Clothing The Portable Environment Part A: Body Responses to Heat and Cold Dr. Jimmy Lam Institute of Textiles & Clothing

Agenda Mechanisms of Heat Transfer Heat balance and the Body Blood flow and Temperature Evaporation Individual response to cold environment

Introduction Clothing is our most intimate environment. What makes it a unique environment is that it is carried everywhere with an individual, creating its own room within a room and its own climate within the larger climate of our surrounding. It is a key factor in modifying energy consumption, it holds an important role in consumer protection.

Heat Transfer Mechanisms

Mechanisms of Heat Transfer Heat is the transfer of energy from one object to another by virtue of temperature difference between them. There are four general mechanisms by which heat transfer occurs in the environment. They are conduction, convection, radiation and evaporation.

Mechanisms of Conduction Conduction of heat occurs when the surfaces of two objects touch. The surfaces of objects often act as conduits for heat exchange for the whole object, with heat flowing from the warmer object into the cooler object until both are at equal temperatures. If conduction occurs because of contact, the more contact between two surfaces, the more conduction takes place.

Convection & radiation Convention relies on some sort of fluid medium, either gas or liquid, for heat transfer. Unlike conduction, convection involves material transfer. Radiation involves the transfer of heat by electromagnetic waves. Electromagnetic waves are the result of interaction between the electric field and a magnetic field as they move through space. Radiant heat transfer does not require air (as convention does), nor requires any sort of matter for its transfer (as conduction).

Evaporation Evaporation is an extremely effective means of heat transfer that occurs when a liquid changes in a gas (water to stream). This change in state requires a great deal of heat, as is evident in the boiling water. The evaporate heat exchange between human body and the environment involves much lower temperature than does boiling water; nevertheless, the vaporization of liquid provides substantial heat transfer.

Heat Balance and the Body

Heat balance and body The ways the body reacts to temperature conditions in the environment are fascinating. The human adjusts to temperature change by producing or releasing heat in specific ways. Two ways the body can produce heat: –By absorption of heat from the environment (standing in front of fire, the body absorbs heat through radiation, conduction and convection.) –By the body’s chemical processes (increase body metabolism.) When food is digested, the burning of food provides energy in the form of mechanical energy, stored energy and heat.

Metabloism Metabolism is affected by many things (eg food or activity) The metabolism is simulated by exercise. Age, sex, emotional state, height-weight ratio, disease also play a part in determining the metabolism rate of an individual. Heat production in an indivdual is expressed as calories per hour. A calorie is the amount of heat required to raise the temp. of 1 gm of water at 1 0 C.

Blow flow and temperature Blood is an important vehicle that carries heat throughout the body Blood carries heat from the deeper body tissues to the surface tissues, where it can be released to the environment. Changes in body temp. occur when something happens to bring greater or lesser amount of blood near the skin surface. There are several ways to do these: –Increase/decrease the rate of blood flow; –Increase/decrease the volume of blood flow; and –Opened up/ blocked out the assess of blood to the outer body surface.

Body Core The circulatory of blood flow is to protect the body core, the inner body that houses the vital organs (brain, heart, lung etc.) The body works to maintain body supply to the core at all times, even if it must sacrifice it for the extremities (hands, foot). So each individual’s vital heat-balance equation represents the body’s attempt to protect its core, to keep it within the temperature range in which it can continue to function.

Evaporation If outside temperature is very hot (eg dissert), conduction, convention and radiation cannot carry the heat away from our body, the last resort is EVAPORATION cooling. The body experiences evaporation cooling by 1) insensible perspiration and 2) sweating. Insensible perspiration is to dry out the skin from the environment (uncontrollable) Sweating is a body-controlled process that is activated by hypothalamus when blood temp rises. Sweating is an extremely efficient means of dissipating body heat by vaporization (liquid to gas).

Evaporation Evaporation takes place at a more rapid rate if the environment is relatively dry. (RH=20%) When air is completely dry, survival is possible at a temp of 130 o C for 20 mins or longer. In very moist air (wet), a temp of 45 o C may not be bearable for a few mins.

Summary It is important to recognize that the thermal balance of the body with the environment depends on the net heat gain and heat loss by each of the mechanisms (conduction, convection, radiation and evaporation) Heat may loss through conduction and gaining through radiation if one swims in cool water in a sunny day. The body gains heat by generating it through its metabolic processes or absorbing it from the environment. Heat is transferred toward or away from the body by radiation, conduction and convection. Evaporation of moisture from the skin surface provides an additional means of heat loss.