Heat stress compatibility of WBGT limits for Chinese migrant construction workers in tropical climates

WBGT Limits for Chinese Migrant Workforce

The effects of heat stress on workers has been well documented (1-4), especially in the construction industry (5-6), but widely compatible standards for determining safe limits for heat exposure have yet to arisen, which makes a difficult task of determining the compatibility of Wet Bulb Globe Temperature (WBGT) limits with a Chinese migrant workforce in the construction industry, working primarily in a tropical climate. Each population and climate requires specific considerations when determining the risk of heat stress, and these specific considerations ultimately demonstrate certain gaps in the WBGT heat index that makes it incompatible for deployment with the previously mentioned workforce. A number of regulations use the Wet Bulb Globe Temperature as a standard for determining heat thresholds (7), but recent research (8-9) has shown the WBGT to be overly conservative in certain situations, particularly in areas with high humidity levels such as tropical climates. With this is in mind, this study seeks to examine the compatibility of WBGT limits with a Chinese migrant construction workforce in a tropical climate by looking at the relative weaknesses of WBGT limits in comparison to alternative heat indices as well as the unique requirements for managing heat exposure in tropical areas with both acclimatized and unacclimatized workers. In turn, this will demonstrate the ultimate goal of this proposed research, which is to determine a widely applicable, accurate in varying climates, and easy-to-use heat index for the mitigation of heat stress in tropical climates.

In order to understand the relative strengths and weaknesses of WBGT limits in relation to tropical climates, it will be necessary to briefly address the numerous variables that must be taken into account when monitoring and mitigating heat stress in workers more generally, before examining heat stress in the construction industry, tropical climates, and Asian workforces more specifically. These discussions will culminate in an analysis of WBGT limits that bears in mind the unique requirements on the aforementioned conditions and demographics, ultimately demonstrating the compatibility problem this proposed research intends to address. Heat stress mitigation and the standards which govern it are a crucial topic of investigation because as Ben, Hashim, and Hamzah (1) note, prolonged exposure to a hot workplace "may result in heat cramps, heat exhaustion, heat syncope, and heat stroke" in addition to milder symptoms, and these problems present themselves in industries the world over, wherever workers are subjected to prolonged heat exposure.

Measuring for heat stress is a nuanced practice, requiring more attention to detail than the common reliance on the WBGT would suggest. First of all, measuring core temperatures is required in order to determine the effects of heat stress on any given individual, and Nagano et al. have found that "monitoring temperatures based on a technique involving an auditory canal plug can be used to estimate rectal temperatures accurately, and thereby to avoid conditions leading to heat stress disorders." This is coupled with observations of heart rate and external temperature in order to give an idea as to the physiological reactions to heat stress. However, getting an accurate reading of the subject's core temperature and heart rate is not enough to analyze the effects of heat stress. For instance, the effects of hydration on the body's reaction to heat stress must be taken into account, because the "homeostasis of body water can be difficult to maintain when challenged by strenuous physical work and heat stress" (4). This fact is often disregarded in considerations of heat stress, however, because "implicit in all indices used for risk assessment in the prevention of heat stress is the assumption that workers are healthy and well hydrated" even when this is clearly not the case in reality (3). In fact, a number of the studies considered here noted that subjects rarely hydrated according to generally accepted minimums, demonstrating that already the risk of heat stress is greater than some studies (which rely on assumptions of good health and hydration) suggest.

This oversight is more egregious considering research demonstrating that "people can work, without adverse physiological effects, in hot conditions if they are provided with the appropriate fluids and are allowed to self-pace" (5). In short, encouraging proper hydration greatly mitigates the effects of heat stress even before it becomes a serious issue, because a well hydrated body, even one not acclimatized to the situation, is far more resilient in the face of heat stress than a dehydrated or otherwise thirsty body. Furthermore, research has demonstrated the importance of clothing weight and consistency in regards to heat stress, further complicating the observation, categorization, and mitigation of heat stress (11). These variables that must be taken into account actually reveal some of the weaknesses of WBGT-based limits, but for now these weaknesses must be put aside in lieu of an examination of the particular effects of heat stress in the construction industry and tropical climates, because the ultimate aim of this research is to determine the efficacy of WBGT limits for a Chinese migrant workforce working in the construction industry in a tropical climate.

As mentioned earlier, the construction industry sees outsized effects of heat stress on worker health and productivity, because "outdoor operations conducted in hot environments, such as construction and waste site activities, are […] likely to cause heat stress among workers" (6). The reasons for this are twofold. Firstly, outdoor construction work means that workers are far more susceptible to climactic influences with little recourse (such as air conditioning) to escape from the heat and humidity. Secondly, the environment of a work site does not encourage careful management of hydration and heat levels, so that workers suffering from heat stress may remain unnoticed or otherwise unrelieved. However, perhaps more important than the specific industry is the climate of the region in which the work is taking place, because humidity plays a crucial role in the complex interactions that contribute to heat stress. In a tropical environment, humidity is necessarily higher, so including the effects of humidity in any study is crucial, and the influence of a warming planet will only increase these effects (12-13).

Combining these two factors results in extreme danger of heat stress, such that "in particular, populations of ageing farmers and physically overloaded construction workers are the two most vulnerable worker categories in which high temperature impacts on health and productivity" (14). Thus, now more than ever a widely applicable standard for determining heat stress limits is needed, and it must be one that can account for not only the effects of humidity but of longer-term climatic changes as well. However, it must also be able to successfully predict and account for the differences between native and migrant populations, because as will be shown, the difference between acclimatized and unacclimatized workers in relation to heat stress can be dramatic and in some cases, deadly.

Before finally addressing the efficacy of WBGT limits for a Chinese migrant workforce directly, examining previous research into the heat acclimatization of migrant and native Asian populations will offer further insights into the unique heat stress measurement and mitigation considerations necessitated by the object of this study. Work by Matsumoto et al. (15) has shown that the "heat tolerance of tropical subjects [in this case Thai males, as opposed to Japanese "temperate" subjects] is due to a more efficient evaporative ability due to greater heat loss brought about by their long-term exposure to heat," and additional research (16) has demonstrated that in general, tropical workers are better able to maintain their body temperature through a more economical pattern of sweating. This phenomenon is called acclimatization, and the differing effects of heat stress on acclimatized and unacclimatized workers must be taken into account when addressing the effects of heat stress on a particular population (in this case Chinese migrant workers) because acclimatized workers actually have more efficient and more useful physiological responses to heat stress to the point that an acclimatized worker and an unacclimatized worker might as well be working in entirely different climates, so different is the physiological response of their respective bodies.

Having looked into the myriad variables that can affect an individual's response to heat stress, and in particular the effects of humidity, clothing, and acclimatization (or lack thereof), it is now possible to consider WBGT limits and whether or not they offer a sufficiently robust means of monitoring and mitigating the effects of heat stress on a population of Chinese migrant workers in the construction industry, within a tropical climate. Although a number of previously mentioned studies point out a weak spot of WBGT limits that would seem to preclude its use in a tropical environment, it will be useful to first examine those arguments in favor of utilizing WBGT limits as a universal standard.

Parsons (7) argues in favor of using WBGT as a worldwide standard, claiming that "as a simple index with face validity, experience of use and 'normal' conditions, the standard can be regarded as 'fit for purpose,'" so long one makes appropriate considerations for clothing as well as "type of person and context of application." While this would seem to be in line with the necessary considerations elucidated above, subsequent research has found the WBGT index lacking in a key area; namely, WBGT produces overly conservative estimations of heat stress in highly humid conditions, necessitating the use of a more accurate heat limit standard for tropical climates.

In contrast, Claasen and Kok (9) examined the accuracy of the WBGT heat stress index at low and high humidity levels in their paper of the same name and ultimately found that "WBGT index values above 30 with HH [high humidity] levels underestimate thermal load and that un-acclimatized employees may be at risk to develop heat illnesses if work schedules are not properly managed" because the WBGT index is so inaccurate at high humidities that "a reduction of at least 3 in the WBGT index value is suggested for WBGT index values above 30 with high relative humidity levels." This coincides with additional research by Miller and Gates (8) that compared the Thermal Work Limit (TWL) heat index with the WBGT, finding that "in the field study, TWL was a more appropriate and realistic index than WBGT, which was found to be excessively conservative." In particular, Miller and Gates found that "in an aboveground environment where the convective and evaporative effect of air movement contributes significantly to cooling, the WBGT is an excessively conservative index of environmental heat stress," so that "although WBGT is the nominal standard for many industries, it is often not in fact used, particularly in industries where heat stress is a significant issue, as its implementation would lead to too much lost production." In short, though WBGT has become widespread in literature and regulations due to its relative ease of use, its actual practical applicability has been limited in the industrial sector, precisely for those reasons listed above.

The researchers suggest the TWL as a viable alternative to WBGT, because "TWL has been shown to perform better than WBGT as a predictor of the impact of environmental heat stress in outdoor work environments." Further research (12) has compared the WBGT index with the Predicted Heat Strain (PHS) index, finding that "analyzing similar climatic conditions with WBGT and PHS indicate that WBGT provides a more conservative assessment philosophy that allows much shorter working time than predicted with PHS" such that the WBGT is shown to be an unreliable standard for determining heat stress in high humidity environments as well as those climates which will be drastically affected by climate change in the future (such as the tropics).

The entire preceding discussion of extant literature regarding heat stress effects on migrant workforces in tropical climates and the construction industry as well as the efficacy of the WBGT index in general has served to outline the questions and considerations surrounding the compatibility of the WBGT index with a Chinese migrant workforce laboring in the construction industry of a tropical climate. Thus, this proposed research will concern itself with using the observations and conclusions produced by this extant literature to demonstrate the need for alternate or hybrid heat indexes in determining heat stress limits for a Chinese migrant workforce in a tropical climate. To see why this is necessarily the case, it will be useful to look back at the necessary considerations for determining heat stress limits elucidated above, this time with an eye towards those considerations which the WBGT index is incapable of taking into account (or else produces inaccurate information when confronted with these considerations).

The first crucial variable to consider when determining the compatibility of the WBGT index to a Chinese migrant workforce in a tropical environment is the influence of acclimatization on heat stress, because as Matsumoto et al. (15) and Lee et al. (16) note, acclimatization greatly reduces the rate of heat exposure and the effects of heat stress on the body through more efficient cooling and use of bodily fluids, bringing with it an attendant reduction in dehydration. Thus, a Chinese migrant workforce would at least initially not be acclimatized to tropical conditions (particularly the high humidity), but the WBGT heat index was found to be inaccurate in high humidity situations, such "that un-acclimatized employees may be at risk to develop heat illnesses if work schedules are not properly managed" with a more accurate heat index (9).

Furthermore, as the tropics will be affected by climate change to a vastly disproportionate degree, any heat index used to create limits for migrant workers in a tropical climate must necessarily take into account the mutability of that climate, something the WBGT has been shown to be less-than-ideal for (12). Although the WBGT may be adapted to account for clothing and different body types and even hydration levels, its inaccuracy at high humidities and short-range predictive power make it an apparently ill fit for determining heat stress limits for a Chinese migrant workforce in the construction industry in a tropical climate. (Although WBGT does not necessarily have any explicit weaknesses when it comes to determining heat stress limits in the construction industry beyond those already demonstrated regarding humidity and climate change, it is worth recalling Miller and Gates' (8) observation that WBGT "is often not in fact used, particularly in industries where heat stress is a significant issue [such as the construction industry], as its implementation would lead to too much lost production.")

If it is not already clear, the WBGT heat index weaknesses are precisely in accounting for those variables which become most relevant when considering unacclimatized migrant workers in a tropical environment, especially in the construction industry in which all the threats from heat stress are amplified and exacerbated. However, this has not kept the WBGT heat index from becoming standardized, as the majority of the studies addressed here use it as their central means of determining heat stress (except for those studies which intentionally compare WBGT against alternate heat indices). In summation, a look at the relative strengths and weaknesses of Wet Bulb Globe Temperature limits have revealed that measurements and predictions based on the WBGT index appear problematically conservative when dealing with unacclimatized workers or climates with particularly high humidity or likelihood of climatic upheaval.

Thus, the ultimate purpose of this proposed research (after having demonstrated the incompatibility of WBGT limits for a Chinese migrant workforce in a tropical climate more robustly) will be to build off of the research already discussed here dealing with alternative and hybrid heat indices as a means of finding the ideal limits for the particular labor demographic under discussion (in this, the aforementioned Chinese migrant workforce, but ideally, determining the best heat index to use with this specific population will reveal standard rules and guidelines for choosing the ideal heat index for any configuration of acclimatization, climate, and relative humidity).