## Convection Radiation and Evaporation Are the Main Avenues of Heat Exchange With the Environment

Convection is the transfer of heat resulting from the movement of a fluid, either liquid or gas. In thermal physiology, the fluid is usually air or water in the environment or blood, in the case of heat transfer inside the body. To illustrate, consider an object immersed in a fluid that is cooler than the object. Heat passes from the object to the immediately adjacent fluid by conduction. If the fluid is stationary, conduction is the only means by which heat can pass through the fluid, and over time, the rate of heat flow from the body to the fluid will diminish as the fluid nearest the object approaches the temperature of the object. In practice, however, fluids are rarely stationary. If the fluid is moving, heat will still be carried from the object into the fluid by conduction, but once the heat has entered the fluid, it will be carried by the movement of the fluid—by convection. The same fluid movement that carries heat away from the surface of the object constantly brings fresh cool fluid to the surface, so the object gives up heat to the fluid much more rapidly than if the fluid were stationary. Although conduction plays a role in this process, convection so dominates the overall heat transfer that we refer to the heat transfer as if it were entirely convection. Therefore, the conduction term (K) in the heat balance equation is restricted to heat flow between the body and other solid objects, and it usually represents only a small part of the total heat exchange with the environment.

Every surface emits energy as electromagnetic radiation, with a power output proportional to the area of the surface, the fourth power of its absolute temperature (i.e., measured from absolute zero), and the emissivity (e) of the surface, a number between 0 and 1 that depends on the nature of the surface and the wavelength of the radiation. (In this discussion, the term surface is broadly defined, so that a flame and the sky, for example, are surfaces.) Such radiation, called thermal radiation, has a characteristic distribution of power as a function of wavelength, which depends on the temperature of the surface. The emissivity of any surface is equal to the absorptivity—the fraction of incident radiant energy the surface absorbs. (For this reason, an ideal emitter, with an emissivity of 1, is called a black body.) If two bodies exchange heat by thermal radiation, radiation travels in both directions, but since each body emits radiation with an in tensity that depends on its temperature, the net heat flow is from the warmer to the cooler body.

At ordinary tissue and environmental temperatures, virtually all thermal radiation is in a region of the infrared range where most surfaces, other than polished metals, have emissivities near 1 and emit with a power output near the theoretical maximum. However, bodies that are hot enough to glow, such as the sun, emit large amounts of radiation in the visible and near-infrared range, in which light-colored surfaces have lower emissivities and absorptivities than dark ones. Therefore, colors of skin and clothing affect heat exchange only in sunlight or bright artificial light.

When 1 g of water is converted into vapor at 30°C, it absorbs 2,425 J (0.58 kcal), the latent heat of evaporation, in the process. Evaporation of water is, thus, an efficient way of losing heat, and it is the body's only means of losing heat when the environment is hotter than the skin, as it usually is when the environment is warmer than 36°C. Evaporation must then dissipate both the heat produced by metabolic processes and any heat gained from the environment by convection and radiation. Most water evaporated in the heat comes from sweat, but even in cold temperatures, the skin loses some water by the evaporation of insensible perspiration, water that diffuses through the skin rather than being secreted. In equation 1, E is nearly always positive, representing heat loss from the body. However, E is negative in the rare circumstances in which water vapor gives up heat to the body by condensing on the skin (as in a steam room).