Definition
THERMAL COMFORT: The term describes a person’s psychological state of mind and is usually referred to in terms of whether someone is feeling too hot or too cold. It is defined in British Standard BS EN ISO 7730 as: “that condition of mind which expresses satisfaction with the thermal environment.”
Where The Occupational Safety and Health Administration (OSHA) does not have a special standard for thermal stress. However, since thermal stress is considered a hazard, the employee is protected under OSHA’s General Duty Clause, which states that “employers shall provide a workplace free of recognized hazards.…
Thermal comfort is difficult to objectively determine because allowances have to be made for a range of environmental and personal factors when deciding what will make people feel comfortable. We all know from personal experience what room temperatures will make us feel most comfortable and we know that this will vary depending on a range of factors, such as the activity that we are performing. For example, as I sit writing these words the air temperature is 18°C and I am feeling a little cold. If I were stood up lecturing that would be a perfect temperature for me, but probably rather cold for those sat listening (metabolic work rate is higher if you are stood than when you are seated). If I were playing five-a-side football it would be unbearably hot in an indoor sports hall with no cooling breeze (running makes metabolic work rate soar). We also know from personal experience that what suits us does not suit other people. People have very different opinions about what constitutes thermal comfort. Some people would happily sit in an office heated to 25°C, a temperature that others would find sleep-inducing. You all have your own opinions on the matter.
In this thermal comfort there are two parts hot and cold weather but in this one We are going to discuss about hot Environment
Heat Stress
Heat stress is of great concern in the workplace, as well as during recreational activities. It is caused by a combination of factors and tends to increase body temperature, heart rate, and sweating
Heat Balance
The body can be considered to be a chemical factory which carries out myriad complex chemical reactions at
a temperature of about 37°C and a pressure of one atmosphere. Normal pressure variations do not affect the
metabolic processes unduly, but the body temperature is critical and does not vary in general outside the range 36-
38°C. Many body reactions involve oxidation where heat will be generated. In order to maintain a stable temperature required for the efficient functioning of the central nervous system and body organs, the metabolic heat loss must be carefully controlled. The body has developed a very sensitive heat control system which is able to react to the considerable variations in environmental temperatures to which the body is exposed and can activate mechanisms which will oppose or allow heat loss in an effort to keep the body temperature stable.
Thermo-Regulatory Mechanisms
As core temperature increases, the body responds by trying to lose heat to the external environment. This is done by transporting heat from the core to the surface of the body (the skin). Blood vessels in the dermal layer of the skin dilate (vasodilation) so that blood can flow close to the surface of the skin. The increase in temperature at the skin leads to heat loss by radiation (infrared radiation) and direct conduction into colder air. At the same time, sweat (a mixture of water and salts extracted from the blood by the sweat glands in the dermal layer of the skin) is excreted up the sweat ducts and onto the surface of the skin. This liquid spreads over the surface of the skin and evaporates. As it does so, it takes heat from the surface of the skin (this is the heat required to change the state of the water from a liquid to a vapour). This lost heat cools the skin, which, in turn, cools the blood. The cooled blood is returned to the core where it picks up more heat; and so the cycle is repeated. As core temperature decreases, the opposite reaction takes place. Blood supply to the skin is reduced by restricting the size of the blood vessels (vasoconstriction). The excretion of sweat onto the surface of the skin stops. Blood is kept in the core of the body and blood supply to the extremities will be reduced to a minimum. Hair on the skin is brought upright. This is done by small erectile muscles in the dermal layer of the skin attached to each hair follicle – this is what creates the puckered skin effect of ‘goose-bumps’. If core temperature cannot be maintained, then the body may respond by trying to create additional heat through muscle activity in the form of uncontrollable shivering.
Sources of Heat Stress
There are four basic sources of heat to the body, namely,
(1) radiation,
(2) convection,
(3) conduction,
(4) metabolic.
It is important to understand the process of heat transfer.
The important heat transfer mechanisms are:
• Radiation.
• Evaporative cooling.
• Convection.
- Metabolic.
Conduction is of limited importance except where there is considerable body contact with a heating or cooling source, e.g. motor fitters lying on concrete floors.
Convection is very important as heat transfer from the body depends on convection currents in the air. Convection currents are also an integral part of evaporative cooling.
Radiation becomes progressively more important as body temperature rises. Evaporative heat loss is the most important heat loss mechanism of all and becomes the most significant heat loss mechanism once air temperature gets to 35oC or more.
Metabolic heat is generated from within the body through work. Metabolic heat is the source of greatest concern, since it has potential to overheat the body.
The body’s thermal balance can be represented by the equation:
M = K + C + R + E
Where:
• M is the rate of metabolic heat production.
• K is the loss or gain of heat by conduction.
• C is the loss or gain of heat by convection.
• R is the loss or gain of heat by radiation.
• E is the heat loss from skin or respiratory tract due to evaporation of moisture (so this is never +ve).
When both sides of the equation balance, thermal balance exists (i.e. the metabolic heat production rate is balanced by the amount of heat being gained or lost through the various mechanisms).
When the equation does not balance, core temperature cannot be maintained and rises or falls to cause heat stress or cold stress respectively.
-
Health Effects of Working in hot Environments
Exposure to Hot Conditions
When air temperature is high, or significant radiant heat is falling on a person, or they are doing heavy work perhaps whilst wearing protective clothing, then core temperature may increase faster than the body can lose heat. If the body is not able to lose heat quickly enough and core temperature rises, this leads to heat stress.
Since one of the major ways that heat is lost by the body is by evaporative heat loss (sweating), the amount of water vapour in the air (humidity level) has a significant effect on rate of heat loss. When the air temperature reaches 35°C or more, the body loses heat by sweating alone. If relative humidity levels are 80% or more then evaporation of sweat virtually stops. This is why tropical climates can be exhausting; the combination of high air temperature and high humidity means you sweat but do not enjoy any cooling effect – you simply get wet and dehydrate but it does not cool you down.
The extent of the effects depends on: the individual (in particular, whether they are acclimatised); environmental conditions, such as temperature, humidity and air movement; clothing; work rate, etc.
Typical symptoms of heat stress :
• Inability to concentrate.
• Muscle cramps (due to salt loss through sweating).
• Heat rash (sometimes called ‘prickly heat’).
• Severe thirst (due to dehydration) – a late symptom.
• Fainting (sometimes called ‘heat syncope’).
• Heat exhaustion – fatigue, giddiness, nausea, headache, moist skin.
• Heat stroke – hot dry skin, confusion, convulsions, loss of consciousness.
This is the most serious effect as it can lead to coma and death.
These symptoms are caused by a combination of dehydration (loss of water and salts from the blood through sweating), loss of blood pressure (caused by the blood vessels vasodilating to bring the blood closer to the surface of the body) and the increased core temperature (which takes metabolic processes away from their optimal rate).
Depending on circumstances, mild heat stress can develop into heat exhaustion and heat stroke over several days, hours or minutes.
Typical work situations causing heat stress:
• Furnace work, handling molten metal.
• Glass-making.
• Welding, brazing.
• Boiler and furnace maintenance, boiler-room work.
• Deep-mining work.
• Laundries.
• Kitchens.
• Fire-fighting.
Environmental Parameters that Affect Comfort
-
Air Temperature
In terms of temperature, heat will always flow from a high temperature to a low temperature. It therefore follows that, as the temperature of the surroundings increases, the body will find it increasingly difficult to lose heat. There will come a point where the body will begin to overheat. Similarly, as air temperature falls, the heat gradient between the body and the surrounding air becomes steeper; this causes heat to flow from the body more rapidly, making it difficult for the body to maintain temperature.
-
Radiant Temperature
Thermal radiation is the heat that radiates from a warm object. Radiant heat may be present if there are heat sources in an environment.
Radiant temperature has a greater influence than air temperature on how we lose/gain heat to/from the environment. Our skin absorbs almost as much radiant energy as a matt black object, although this may be reduced by wearing reflective clothing.
Examples of radiant heat sources :
• The sun.
• Fire.
• Electric fires.
• Furnaces.
• Steam rollers.
• Ovens.
• Walls in kilns.
• Cookers.
• Dryers.
• Hot surfaces.
• Machinery.
• Molten metals, etc
-
Humidity
The amount of water vapour in the air has a bearing on the ability to lose water vapour from sweat on the surface of the skin, so humidity becomes an important factor in determining the amount of evaporative cooling.
Humidity is a measure of the concentration of water vapour in the atmosphere. The amount of water vapour is dependent on two main factors, the:
• Presence of liquid water to supply the water vapour; air over water will have a higher concentration than air over a desert.
• Temperature of the air; the higher the air temperature, the greater the capacity of the air to hold water vapour.
-
Metabolic Rate
The body’s metabolic rate can be expressed in watts (joules of energy per second) or watts per square metre (of body surface area). The metabolic rate in a completely resting body is approximately 45W/m2 of body surface. If the surface area of a typical male is taken as 1.8m2 this amounts to a total heat output of about 80W.
-
Clothing
Clothing, as you might expect, has a significant effect on the ability of the body to lose heat to the external environment. In considering conduction of heat at the body surface through clothing, it is the resistance (or insulation) to heat flow across a given thickness of material that we are concerned about. This parameter is given an arbitrary unit, the ‘clo’, to express the insulation value of clothing; 1.0clo is defined as the insulation provided by clothing which allows comfort in still air at a uniform temperature of 21°C. Thermally insulating polar clothing provides clo values as high as 3 or 4.
-
Sweat Rate
Sweat rate needs to be within certain narrow limits for us to feel comfortable (put simply, sweating helps us cool down but excessive moisture makes us feel uncomfortable – we like to be largely free of sweat).
-
Duration of Exposure
The longer someone is exposed to thermal discomfort, the more severe the effects are likely to be.
Instruments for measuring temperature :
- Thermometer
- Hygrometers
- Anemometer
Wet Bulb Globe Temperature
(WBGT) Index
The purpose of heat stress indices is to estimate the physiological responses of an individual to their environment.
The end result is to provide a value that allows a comparison between environments, different working situations and different types of clothing to be made.
Various types of heat stress index have been developed that summarise environmental, and other, parameters into a single number which can be used to quantify the severity of the thermal environment. The most widely used is the WBGT Index.
The Wet Bulb Globe Temperature (WBGT) heat stress index is the most widely accepted heat stress index and forms the basis of many standards. It has been published as British Standard BS EN ISO 7243:2017.
WBGT is calculated from:
WBGT = 0.7 WB + 0.3 GT indoors
or
WBGT = 0.7 WB + 0.2 GT + 0.1 DB outdoors
Where:
• WB is the wet bulb temperature.
• GT is the globe thermometer temperature.
• DB is the dry bulb temperature.
The outdoor formula reduces the influence of the globe contribution from direct sun.
The index takes account of radiant and air temperatures, humidity and low air velocities. The index value must be used in conjunction with empirical recommendations (set out in ISO 7243) to indicate a level that is safe for most people who are physically fit and in good health. Different values are quoted to distinguish persons acclimatised orunacclimatised to heat. If conditions of exposure fluctuate, a time-weighted average exposure can be derived and used.
Practical Control Measures
Aswith any hazard associated with the workplace, OSHA requires that engineering controls, administrative controls, and personal protective equipment be used in that order to prevent work-related injuries.
• Control Heat Sources
It may be possible to enclose heat sources and provide lagging/insulation to prevent the escape of heat into the wider workplace environment. For example, a large oven or steam pipes passing through a workspace can be lagged/insulated. It may also be practical to separate areas within the workplace where excess heat and/ or humidity are going to occur from other areas where such extremes are not needed, effectively segregating extreme areas from other workspaces.
-
Circulation of Air and Ventilation
– Ventilation can be used to remove or dilute hot/humid air and replace it with cool/dry air. Air conditioning can be used for internal workspaces.
–Increased air velocity can aid bodily cooling through heat loss due to increased sweat evaporation. Air handling units and fans can be used for internal workspaces.
• Workplace Design
Radiation barriers placed between the source of heat and the worker can reduce the level of radiant heat
exposure. (Barriers should be good insulators and high-heat reflectors to prevent them heating up and becoming
radiant heat sources themselves.)
In outdoor workplaces, shade from the heat of the sun can be supplied. If this is not practical then shade and fans
might be supplied in external rest areas to help workers cool down between work rotations.
Cool refuges can be incorporated into very hot and humid environments so that workers can temporarily seek
relief from the extreme environment to cool down.
• Job Design and Job Rotation
For outdoor workers, restrict work to the cooler parts of the day or work at night. Avoid work during the hottest
parts of the day:
– Control the duration of each work period and ensure that sufficient rest breaks are incorporated into work
routines so that workers can cool down. Heat stress indices, such as the WBGT index, can be used to make calculations of the maximum allowable work period and the rest period required to achieve heat balance.
– Supervision is necessary to ensure that the work regimes are followed and that potential heat stress is detected at an early stage.
– Acclimatisation is important to enable workers to become used to the more extreme thermal environments.
Acclimatisation allows the worker’s body to adjust physiologically and it also allows workers to adjust mentally
how they plan and manage their work.
– Easy access to drinking water or other cold drinks is important for workers in hot environments, as is access to
electrolytes (salt) so that salts lost by sweating can be replaced.
• Personal Protective Equipment
Personal protective equipment insulates the body and reduces evaporative heat loss which increases the risk of heat stress if physically demanding work is carried out. If protection is needed against radiant heat, heat-resistant clothing gives limited protection with ice-cooled jackets and air-cooled or water-cooled suits needed for longer periods of exposure. However, the use of such personal protective equipment is likely to increase the metabolic rate and possibly lead to thermal strain.
• Information, Instruction, Training and Supervision
Workers need to understand the hazards and risk associated with heat stress, dehydration and heat stroke. They must be trained to understand the uses and limitations of the control measures necessary and how to recognise the signs and symptoms of heat stress in themselves and their fellow workers, so that they can react appropriately.
• Health Surveillance
The regular monitoring of workers is important for those who work in extreme temperatures. The testing of kidney, respiratory and cardiovascular functions, together with general health, are relevant.
