Impact of Global Warming on severity of Heat Stroke

Harishankar.N B.E Mechanical Engineering, P.S.G College of Technology,

Coimbatore, India. e-mail: hsharishankar90@gmail.com

Prof.J.Srinivasan Divecha Centre for Climate Change,

Indian Institute of Science, Bangalore, India.

e-mail:jayes@caos.iisc.ernet.in

Abstract— Heat stroke is defined as a condition in which the core body temperature rises beyond 40.6 °C (105.1 °F) due to environmental heat exposure with lack of thermoregulation. IPCC (Intergovernmental Panel on Climate Change) has reported that global mean temperature will rise by 30 C by 2100 which implies that heat exposure might be an important factor of concern in the near future. The industrial workers in tropical climate region are exposed to high heat during the months of summer. Hence to protect the workers, a study on the ill-effects and severity due to heat stroke and also development of guidelines for safe human exposure is imperative. Heat stress index predicts heat stroke by integrating the factors that defines thermal environment. WBGT (Wet-Bulb Globe Temperature) is the most commonly used heat stress index at present. But it does not take into account the metabolic rate of individuals and also the latent heat loss. Latent heat loss by evaporation becomes a major source of thermoregulation at high temperatures. This paper focuses on the prediction of heat stroke from the perspective of latent heat loss due to sweat evaporation. It also deals with the significance of wind speed in industries and effects of global warming on heat stroke. The effects of free and forced convection for both present and future conditions are separately studied.

Keywords: Heat stress index, Latent heat loss by sweat evaporation, free convection, forced convection

I. INTRODUCTION

A. Heat Stroke Heat stroke is defined as a condition in which the core

body temperature rises beyond 40.6 0C (105.10F) due to environmental heat exposure with lack of thermoregulation. Humans are warm-blooded animals. We maintain heat by thermoregulation in our body. The food we intake is converted into energy. By second law of thermodynamics, when we work, heat is also produced by our body. To maintain the core body temperature, our body must lose heat continuously. We lose heat through skin. Heat loss from the skin takes place by various ways.

• Latent heat loss by sweat evaporation • Convection (free and forced) • Radiation • Conduction with surfaces in contact

Heat loss by convection and radiation depends on both the magnitude and direction of temperature gradient. Our skin needs to be at a higher temperature compared to the ambience for heat to flow from the skin. So latent heat loss by sweat evaporation turns out to be a major source of heat loss at elevated temperatures.

B. Severity Of Heat Stroke In Future Humans, being the most dominant species on Earth, in

the name of developing mankind, have polluted the environment. The effects are already visible. Changes in weather patterns, frequent floods and droughts, hurricanes, melting ice glaciers have already shaken the world. Research has already been started on these problems. But Heat stroke and its effects being one of the major impacts of climate change are neglected. IPCC (Intergovernmental Panel for Climate Change) has recently reported that global temperature will rise by 30 C in next hundred years [1]. Farmers, industrial workers, soldiers, sports persons and millions of people below the poverty line are highly vulnerable to such temperature rise. At high temperatures heat loss by convection and radiation becomes dormant. In fact, it increases the heat load on human body. When temperature raises by 30 C in future, human body (all warm blooded animals) will struggle to release heat. Hence developing precautions against heat stroke is highly imperative. In this paper we focus on the region Chennai (Lat Long 13.04 0 N, 80.17 0 E), India.

C. Heat Stress Index Heat stress index is used to measure the severity of heat

stroke. It integrates various factors that affects thermal environment. The factors [2] are

• Air temperature • Relative humidity • Sun radiation • Wind speed • Individual metabolic rate • Clothing

D. Existing Methodology 1) Wet-Bulb Globe Temperature (WBGT)

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2011 2nd International Conference on Environmental Science and Development IPCBEE vol.4 (2011) © (2011) IACSIT Press, Singapore

There are lots of guidelines being developed for safe human exposure. WBGT is the most widely used heat stress index. It integrates air temperature, relative humidity, sun radiation and wind speed. WBGT is formulated as [3]

0.7 0.2 0.1 (1)

Natural wet bulb temperature (o C) Globe temperature (o C) Ambient temperature (o C)

As shown in (1), it is simple and easy to calculate.

Figure 1.Plot of WBGT (Wet-Bulb Globe Temperature) for the past 10

years data of Chennai

E. Inference • In the past 3 years, the number of days crossing

WBGT of 390 C is 8 days • With 30 C rise in temperature, the number of days

crossing the limit is 28 days. • According to Fig 1, the number of days that can

result in heat stroke increases by a factor of 3.5 due to 30 C rise in temperature.

F. Limitations 1) Inadequate response to humidity and air movement

[4]: At very high temperatures wetbulb temperature is more

predominant in causing heat stroke and at low temperatures it is less effective. But WBGT gives a constant weighing for wetbulb temperature at all environments. Hence it overestimates at low temperatures and underestimates at high temperatures. It also responds less to air movement. For example, clothed men alternately exercising and resting for 4 hours in hot and humid conditions reported that they had tolerated ‘with ease’ when wind speed was 0.8 m/s became incapacitated when the experiment was repeated with wind speed 0.1 m/s [5] evidently because the reduction in wind speed had reduced the evaporative capacity of the air (Emax)

to one-third of its former value. The accompanying trivial increases in WBGT (from 37.40 C to 37.6 0 C) gave no indication of this disabling increase in heat stress. WBGT does not respond the same way to hot, humid and hot, dry areas. Laboratory tests and industrial experience revealed that WBGT suffers from this limitation. Ramanathan and Belding [6] demonstrated, in controlled laboratory tests, that at WBGT 890F (31.7 0C) the heart rate, rectal and skin temperatures, sweat rate, and subjective distress of exercising men were much greater in environments of high humidity and low air movement (hence restricted evaporation) than in those of low humidity and high air movement (free evaporation). They recommended establishing two sets of WBGT limit levels, one for humid conditions and the other for dry, and they concluded that ‘WBGT has limited value as a predictor of physiological strain at the higher heat stress levels which may be encountered in industry’. Our data belongs to Chennai. Unfortunately Chennai is a hot and humid region. It lies along the coast. So it enjoys wind throughout the year. Due to WBGT’s inadequate response to humidity and air movement, it may have underestimated the humid condition and overestimated the air movement.

II. PRESENT WORK In Chennai the number of days with ambient temperature

crossing the skin temperature (350C)[7] increases by a factor of 4, as shown in Fig 2 and this can be attributed to global

warming. In order to measure the impact of global warming on severity of heat stroke, we analyze the heat stress on humans (nude) from the amount of possible heat loss, as

WBGT may not give a correct idea.

Figure 2. Probability density functions of ambient temperature in Chennai for the past 10 years and with 30C rise in temperature. A Gaussian

curve fit is used here.

III. METHODOLOGY When the ambient temperature exceeds skin temperature

(350C), our body releases heat only by latent heat loss by sweat evaporation. Hence heat stress is measured from the perspective of latent heat loss alone. When our body is unable to release heat by radiation and convection it increases the sweat excretion rate. But it is the sweat

Number of days=820

Number of days=210

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evaporation rate that actually causes heat loss from our body. Sweat evaporation rate depends on the temperature, relative humidity and specific humidity and bulk velocity of air. Except for the last factor, all the other factors do not change rapidly. Hence to clearly analyze the effect of air velocity on heat stroke we consider two cases.

• Free convection (still air) • Forced convection (wind)

A. Free Convection In free convection, the bulk velocity of air is zero. As

mass transfer of sweat is accompanied with heat loss from the skin, it is a combined heat and mass transfer problem. Hence Grashoff number has to be calculated separately for heat and mass transfer. According to Gebhart [8], , (2)

, ∞ (3) , , (4) |∆ | (5)

g Acceleration due to gravity m s =9.81 m s Gr Grashoff number x Characteristic length (m) = 1m

Temperature near the skin (o C) Ambient temperature (o C) C Concentration of water vapor near the skin Concentration of water vapor in the ambience

Kinematic viscosity = 15.68e-6

From (5) we understand that as though there is no air movement, local change in density causes the formation of eddies near the skin. These eddies are responsible for heat and mass transfer from our skin.

, 0.59 . (6) , (7)

(8) (9)

Latent heat loss (10)

Evaporation rate (kg/s) Heat transfer coefficient (W/ .K)

Mass transfer coefficient (m/s) Heat capacity of air (kJ/kg.K) = 1.007 kJ/kg.K

Conductivity of air (W/m.K) Schmidt number = 0.61

Pr Prandtl number = 0.69 , Nusselt number

According to Fig 3, eddies are formed on both sides of the threshold line. When density of ambient air is below density of air near the skin, air circulates away from the skin. When density of air near the skin is less than that of ambient air, air circulates into the skin. Hence eddies are always formed except when density of ambient air and density near the skin equals to the same value.

B. Forced Convection It implies the presence of bulk velocity of air in the

atmosphere. Latent heat loss under the condition of forced convection is formulated as , , (11)

Specific humidity of air near the skin Specific humidity of ambient air

Latent heat of vaporization = 2450 kJ/kg.K Air density Coefficient of drag = 2e-3

U Wind velocity (m/s) RH Relative humidity (%)

Figure 3. Change in density of ambient air with respect to ambient temperature and relative humidity is shown. All possible temperatures

between 300 C and 45 0 C are considered for the plot.

Air near the skin becomes hot as it absorbs heat from our

body. Once this air becomes saturated, heat loss is not possible. Unlike free convection, in forced convection, bulk velocity of air forces saturated air near our skin to move away from our body allowing fresh and unsaturated air to occupy the area near our skin. So our body is able to release more heat under forced convection. According to (11), forced convection mainly depends on difference in specific humidity. According to Fig 4, heat loss is possible only below the threshold line. It is observed that difference in specific humidity between air and the skin decreases with increase in temperature and relative humidity.

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So it indicates that coastal areas are more prone to heat stroke because of their high temperature and relative humidity.

IV. RESULTS

A. Free Convection As saturated air near our skin is not replenished by strong

air circulation, heat loss in this case is less. Hence in still air, humans will find it very difficult to lose heat. In Fig.5, 100 W is taken as threshold value. It represents the minimum heat production from our body when we take rest. According to Fig.5, the curve indicating 30C rise in temperature shifts to the left. The maximum of this curve is less than 100 W. It indicates that latent heat loss due to temperature rise will decrease dramatically. Number of days with latent heat loss<100 W, increases by a factor of 5.This shows that in future, people working outdoors are at high risk due to heat stroke.

Figure 4. Change in specific humidity with respect to relative humidity and ambient temperature. Unlike density, specific humidity increases with

increase in RH and ambient temperature.

B. Forced Convection Compared to free convection, heat loss is high in the case

of forced convection. Though the curve indicating temperature rise shifts to the left (Fig: 6) latent heat loss does not fall as dramatically as free convection problem. Though the number of days with LH<100 W increased by a factor of 8, the difference between the present and future conditions is just 104. This indicates the importance of fans and air conditioners in future. In industries, fans will play a vital role in reducing heat stress on workers. According to Fig.6, as wind speed is increased, number of days with LH<100 W has decreased dramatically from 453 to 23.

We have compared free and forced convection in Fig.8 to bring out the importance of air circulation in industries. ISO has stated that metabolism rate of workers when working with medium tool is 170 W. According to Fig.8, number of days with LH<170 W [9] decreases from 5793 to 453 with air circulation of 2m/s. The number of days decreased dramatically by a factor of 12.7.

Figure 5. Normalized frequency distribution of latent heat loss under

free convection. A Gaussian curve fit is used here.

Figure 6. Nomalised frequency distribution of latent heat loss under forced convection. A Gaussian curve fit is used here.

Figure 7. Normalized frequency distribution of latent heat loss under

forced convection for various wind speed. A Gaussian curve fit is used here.

Number of days=453

Number of

days=766

Number of days=3927

Number of days=119

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Figure 8. Normalized frequency distribution of latent heat loss under forced convection for various wind speed. A Gaussian curve fit is used here

V. CONCLUSION Our work considers latent heat loss during free and

forced convection separately and it has been understood that heat loss is more during forced convection. This implies that industries will have to improve the internal air circulation in order to reduce heat stress. There will be an increased usage in air conditioners and electric fans which will increase the power load of industries thereby directly affecting the GDP of the world and in long run it might lead to power shortage. People under the poverty line will be highly vulnerable to heat stress. With 60 C rise in temperature coastal areas will become extremely unsuitable for heat dissipation, questioning the possibility of human habitation at these places. Heat stress should be categorized as a natural calamity and steps should be taken to control human exposure else temperature rise will slowly wash out millions of lives in the future.

VI. SCOPE FOR FUTURE WORK We intend to calculate the effect of clothing on heat

stress. We would like to create a model of human body and numerically analyze the effect of sweat evaporation. In addition we would like to analyze the variation of core body temperature with time during exposure to sun rays. We are interested to make a study of the effect of ‘salt content in sweat’ on its latent heat of vaporization.

ACKNOWLEDGMENT I wish to express my gratitude to Mechanical

Engineering Department, Indian Institute of Science (IISc), Bangalore for providing support and encouragement throughout this work.

REFERENCES [1] IPCC. Fourth assessment report. Geneva, Inter-governmental Panel

on Climate Change. Cambridge: Cambridge University Press; 2007.

[2] Tord Kjellstrom, Ingvar Holmer and Bruno Lemk, Global Health Action 2009, 11 November 2009

[3] NIOSH.; Criteria for a recommended standard: occupational exposure to hot environments, Publication No. 86_113. Atlanta, GA: National Institute of Occupational Health, 1986.

[4] G.M. Budd, Wet-bulb globe temperature (WBGT)—–its history and its limitations, Elsevier on 8 July 2007

[5] Macpherson RK. Physiological responses to hot environments. An account of work done in Singapore,1948—1953, at the Royal Naval Tropical Research Unit, with an appendix on preliminary work done at the National Hospital for Nervous Diseases, London; 1960

[6] Ramanathan NL, Belding HS. Physiological evaluation of the WBGT index for occupational heat stress. Am Ind Hyg Assoc J; pp34:375, 1973.

[7] Naoshi Kakitsuba, Dynamic changes in sweat rates and evaporation rates through clothing during hot exposure, Journal of Thermal Biology Vol 29, pp 739–742, 2004

[8] B. Gebhart and l. Pera, The nature of vertical natural convection flows resulting from the combined buoyancy effects of thermal and mass diffusion, Vol 14,pp 2025, 1971

[9] ISO 7243,Hot environments-Estimation of the heat stress on working man, based on WBGT index(wet bulb globe temperature)

Number of days=493

Number of days=5793

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