Ann. Occup. Hyg., 2014, Vol. 58, No. 8, 936–942
doi:10.1093/annhyg/meu073
Commentary
Advance Access publication 26 September 2014
Co m m e n ta ry
Re-evaluating Occupational Heat Stress
in a Changing Climate
June T. Spector1* and Perry E. Sheffield2
1.Departments of Environmental and Occupational Health Sciences and Medicine, University of Washington, Seattle, WA 98105, USA
2.Departments of Preventive Medicine and Pediatrics, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA
*Author to whom correspondence should be addressed. Tel: +1-206-897-1979; fax: +1-206-897-1991; e-mail: [email protected]
Submitted 3 August 2014; revised 22 August 2014; revised version accepted 22 August 2014.
A b st r a ct
The potential consequences of occupational heat stress in a changing climate on workers, workplaces,
and global economies are substantial. Occupational heat stress risk is projected to become particularly high in middle- and low-income tropical and subtropical regions, where optimal controls may
not be readily available. This commentary presents occupational heat stress in the context of climate
change, reviews its impacts, and reflects on implications for heat stress assessment and control. Future
efforts should address limitations of existing heat stress assessment methods and generate economical,
practical, and universal approaches that can incorporate data of varying levels of detail, depending on
resources. Validation of these methods should be performed in a wider variety of environments, and
data should be collected and analyzed centrally for both local and large-scale hazard assessments and
to guide heat stress adaptation planning. Heat stress standards should take into account variability in
worker acclimatization, other vulnerabilities, and workplace resources. The effectiveness of controls
that are feasible and acceptable should be evaluated. Exposure scientists are needed, in collaboration
with experts in other areas, to effectively prevent and control occupational heat stress in a changing
climate.
R e - co n t e x t ua l i z i n g O cc u pat i o n a l
H e at St r e s s
Heat stress in occupational populations is a moving target. Global climate change, which includes
both changes in mean weather conditions and the
frequency and intensity of extreme weather events
(IPCC, 2013), will change the pattern of occupational
heat stress risks over time. Workers in tropical and subtropical regions, many of whom are involved in heavy
physical work outdoors or indoors without effective
cooling, are at particularly high risk (Kjellstrom et al.,
2013). Unacclimatized workers in temperate regions
are also at increased risk of heat stress, especially during heat waves (Adam-Poupart et al., 2013). Workers
in large cities may experience greater heat exposures,
relative to their rural counterparts, as a result of urban
heat island effects (United States Environmental
Protection Agency, 2014).
A complex picture emerges when work is viewed in
the context of climate change. Heat stress risk will vary
by industry and occupation over time not only as a
result of spatiotemporal changes in weather conditions
© The Author 2014. Published by Oxford University Press on behalf of the British Occupational Hygiene Society.
936 •
Occupational heat stress • 937
but also as a result of personal and work characteristics
that themselves may be influenced by climate change.
Outdoor industries, such as construction, agriculture,
forestry, fishing, and mining (Bonauto et al., 2007;
Centers for Disease Control and Prevention, 2008;
Schulte and Chun, 2009; Jackson and Rosenberg,
2010; Adam-Poupart et al., 2013), and certain indoor
industries (Favata et al., 1990; Srivastava et al., 2000;
Schulte and Chun, 2009) have already been identified
as high risk for heat stress. This risk will increase further, particularly if there develops a confluence of (1)
local increases in mean temperatures and/or frequencies of heat events, (2) lack of controls such as shade,
fans, air-conditioning, and other adaptive capacities,
(3) more intense physical work demands and/or
fewer rest breaks, and (4) workforces that are more
susceptible to internal heat retention such as unacclimatized workers.
Although successful control of occupational heat
stress begins with the early identification of high-risk
workers, this task is not straightforward. Industries
and workforces will change over time, including in
response to climate change (Schulte and Chun, 2009;
Adam-Poupart et al., 2013; IPCC, 2014a). Increased
heat stress is projected to affect productivity, economic development, and job security, which could in
turn affect the underlying health and susceptibility of
workers to heat stress health effects (Kjellstrom et al.,
2011a; Rhodium Group LLC, 2014). Interdisciplinary
and collaborative work, including between exposure
scientists, other occupational and environmental
health professionals, atmospheric scientists, biological scientists, labor studies experts, and economists, in
industry, labor, and academia, is needed to effectively
predict and control occupational heat stress.
I m pa c ts o f O cc u pat i o n a l H e at
St r e s s i n a Cha n g i n g C l i m at e
In addition to heat-related illnesses, which can be fatal,
several studies have reported relationships between
occupational heat stress and increased injury and
accident risk (Fogleman et al., 2005; Morabito et al.,
2006; Tawatsupa et al., 2013). The interaction of occupational heat stress and certain chemical exposures
may impact the absorption and metabolism of chemicals and thermoregulatory responses to heat exposure
(Bourbonnais et al., 2013). Heat stress is also hypothesized to contribute, in conjunction with other factors
such as pesticide exposure, to chronic kidney disease
in sugarcane workers (Raines et al., 2014). Climate
change will increase the risk of these heat health
effects in workers.
The impacts of occupational heat stress extend
far beyond exposed workers. Heat stress is estimated
to lead to decreased work productivity (Kjellstrom
et al., 2011a) by triggering the body’s natural response
to reduce physical work intensity and internal heat
generation and also through deliberate efforts to
adhere to heat stress exposure limits (Lundgren et al.,
2013). Heat stress may also serve as an indicator of
other climate change-related occupational hazards
and health effects that impact work productivity.
For example, certain agricultural workers exposed to
heat stress may also be affected by new or increased
exposures to water- and vector-borne diseases and
pesticides given their direct contact with irrigation
water and associated disease vectors, and the emergence of new pest-control issues in a changing climate
(Sareen and Nagata, 2011). Air pollution and ultraviolet radiation exposures also relate to heat stress
levels (Lundgren et al., 2013). Although quantitative
exposure–response relationships are not available for
all heat health effects, work has been done to evaluate the effect of heat exposure directly and indirectly
(e.g. via changes in air pollution) on such outcomes
as cardiovascular and respiratory diseases, and on
mental health (Kjellstrom, 2009; Berry et al., 2010).
Productivity losses from these and other health conditions are estimated to cost US employers alone $225.8
billion annually (Stewart et al., 2003).
The consequences of occupational heat stress in a
changing climate, and its associated hazards, on workers, workplaces, and global economies provide strong
motivation for a re-evaluation of occupational heat
stress assessment and control strategies. Further, the
International Organization for Standardization (ISO)
7243 (1989) hot environments standard (ISO, 1989)
is undergoing revision (d’Ambrosio Alfano et al.,
2014), so a re-evaluation of heat stress assessment and
control is timely.
R e -t h i n k i n g H e at St r e s s
A ssessment
A multitude of empirical (Bedford, 1946; McArdle
et al., 1947; Yaglou and Minard, 1957) and rational
(Belding and Hatch, 1955; Vogt et al., 1981; Malchaire,
938 • Occupational heat stress
2006; Bröde et al., 2012) heat stress indices have been
developed, each with different strengths and limitations. The High Occupational Temperature, Health,
and Productivity Suppression (Hothaps) Program
is an international effort aimed at quantifying the
extent to which workers are affected by, and adapt
to, heat exposure, and the effect of climate change on
these estimates (Kjellstrom et al., 2011b). One aim of
the Hothaps program has been to develop improved
methods for human heat exposure assessment. The
following characteristics of optimal occupational heat
stress assessment methods, in the context of a changing climate, have been described (Kjellstrom et al.,
2009; Lemke and Kjellstrom, 2012; Parsons, 2013).
Practical
The need for a practical and economical heat stress
assessment method, given greater projected impacts of
climate change on workers in low- and middle-income
countries, raises three important considerations: (1)
the level of detail of the assessment method; (2) the
anticipated audience of the method; and (3) the actual
measurement method. The ISO scheme for heat stress
assessment includes the use of a relatively simple index
(ISO, 1989) and, when needed, more sophisticated
resource-intensive methods (ISO, 2004a,b). The focus
for high-risk areas of the world that have not yet been
extensively studied should be on the optimization of a
simple index, using more detailed assessments in later
stages as needed. The rationale for a simpler, more
practical index includes the need for use by workers,
managers, and other key stakeholders in areas where
access to advanced training may be limited, and the
lower cost of a simpler approach (Parsons, 2013).
The practicality of the actual measurement method
must also be considered. The wet bulb globe temperature (WBGT) instrument, which measures natural wet bulb temperature, globe temperature, and air
temperature, indirectly captures information about
radiant temperature, air velocity, and humidity. The
WBGT itself is really a measure of heat exposure and
not heat stress unless considered with activity and
clothing (Parsons, 2013), as in ISO standard 7243
(1989) (ISO, 1989). As D’Ambrosio Alfano et al.
(2014) point out in this issue, the WBGT has a number of limitations, including limited use in high humidity, low air movement environments, lack of a direct
measure of air velocity, and a lack of standardization of
instruments to measure WBGT. However, its relative
ease of use and widespread adoption are advantages
(Parsons, 2013).
An alternate approach is to estimate WBGT from
standard meteorological data (Lemke and Kjellstrom,
2012). Two such methods were reported by Lemke
et al. (2011) to meet pre-specified criteria for a “valid”
method: Bernard’s empirical method for indoor
WBGT estimation, which does not incorporate solar
data (Bernard and Pourmoghani, 1999), and Liljegren’s
rational method for outdoor WBGT estimation
(Liljegren et al., 2008). These methods have the capacity to incorporate climate change models to estimate
future WBGT values (Lemke and Kjellstrom, 2012).
There are several important limitations of the use
of meteorological data to estimate WBGT. First, these
methods are limited in environments where there
is a large differential between local work conditions,
for example, near an indoor heat source such as a
smelter, and ambient outdoor conditions. Estimation
of WBGT from meteorological data is more likely
to be useful in outdoor workplaces or indoor work
environments where outdoor conditions consistently
approximate indoor conditions, such as those without heat sources and air-conditioning and with open
windows, a common scenario in low-income countries (Lemke and Kjellstrom, 2012). Second, data
from weather stations may not correspond to specific
workplace conditions, depending on the proximity and elevation of the weather station in relation to
the worksite. If traditional WBGT meters, which cost
thousands of US dollars, are too expensive and not
feasible, cheaper hand-held electronic meters and data
loggers could be considered for use at the workplace if
certain assumptions are met (Lemke and Kjellstrom,
2012; d’Ambrosio Alfano et al., 2014). Although heat
stress assessments using estimates from weather station data or hand-held devices may not be as accurate
or detailed as traditional methods, these methods may
be useful for initial assessments of heat stress risk, particularly in settings with limited resources.
Automated
Assuming adequate computing power and/or access
to internet-based calculators, simple indices need not
be simple in their approach, but only simple from a
practical standpoint. For example, one limitation of the
WBGT, as applied in the ISO standard 7243 (1989)
Occupational heat stress • 939
(ISO, 1989), is its adjustment for clothing properties
(d’Ambrosio Alfano et al., 2014). More sophisticated
clothing adjustment models could be incorporated
into existing methods, using an automated tool that
computes an index and provides an interpretation,
given the necessary inputs. The Universal Thermal
Climate Index provides an automated approach to
incorporate complex physiological models but is limited in its assessment of metabolic rate and occupationally relevant clothing (Bröde et al., 2012).
Valid in high-risk zones
An ideal occupational heat stress index should apply
in a variety of climates and contexts, including moist
tropical climates, where the risk of occupational heat
stress is projected to become dangerously high (Lemke
et al., 2011). In these climate zones, limitations in cases
of high humidity may preclude direct measurement
using traditional approaches (d’Ambrosio Alfano et al.,
2014), and alternative methods will need to be developed and validated.
Interpretable and translatable outputs
Outputs should have meaningful interpretations
and be translatable into heat stress control strategies. One limitation of the WBGT, as applied in ISO
standard 7243 (1989) (ISO, 1989), is that in highrisk hot countries, where conditions will become
even more extreme over time, WBGT values are
close to or exceed reference values (Parsons, 2013).
Although preferable in extreme conditions, followup with detailed heat stress and strain assessments
may require more expertise and resources than are
currently available in certain settings. Further, ISO
standard 7933 (2004), which uses information
about heat transfer between workers and the environment to predict likely heat strain, only considers
standard subjects in good health that are fit for work
(ISO, 2004a). Workforces in different geographical
areas, climates, and workplaces likely differ in levels
of acclimatization and prevalences of other factors
that may increase the risk of heat strain. Further
work is needed to determine how this information
could inform the interpretation of heat stress assessment method outputs and control recommendations, when individual heat strain monitoring is not
possible, so that work is not completely restricted
but health remains protected.
Technological developments may allow for cheaper
and more efficient physiological monitoring of groups
of workers in the future (Parsons, 2013). Individual
heat strain monitoring will be particularly useful in
groups of workers with variability in heat strain risk.
If workplace physiological monitoring becomes more
prevalent, further work on how best to translate these
physiological data into useful control recommendations will be needed (Parsons, 2013).
Acceptable and accessible
Global acceptability of an occupational heat stress
assessment method is likely related to its practicality and cost. Accessibility is aided by infrastructure
such as the ISO, which helps distribute standards to
key stakeholders. An affordable price to access these
standards, however, is needed to ensure accessibility
(Parsons, 2013).
An acceptable and accessible universal index is
a first step toward generating large-scale heat stress
databases that address high-level trends as well as local
heat stress risks (Kjellstrom et al., 2011b). Such an
index could have various levels of inputs, from individual workplace to regional data. Inputs could include
basic parameters and, when possible, more detailed
workplace-level data could be added. Data could then
be collected and analyzed centrally for climate change
impacts assessments and adaptation strategy planning, an aim that has been articulated by Hothaps
(Kjellstrom et al., 2011b).
L i m i tat i o n s o f H e at St r e s s
Co n t ro l s i n a Cha n g i n g C l i m at e
The climate change community has outlined a need for
both mitigation, or slowing, of climate change (IPCC,
2014b) and adaptation, or actions that can be taken to
reduce heat stress and associated health effects (IPCC,
2014a). Mitigation is outside of the scope of this commentary. However, awareness of the negative effects of
potential control strategies, such as traditional air-conditioning, which can involve burning of fossil fuels to
produce electricity and greenhouse gases and worsen
climate change, is critical.
There are a number of limitations of elements of the
traditional hierarchy of heat stress controls in the context of climate change. Although elimination or reduction of heat exposure, including through the design
of workplace buildings, the use of trees for shade, and
940 • Occupational heat stress
the reduction of heat island effects (Lundgren et al.,
2013), may be the ultimate goal for climate change
adaptation, these longer term adaptations require
substantial resources and planning. Yet, sufficient
resources may not be available in high-risk, low- and
middle-income countries, where the majority of the
global population lives and works and where the risk
of heat stress is projected to increase (Kjellstrom et al.,
2013). Engineering controls such as job automation
and cooling systems may also not be available in these
settings. Reliance may instead be on administrative
controls, such as organization of work around cooler
periods of the day and season and work-rest regimens.
Unfortunately, cooler periods may diminish with climate change, and there are barriers to effective workrest regimens, particularly in settings where workers
are incentivized per unit of work (Lundgren et al.,
2013). Further, work-rest regimens have implications for productivity. Current efforts to understand
the relationship between heat stress, acclimatization,
heat-related health effects, productivity, and economic
impacts may help inform the development of regionand industry-specific work-rest recommendations
(Kjellstrom et al., 2009).
The confluence of globalization and climate change
will lead to increasingly diverse working populations
being exposed to more varied and extreme climates
(Parsons, 2013). If climate change leads to increased
chronic disease risk, affected workers may become
more vulnerable to heat stress effects. Control strategies such as education programs, early warning
systems, and hydration schemes may show different effectiveness in different populations, depending
on cultural beliefs and other barriers and facilitators
(Lam et al., 2013; Stoecklin-Marois et al., 2013),
even when incorporated into regulations (California
Division of Occupational Safety and Health, 2006;
Washington State Legislature, 2012). Certain workers
whose home and leisure circumstances do not facilitate cooling outside of work may be vulnerable to heat
effects at work despite workplace controls (Quandt
et al., 2013). Controls may need to be tailored to specific working populations.
Finally, climate change may create more profound
“competition” between hazards. For example, changes
in crop pests and increases in pesticide impermeable personal protective equipment use may occur in
increasingly hot environments (Sareen and Nagata,
2011). Overall, although workplaces should aim to follow the hierarchy of controls, practicality limitations of
certain approaches may preclude their implementation.
Co n c lu s i o n s a n d F u t u r e
D i r e ct i o n s
In summary, climate change will affect occupational
heat stress risk, particularly in low- and middle-income
tropical and subtropical regions. The potential for
profound consequences of occupational heat stress
on workers, workplaces, and economies in a changing climate merits a re-evaluation of heat stress assessment and control strategies. Future efforts should build
upon relatively straightforward and accepted heat stress
indices such as the WBGT, as applied in an improved
ISO standard 7243, to generate economical, practical,
and universal approaches to heat stress assessments.
Limitations of existing methods should be addressed,
and the possibility of including inputs of varying levels
of detail, depending on resources, should be incorporated. Validation of these methods should be performed
in a wider variety of contexts, including high-risk climate
zones. Heat stress data should be collected and analyzed
centrally for both local and large-scale hazard assessments and to guide heat stress adaptation and control
efforts. Although workplaces should aim to follow the
hierarchy of controls, flexibility should be built into the
interpretation of heat stress standard recommendations
to allow for variability in worker acclimatization, other
vulnerabilities, and in workplace resources, while still
protecting health. The effectiveness of controls that are
feasible and acceptable should be evaluated. An ultimate goal should be to prevent heat stress through the
design of workplaces and communities for longer term
climate change adaptation. Occupational exposure scientists should play a key role in these efforts, in collaboration with experts in other disciplines.
Discl aimer
The authors report no direct or indirect sources of
support or competing interests. The authors report no
conflicts of interest.
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