THERMAL ANALYSIS OF HUMAN BODY

The rate at which body produces heat is called the metabolic rate. The heat produced by a normal healthy person while sleeping is called the basal metabolic rate, which is of the order of 60 W. The maximum value may be 10 times as much as this for a person engaged in sustained hard work. Human comfort is influenced by physiological factors determined by the rate of heat generation within the body and the rate of heat dissipation to the environment.

The body loses heat to the environment mainly by convection (Q_conv), radiation (Q_rad) and evaporation of moisture (Q_evap). In addition, there is heat loss by respiration having sensible component (Q_Sr) and latent component (Q_Lr) the total heat loss from the body is thus,

Q = Q_conv + Q_rad + Q_evap + (Q_Sr + Q_Lr)

= (Q_conv + Q_rad + Q_Sr) + (Q_evap + Q_Lr)

= Q_ST + Q_LT

where   Q_ST = total sensible heat loss

Q_LT = total latent heat loss.

The total sensible heat component depends on the temperature difference between the surface of the body and the surroundings. The latent heat component depends in the difference in the water vapour pressures.

In summer, the temperature difference available for sensible heat transfer is less. Thus, the convective and radiative heat losses are reduced. To maintain thermal equilibrium, the body starts perspiring to increase the evaporative loss. On the other hand in winter the sensible heat transfer is increased so that the evaporative losses tend towards zero.

The heat exchange between human body and environment can be expressed by the following energy balance equation:

where  Q_M = metabolic heat produced within the body

W= useful work done by human being

Q_M − W = heat to be dissipated to environment

Q = Q_conv + Q_rad + Q_evap+ (Q_Sr+ Q_Lr)

Q_s = heat stored in the body

1. The metabolic heat produced depends upon the rate of food energy consumption in the body.
2. The heat loss by evaporation is always positive. It depends upon the vapour pressure difference between the skin surface and the surrounding air. where C_d = diffusion coefficient, kg of water evaporated per unit surface area and pressure difference per hour.A = skin surface area = 1.8 m² for a normal human beingp_s = saturation vapour pressure corresponding to skin temperaturep_v = vapour pressure of surrounding airh_fg = latent heat of vaporisation = 2450 kJ/kgC_c = factor which accounts for clothing worn.
3. The heat loss or gain by radiation from the body to the surroundings depends upon the mean radiant temperature. It is the average surface temperature of the surrounding objects. Q_rad is positive when the mean radiant temperature is lower than the DBT of room air, i.e. the human body will undergo a radiant heat loss. When Q_rad is negative, the body will undergo a radiant heat gain.
4. The heat loss by convection from the body to the surroundings is given by where   U = body film coefficient of heat transferA = body surface area = 1.8 m² for normal human beingt_b = body temperaturet_s = surroundings temperature.The heat will be gained by body when t_s > t_b and Q_conv is negative
5. Q_S is negative when Q > (Q_M − W), i.e. body temperature falls down. Q_S is positive whenQ < (Q_M − W), i.e. a human being gets fever.A human body feels comfortable when there is no change in the body temperature, i.e. when Q_S = 0. Any variation in the body temperature acts as a stress on the brain which results in either perspiration or shivering.

1 Factors Affecting Human Comfort

The following factors affect human comfort:

1. Effective temperature.
2. Heat production and regulation in human body.
3. Heat and moisture losses from the human body.
4. Moisture content of air.
5. Quality and quantity of air.
6. Air velocity.
7. Hot and cold surfaces.
8. Air stratification.

2 Physiological Hazards Resulting from Heat

The physiological hazards resulting from rise in body temperature are:

1. Heat exhaustion: It is due to the failure of normal blood circulation. The symptoms of heat exhaustion include fatigue, headache, dizziness, vomiting and abnormal mental reactions such as irritation. Severe heat exhaustion may cause fainting. It does not cause permanent injury to the body and recovery is usually rapid when the person is shifted to a cool place.
2. Heat Cramp: It results from loss of salt due to an excessive rate of body perspiration. It causes severe pain in high muscles. This may be avoided by using salt tablets.
3. Heat stroke: It occurs when a person is exposed to excessive heat and work. If the body temperature rises rapidly to 40.5°C or 105°F, sweating ceases and a person may enter a coma, with imminent death. It may lead to permanent damage to brain. Heat stroke can be a avoided by drinking sufficient water at frequent intervals.