investigated different thermal parameters with specialized types of headgear, including safety helmets,3,4 bicycle helmets,5,6 equestrian helmets,7 and rowing headgear.8 However these instances are highly specific and do not provide much useful information for the public regarding general hat wear during daily outdoor activities.
Effects of Solar Radiation
One study9 examined the ultraviolet protection provided by several styles of hats at various anatomical sites of the head (forehead, nose, cheeks, chin, and back of neck). It was found that hats with a small brim provided negligible protection at all sites apart from the forehead. Peaked baseball caps offered protection to the nose and forehead but were relatively ineffective at other sites on the face. A hat with a brim of at least 7.5 cm was found necessary in order to provide reasonable protection to the nose and cheeks. Although this study provides evidence regarding the solar protection of hats, there was no investigation into physiological indicators of thermoregulation.
Heat Transfer
Both radiant and convective heat flow have been measured and compared for rowing headgear, white and black baseball-style caps (100% cotton), and a head of hair (wig) as compared to a nude headform.8 There was a notably higher transfer of heat when the radiant source was applied directly above the head, as opposed to an angle, congruent with the greatest risk of heat illness and sunburn, often occurring at midday. All types of headgear studied reduced forced convective heat loss (heat dissipation due to forced air, such as an outdoor breeze or a fan), but baseball caps impeded this heat loss the most, by 60%. However the same baseball caps were also effective in reducing radiant heat flow by approximately 80%. These studies did not examine the subsequent effects on human skin or core temperature, therefore, it is difficult to assess whether reduced convective heat loss with a high protection from solar radiation is effective at lessening a person’s risk of developing a heat-illness.
Occupational and Athletic Considerations
When evaluating headgear, working in environments that require the use of safety helmets involves different considerations than general outdoor protection from the heat. These considerations should also be noted for athletes who participate in contact sports, as helmets are a requirement. Workers and athletes are unique because they often undergo physical exertion in hot environments and the body’s thermal balance becomes increasingly dependent on evaporative heat loss from sweating.2 While a helmet is intended to protect against the impact of falling objects in the workplace, or to protect from collision in the case of contact sports, it may cause increased thermal stress by interfering with air circulation over the head, consequently affecting work efficiency.4 During these instances, changes in physical discomfort and mental status may increase the potential for accidents and injuries so it is important that heat strain be kept to a minimum. Heat stress has been shown to decrease physical performance by impairing mental function, alertness, and motor control and by contributing to muscle fatigue.10,11In addition, with increased core body temperature a person becomes more irritable, angry, and can have other emotional states which may cause workers to overlook safety procedures or have decreased attention when performing hazardous tasks, thus increasing the likelihood of an accident.12
Studies have shown that ventilated, as compared to non-ventilated, safety hats are physiologically more effective in maintaining a worker’s state of homeostasis.3 Sweat rate was also found to be 1.2 times higher in non-ventilated, as compared with ventilated, safety hats.3 Similar studies, evaluating the effects of radiant heat on bicycle helmets, found that strong variations exist in total radiant heating.5,13 The unique designs of helmets can contribute negatively and positively to overall heat loss,5 suggesting that most helmets can be improved for use in hot outdoor environments. Conversely, Sheffield-Moore et al. (1997) found that wearing a cycling helmet had no additional impact on core temperature, head skin temperature, thermal sensation, heart rate, sweat rate or perceived exertion, when compared to cycling without a helmet.6 Therefore, additional research is required to determine the thermal properties of different forms of headgear.
It is also speculated that dark colours have a tendency to absorb the sun’s radiation and should be avoided, unlike light colours which have reflective properties. In the case of headwear, these relationships have not been extensively studied, however, one study found no differences in heat gain between a white and a black baseball cap,8 suggesting that hat colour is negligible.
Conclusions and Recommendations
Few studies have examined the thermal parameters and subsequent physiological stress generated by different types of common headgear, such as hats. Although studies have been conducted on particular aspects of heat transfer, most have focused on the effects of specific types of headgear designed for specialized activities, such as, bicycling, horseback riding, or in occupational environments where safety is of primary importance. Further research, on thermal stress associated with headgear used by the general public, is needed to adequately promote safe behaviour during extreme heat. However, based on the above information, some general recommendations can be made. Hats that are worn to protect against the sun should be wide brimmed (at least 7.5 cm) to adequately protect the face, ears, and back of the neck and well-ventilated to enhance both convective and evaporative heat loss from the body. If it is impossible to adhere to these guidelines, a person should stay out of the sun on hot days (i.e., indoors or in a shaded area) or use an umbrella as a reasonable alternative.
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  8. Bogerd CP, Brühwiler PA, Heus R. The effect of rowing headgear on forced convective heat loss and radiant heat gain on a thermal manikin headform. J Sports Sci. 2008;26(7):733 - 41.
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  10. Razmjou S. Mental workload in heat: toward a framework for analyses of stress states. Aviat Space Environ Med. 1996;67:530-8.
  11. Hunter AM, St Clair Gibson A, Mbambo Z, Lambert MI, Noakes TD. The effects of heat stress on neuromuscular activity during endurance exercise. Pflugers Arch. 2002 Sep;444(6):738-43.
  12. Jay O, Kenny GP. Heat exposure in the Canadian workplace. Am J Ind Med. 2010;53(8):842-53.
  13. Davis GA, Edmisten ED, Thomas RE, Rummer RB, Pascoe DD. Effects of ventilated safety helmets in a hot environment. Int J Ind Ergonomics. 2001;27:321-9.

January 2011