Microclimate - definition Work environment Microclimate A local atmospheric zone where the climate differs from the surrounding area A small-scale site of special conditions within a larger climate Can also refer to purpose made environments, such as those in a room or other enclosed spaces 1 2 Main microclimate elements Temperature Mean Radiant Temperature Air Velocity Air Humidity Atmospheric pressure Main microclimate elements Temperature - the average air temperature from the floor at a height of 1.1 m. Mean Radiant Temperature - the average temperature of the surrounding surfaces, which includes the effect of the incident solar radiation. Air Velocity which affects convective heat loss from the body, i.e. air at a greater velocity will seem cooler. Air Humidity which affects the latent heat losses and has a particularly important impact in warm and humid environments 3 4 Core temperature About 37 C includes the organs of the thorax, abdomen and the head This is where the vital organs are located Their enzyme systems must operate in optimal conditions The peripheral parts of the body can withstand some deviation from the core temperature 2008 Paul Billiet ODWS chemical Alternation of heat produced Thermal regulation active Sweating and evaporation physical Alternation of heat exchanged with environment Radiation Convection Conduction passive 6 1
Physical thermal regulation conduction energy is transferred by a direct contact of molecules, not by a movement of the material depends on a joint surface area and how conductive the material is convection heat transfer by motions of a medium such as air or water when the heated medium particles are caused to move away from the source of heat, carrying energy with themselves Physical thermal regulation radiation Heat transfer by the emission of electromagnetic waves which carry energy away from an emitting object No medium is needed Depends on absorbing features of the object Evaporation Change of state Direct one-way process Depends on humidity 7 8 Evaporation Latent heat - heat energy required to change a substance from one state into another. Latent heat of evaporation is the energy used to change a liquid into vapor Heat balance ΔQ M C R E heat alternation (increase / decrease) metabolic heat ΔQ = M ± C ± R E convection or conduction heat (supplied or lost) radiation heat (supplied or lost) lost evaporation heat 9 10 The body must balance its heat budget Heat is gained: by conduction from warm air surrounding the body by the body s metabolic activity which generates heat e.g. when muscle move Heat is lost: by conduction and radiation to cold air (or water) by evaporation of sweat from the body surface (c.f. properties of water) Humans can also affect their body temperature by changing their behaviour e.g. wearing different clothes, seeking shade Responses to a hot microclimate Vasodilation: the blood supply to the periphery is increased Increasing temperature Sweating increase Quickened pulse skin http://www.youtube.com/watch?v=e7kmmj_qrss 2008 Paul Billiet ODWS 12 2
Psychological responses to a hot microclimate Sleepness Work capacity decrease Longer reaction time Lower perception Increase of error and accidents number Responses to a cool microclimate Vasoconstriction decreasing the heat loss from the skin surface Cessation of sweating Shivering increasing heat production in muscles. Goose pimples Skin decrease temperature 13 14 Psychological responses to a cool microclimate Nervousness Lower concentration Work capacity decrease Maintaining the body temperature Keeping warm Increased insulation, subcutaneous fat reduces the conduction of heat from the body Staying cool Increase blood flow to skin, increases conduction and radiation of heat from the body Reduced sweating decreases evaporation Increased sweat secretion, increases evaporation 15 Increased shivering, increases heat produced by muscle tissue 2 to 5 times Reduced activity Air humidity Amount of water vapor in air Absolute water content in air (g/m 3 ) Relative current absolute humidity ralative to a maximum Maximum saturation vapor humidity Response <30% feeling dry depends on air temperature Air velocity Air flow around the object 0 m/s no air flow (motionlessness) 0.2 m/s noticeable air flow 1 m/s wind Influence on Convection and conduction exchange speed Blood pressure 17 18 3
Thermal comfort Thermal neutrality, where an individual desires neither a warmer nor a colder environment, is a necessary condition for thermal comfort. The sensation of complete physical and mental well being /ISO 7730/ Thermal comfort Zero heat balance Optimal body temperature Optimal skin temperature No sweat secretion No discomfort feeling 19 20 Comfort Assessment Comfort assessment according to ISO 7730 PPD PMV PMV (Predicted Mean Vote) from -0.5 to +0.5 WCI (Wind Chill Index) HI (Heat Index) PPD (Predicted Percentage of Dissatisfied) below 10% 21 22 PMV & PPD relation Hot Warm Slighty warm Neutral Slighty cool Cool Cold 23 4
PMV factors Personal factors Insulative clothing (Clo Value) Activity levels (Met Rate) General Factors ( microclimate elements) Localized factors Physical activity The metabolic rate is the amount of energy produced per unit of time by the conversion of food. It is influenced by activity level and is expressed in mets (1 met = seated relaxing person). Air movement/velocity Radiant asymmetry Floor surface temperatures Air temperature stratification 25 26 Activity Metabolism rate [met] Heat loss [kcal/h] Resting (laying) 0.8 70 Resting (sitting) 1.0 90 Clothing insulation describes the occupant s thermal insulation against the environment expressed in clo units. Active sitting (i.e. at a computer) 1.2 110 Active standing (i.e. shopping) 1.6 145 Moderate physical activity (home works) 2.0 180 Intensive physical activity (i.e. industry workstations) 3.0 270 Very intensive physical activity (building works) 4.0 6.0 360 540 27 28 Thermal comfort for different clothing insulation and physical activity levels Wind Chill Index t wc = 13.12 + 0.6215 t a - 11.37 v 0.16 + 0.3965 t a v 0.16 where t wc = effective "wind" temperature ( o C) t a = air temperature ( o C) v = wind velocity (km/h) 29 30 5
Wind Chill Index effects Heat Index Wind Chill Value Possible Effects 0 o C or higher Generally unpleasent 0 o to -9,5 o C Unpleasent -10 o to -17 o C Very unpleasent -18 o to -29 o C Frostbite possible -30 o to -50 o C -51 o or lower Frostbite likely - outdoor activities dangerous Exposed flesh freezes within 30 seconds 31 32 Bibliography Termofizjologia, mikroklimat i komfort cieplny, http://www.ergonomia.ioz.pwr.wroc.pl Rozporządzenie Ministra Pracy i Polityki Socjalnej z dnia 17 czerwca 1998 r. w sprawie najwyższych dopuszczalnych stężeń i natężeń czynników szkodliwych dla zdrowia w środowisku pracy. Dz.U. nr 79, poz. 513. LIFE: The science of biology, http://bcs.whfreeman.com/thelifewire8e ISO 7730: Moderate thermal environments - Determination of the PMV and PPD indices and specification of the conditions for thermal comfort. 1984 33 34 6