ergonomics (thermal stress)

THERMAL STRESS

PRESENTED BY:

Arundhati P Dolas

Roll No.3

M.Sc-I RM-ERGO

2013-14

Wednesday, April 12, 2023 THERMAL STRESS

CONTENT§ Definition§ Thermo-regulation of the human body§ Variables which affect thermal sensation§ Heat stress§ Heat acclimatization§ Factors causing heat stress§ Types of heat transfer§ Health effects of heat stress§ Methods for measuring heat stress§ Control of heat stress§ Prevention of heat stress§ Conclusion

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Definition

Thermal stress is defined as the

physical and physiological reactions

of the human body to temperatures

that fall outside of the human

normal comfort zone.

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Thermo-Regulation Of The Human Body

• Humans are warm-blooded animals, meaning they maintain their body temperature internally. This control of body temperature is referred to as thermoregulation.

• The human body regulates temperature by keeping a tight balance between heat gain and heat loss.

• Humans regulate heat generation and preservation to maintain internal body temperature or core temperature. Normal core temperature at rest varies between 36.5 and 37.5 °Celsius (°C), which is 97.7 to 99.5 °Fahrenheit (°F). Core temperature is regulated by the hypothalamus (in the brain), which is often called the body’s thermostat.

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Temperature Detection

When met with environments that threaten the internal temperature of the body, such as cold or hot conditions, the human body is triggered into response. For example, nerve endings in the skin detect temperature changes in the environment outside the body and signal the brain to either increase or decrease the heat inside the body.

Reactions to Hot Temperatures

When the body senses warm temperatures any number of reactions may occur. Sweating is one method for reducing body temperature. The sweat glands in the skin excrete sweat, which contains water and some nutrients. The process of evaporation then works to cool the body. The drawback to perspiration is the loss of water. Maintaining water within the body is especially important in high temperatures.

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Hypothalamus Regulation of Temperature

Hypothalamus acts as

“thermostat” that makes

thermoregulatory adjustments

to deviations from

temperature norm in the brain

(37 C ± 1 C or 98.6

±1.8 F ).

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Variables Which Affect Thermal Sensation

Age, weight, degree of physical fitness

Degree of acclimatization, metabolism

Use of alcohol or drugs

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HEAT STRESS

Hot conditions put your

body under a lot of stress. Heat stress is commonly

associated with warm weather. When heat is combined with physical activity, loss of

fluids, fatigue, and other conditions it can lead to a number of heat-related illnesses and injuries. Death is even possible.

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HEAT ACCLIMATIZATION

• The body adapts to a new thermal environment by a process called acclimatization.

• Complete heat acclimatization generally takes six to seven days, but some individuals may need longer. Loss of acclimatization occurs gradually when a person is moved permanently away from a hot environment.

• Repeated exposure to heat stress during exercise improves your ability to get rid of excess heat.

• Muscle glycogen use is reduced to delay onset of fatigue.• Amount of heat acclimation depends on environmental

conditions and duration of exposure and intensity of exercise.

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FACTORS CAUSING HEAT STRESS

Environmental factors:

Air Temperature Humidity Air Movement Radiant Heat

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TYPES OF HEAT TRANSFER

Conduction. Evaporation. Convection. Radiation.

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CONDUCTION

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CONVECTION

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RADIATION

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EVAPORATION

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Workplaces with heat stress conditions may include iron and steel foundries, brick-firing and ceramic plants, glass products facilities, rubber products factories, electrical utilities (particularly boiler rooms), bakeries, confectioneries, commercial kitchens, laundries, food canneries, chemical plants, mining sites, smelters, and steam tunnels.

WORKPLACES WITH HEAT STRESS

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HEALTH EFFECTS OF HEAT STRESS

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HEALTH EFFECTS OF HEAT STRESS

Heat Stroke Heat Exhaustion Heat Syncope Heat Cramps Heat Rash

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HEAT STROKE (Hyperthermia)

Symptoms• Hot, dry skin or profuse sweating• Hallucinations• Chills• Throbbing headache• High body temperature• Confusion/dizziness• Slurred speech Causes • Due to the failure of the heat-regulating

mechanisms of the body.• Due to high heat and humidity.

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First Aid• Move the sick worker to a cool shaded area.• Cool the worker using methods such as:

• Soaking their clothes with water.• Spraying, sponging, or showering them

with water.• Fanning their body.

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HEAT EXHAUSTION

Symptoms• Heavy sweating• Extreme weakness or fatigue• Dizziness, confusion• Nausea• Clammy, moist skin• Pale or flushed complexion• Muscle cramps• Slightly elevated body temperature• Fast and shallow breathing

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Causes• Loss of water/or salt• Loss of blood plasma• Strain on circulatory system First Aid• Have them rest in a cool,

shaded or air-conditioned area.• Have them drink plenty of

water or other cool,

nonalcoholic beverages.• Have them take a cool

shower, bath, or sponge bath.

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HEAT SYNCOPE

Symptoms• Light-headedness• Dizziness• Fainting Causes• Pooling of blood in legs causing drop in blood

pressure. • Lack of acclimatization. • Loss of body fluid from sweating.

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First Aid• Sit or lie down in a cool place when they begin

to feel symptoms.• Slowly drink water, clear juice, or a sports

beverage.

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HEAT CRAMPS

Symptoms• Muscle pain or spasms usually

in the abdomen, arms, or legs. Causes • This normally happens after exercise. • Most often to people who aren’t used

to the heat, who sweat a lot or don’t

drink enough fluids.

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First Aid Stop all activity, and sit in a cool place. Drink clear juice or a sports beverage. Do not return to strenuous work for a few

hours after the cramps subside because further exertion may lead to heat exhaustion or heat stroke.

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HEAT RASH

Symptoms Heat rash looks like a red cluster of

pimples or small blisters. It is more likely to occur on the neck and

upper chest, in the groin, under the breasts,

and in elbow creases. Causes Heat rash normally happens most often in hot, humid

conditions, particularly wearing heavy clothing and with excessive sweating.

Fair skinned individuals are more prone to develop heat rash.

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First Aid• Try to work in a cooler, less humid environment

when possible.• Keep the affected area dry.• Dusting powder may be used to increase comfort.

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METHODS FOR MEASURING HEAT

STRESS WBGT

The Wet-Bulb Globe Temperature device (WBGT) takes into account four variables: air temperature, humidity, radiant heat and air movement. This reading gives a more accurate measurement of heat stress than any one reading alone.

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The WBGT is measured by a simple three-temperature

element device: The natural wet-bulb temperature (Tw),

which consists of a thermometer with its bulb

covered with a wettened cotton wick supplied

with distilled water from a reservoir.

The cotton wick will always be wet, allowing

continuous evaporative cooling of the

thermometer's bulb, simulating the evaporation

of sweat. This thermometer represents the

integrated effect of humidity, wind and radiation.

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The black globe temperature (Tg), which usually consists of a 150 mm (6 inch) black globe with a thermometer located at the center. The black globe temperature represents the integrated effects of radiation and wind.

The (shade) air temperature (Ta), which consists of a thermometer shielded from radiation - generally by being placed in a weather screen. It is the standard temperature normally quoted in weather observations and forecasts.

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For outdoor use in sunshine: WBGT out = 0.7(T nwb )+0.2(T g )+0.1(T

db ) in ˚F or ˚C

For indoor measurements or outdoor measurements in the shade: WBGT=0.7(T nwb )+0.3(T g ) in ˚F or ˚C

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HEAT STRESS INDEX The Heat Stress Index (HSI)

was developed by Belding

and Hatch (1995). The HSI is the ratio of the body’s

heat load from metabolism, convection, and radiation to evaporate cooling capacity of the environment.

It is predicted on the assumption that the heat load must be dissipated through evaporation.

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The HSI compares the amount of sweat that must be evaporated to balance the heat loss equation for given a set of environmental conditions to the maximum amount of sweat that can actually be evaporated for these conditions.

HSI= 100 Ereq / Emax

Where HSI = a dimension less index number

Ereq = evaporative heat loss required (kcl/hr)

Emax = maximum evaporative heat loss (kcl/hr)

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EFFECTIVE TEMPERATURE OR CORRECTIVE EFFECTIVE TEMPERATURE

Effective Temperature (ET) Or

Corrective Effective Temperature (CET) can be determined from the nomogram.

WBGT can be approximated from effective temperature by using the following relationship: WBGT = 1.102ET - 9.1 in ˚F

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Wednesday, April 12, 2023 THERMAL STRESS 36


SLING PSYCHROMETER

The sling psychrometer

measures dry-bulb

temperature (Tdb )

and thermodynamic

wet-bulb temperature

(T wb ).

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PREDICTED FOUR HOUR SWEAT RATE

The predicted four hour sweat rate (P4 SR) index was developed by Mac Ardle et al (1947). The index taken into account metabolic level and two types of clothing (shorts and overall), in addition to the climate factors.

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CONTROL OF HEAT STRESS

Reducing Metabolic Heat Production (heat produced by the body)

Reducing the Humidity Insulating Hot Surfaces Shielding Ventilation and Air Conditioning Reducing the Radiant Heat Emission from Hot

Surfaces

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http://www.osha.gov/Publications/OSHA3108/images/osha3108_06.jpg


Changing the rate of work Schedule hot jobs to cooler times of the day. Increase the frequency and length of rest breaks if possible Allow for slower-paced work during the hottest periods of

the day. Limiting duration of exposure time. Rotate work activities. Take breaks in cool shaded areas. Avoid eating large meals before working in hot

environments. Avoid caffeine and alcohol (these beverages make the body

lose water and increase your risk for heat illnesses).

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Protective Clothing In some workplaces, insulated gloves, insulated suits, reflective clothing, or infrared

reflecting face shields may be needed. Thermally conditioned clothing might be used for extremely hot conditions; for example:

A garment with a self-contained air conditioner in a

backpack. A garment with a compressed air source that feeds

cool air through a vortex tube. A plastic jacket whose pockets can be filled with dry ice

or containers of ice.

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Prevention of Heat Stress

Learn to recognize the symptoms of heat stress. Change work location, taking adequate rest periods (in shade or cooler environment).

Become familiar with the hazards associated with working in hot environments

Use adequate fans for ventilation and cooling, especially when wearing personal protective equipment (PPE).

Wear light colored, loose (unless working around equipment with moving parts) clothing.

Keep shaded from direct heat where possible (e.g., wear a hat in direct sunlight).

Drink plenty of water. In hot environments the body requires more water than it takes to satisfy thirst.

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CONCLUSION Heat stress cannot be eliminated completely. Simple precautionary measures can

considerably reduce heat strain to acceptable levels

Proper thermal control helps a long way in success of missions.

There is a necessicity for maintaining a suitable temperature to protect the health of the workers and maximize efficiency and productivity . 43


BIBLIOGRAPHY

http://www.osha.gov http:// www.cdc.gov en.wikipedia.org/wiki www.osach.ca National workshop on Ergonomics retrospective and

prospective.

July 1993, Andhra University, Visakhapatnam.

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THANK-YOU

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ergonomics (thermal stress)

Documents

heat loss

high heat

number of heat

heat stress conditions

heat generation

heat gain

heat acclimation

heat stress workplaces