The Thermal Environment

Existing methods for evaluation of the general thermal state of the body, both in comfort and under heat or cold stress, are based on an analysis of the heat balance for the human body:

S = M — W-C-R-Јsk — Crcs — Eres — K, (6.1)


S = heat storage in body

M = metabolic heat production

W — external work

C = heat loss by convection

R = heat loss by radiation

Esk = evaporative heat loss from skin

Cres = convective heat loss from respiration

Em — evaporative heat loss from respiration

K = heat loss by conduction

The factors influencing this heat balance are activity level (metabolic rate, met or W/m2), thermal resistance of clothing Id, (clo or m2 °C/W), evapo­rative resistance of clothing Rec[ (m2 Pa/W), air temperature f. (°C), mean radiant temperature t, (°C), air speed va (m/s), and partial water vapor pressure pa (Pa). These parameters must be combined so that the thermal storage is 0, or else the working time has to be limited to avoid too much strain on the body. For comfort, the mean skin temperature also has to be within certain limits and the evaporative heat loss must be low. In existing standards, guidelines, or handbooks, different methods are used to evalu­ate the general thermal state of the body in moderate environments, cold environments, and hot environments; however, all are based on the above heat balance and the listed factors.

Aside from the general thermal state of the body, a person may find the thermal environment unacceptable or intolerable if local influences on the body from asymmetric radiation, air velocities, vertical air temperature dif­ferences, or contact with hot or cold surfaces (floors, machinery, tools, etc.) are experienced.

TABLE 6.2 Developments of International Standards for the Ergonomics of the Thermal Environment


Aims of the standard

Title of the document

ISO EN 11399

General presentation of the set of stan­dards in terms of principles and applica­tion

Ergonomics of the thermal environment; Principles and application of internanona!


ISO 13731

Standardization of quantities, symbols, and units used in the standards

Ergonomics of the thermal environment: Vocabulary and symbols

ISO 7933 |1:N 1251.5)

Thermal stress evaluation in hot environ­ments. Analytical method

Hot environments: Analytical determination and interpretation of thermal stress using calculation of required sweat

ISO EN 7243

Diagnostic method for hot environments

Hot environments: Estimation of the heat stress on working man, based on the WBGT index (wet bulb globe temperature;

ISO F. N 7730

Comfort evaluation in moderate environ­ments

Moderate thermal environments: Dйtermina tion of the PMV and PPD index and specifi­cation of the conditions for thermal comfort



Thermal stress evaluation in cold environments

Evaluation of cold environments: Dйtermi­nation of required clothing insulation, I Ivl

ISO EN 8 9.96 (EN 28996)

Data collection standards for metabolic rate

Ergonomics: Determination of metabolic heat production

ISO F. N 7726

Requirements for measuring instruments

Thermal environments: Instruments for measuring physical quantities

ISO EN 9920

Clothing insulation

Estimation of the thermal insulation and evaporative resistance of clothing ensemble

ISO EN 9886

Evaluation of thermal strain using physiological measures

Evaluation of thermal strain by phystoiogicul measu rements

ISO EN 10551

Subjective assessment’ of the thermal environment

Assessment of the influence of the thermal environment using subjective judgment scales

ISO DIS 12894

Selection of an appropriate system of medical supervision for different types of thermal exposure

Ergonomics of the thermal environment: Medical supervision of individuals exposed to hot or cold environments

ISO DIS 13732

Contact with hot, moderate, and cold surfaces

Ergonomics of the thermal environment: Methods for assessment of human responses to contact with surfaces

NT 14505

Vehicle environments

Evaluation of the thermal environments ;n vehicles

ISO DIS 344.15

People with special requirements

Ergonomics of the thermal environment: The application of international standards for people with special requirements

ISO CD 15265

Assessment of risk in moderate, hot and cold environments

Risk assessment

NP 15743

Work practice in cold environments