The combined effect of temperature, relative humidity and work intensity on human strain in hot and humid environments

Highlights

• We built a climate chamber to simulate hot and humid environments.

• We proved the existence of combined effect caused by temperature and humidity.

• Combined effect of temperature, relative humidity and work intensity was quantified.

• The enthalpy was adopted to reflect the environment intensity.

• The combined effect of enthalpy and work intensity was quantified.

Abstract

Hot and humid environments are prevalent in certain workplaces, such as deep mines, aviations, military facilities and so on. When working in these environments, heat strain may be produced on the human body, and the work productivity and human health will be affected, so it is necessary to study the impact mechanism caused by the influencing factors on the human body. In this paper, a climate chamber was built to simulate the hot and humid environments. The multiple regressions of physiological parameters related to temperature and relative humidity were obtained, and the combined effect caused by temperature and relative humidity on the human physiological responses was proven. The values of combined effect, temperature effect and humidity effect were quantified, and then the work effect was added. Finally, considering the combined effect between temperature and relative humidity, the enthalpy of air was adopted to reflect the effect of these two environment factors comprehensively. The study in this paper helps people understand the effects caused by environment and work intensity on human strain better and it provides a practical method for the future research.

Introduction

Environment with the temperature above 32 °C and relative humidity above 60% can be considered as hot and humid environment [1]. With the rapid developments of economy and technology, hot and humid environments are prevalent in workplaces, such as steel mills, laundries, glass and ceramic units, deep mines, aviations, metallurgy manufactures, rubber foundries, dye houses, military facilities and some outdoor sport activities [2], [3], [4].

When working in hot and humid environments, the physiological parameters such as heart rate, body temperature, blood pressure and sweat production will obviously change [5]. In hot and humid environments, more blood is provided by the heart to the skin to discharge the extra heat to the relative cooler environment, and the sweating is strengthened to cool the human body. If the extra heat is not fully dissipated, heat storage will occur and the core temperature will increase [6], [7], [8]. Heat exposure will cause cramps, heat stroke, heat exhaustion, and even exacerbate the underlying medical conditions, such as lung or heart disease [9], [10].

There are a lot of attempts to study the human responses in hot and humid environments. Epstein et al. [11] studied the effect of different heat loads on vigilance and complex cognitive tasks. 9 subjects were asked to expose to three climatic conditions and shoot at targets of three different sizes randomly. The results indicated that the heat load had no effect on the easy work but had a great effect on the difficult work, in addition, the effect of the task intensity and heat load on deteriorating performance are synergistic. Vangelova et al. [12] investigated the dyslipidemia (abnormal blood lipids) of the industrial workers in hot environments. 102 male industrial workers exposed to heat and a control group of 102 male workers were studied. The results showed that the heat exposed workers run a higher risk of becoming dyslipidemic than the control group. Brake and Bates [13] monitored the deep body core temperatures of 36 industrial workers continuously, and the results indicated that the values regularly exceeded the commonly recommended limits (a temperature of 38.0 °C or an increase of +1 °C) with the average maximum deep body core temperature was 38.3 °C, the maximum temperature rise was 1.4 °C, and the maximum heat storage was 431 kJ. Kalkowsky and Kampmann [14] carried out a study to record the physiological strain of miners. 38 miners were asked to participate during 125 shifts, and the heart rate and rectal temperature were measured continuously. The results showed that the average heart rate and rectal temperature during all shifts were 102.8 beats/min and 37.7 °C, the mean sweat loss per shift was 3436 g and the mean sweat rate was 494 g/h. Saha et al. [15] evaluated the physiological strain of different categories of miners. The results indicated that the oxygen consumption ranged from 4.96 kcal/min to 5.47 kcal/min, and the heart rate revealed that the intensity of the work was classified into heavy and very heavy for the miners.

Among the previous studies, people always focused on the physiological responses and subsequent dangers when working in hot and humid environments and ignored the influencing factors. The physiological responses of human body to the thermal stress were summarized as heat strain, and the heat strain is affected by environment factors and muscle activities [16], [17]. So in this paper, the influence mechanism of the environment and work factors on the human body will be studied to help people understand the effects better.

The structure of this paper is divided into three parts: one is to prove the existence of combined effect caused by temperature and relative humidity, one is to quantify the effects caused by environment factors and work factors, and the last one is to find an index to reflect the temperature and relative humidity comprehensively.