Elsevier

Building and Environment

Volume 116, 1 May 2017, Pages 17-29
Building and Environment

Short- and long-term acclimatization in outdoor spaces: Exposure time, seasonal and heatwave adaptation effects

https://doi.org/10.1016/j.buildenv.2017.02.001Get rights and content

Highlights

  • ANOVA results showed that UTCI conditions remained unchanged throughout exposure time.

  • ANOVA results showed that outdoor thermal perception differed between seasons and exposure time.

  • Seasonal differences lead to overshooting response in summer.

  • Acclimatization to a heatwave in summer lead to enhanced heat tolerance.

Abstract

We investigate acclimatization effects on outdoor thermal perception. Steady-state conditions were ensured by a prolonged stay of participants (N = 16) in a test chamber prior to the subjects' exposure to outdoors, i.e. after five consecutive hours under thermal comfort conditions indoors. After that, subjects walked in a controlled pace around the external precincts of the facility and were asked to vote on their thermal sensation and preference according to a standard questionnaire: a) immediately, b) 15 min and c) 30 min after they left the controlled indoor environment. Altogether 36 sessions were performed with varying outdoor conditions over winter, spring, and summer 2015. We evaluate acclimatization effects on the subjects' thermal perception against predictions of the outdoor thermal conditions in terms of UTCI (Universal Thermal Climate Index) and the derived DTS (Dynamic Thermal Sensation). ANOVA results showed that UTCI conditions remained unchanged throughout the 30-min exposure time outdoors, but differed between seasons, whereas the subjects' thermal perception votes differed both between seasons and the times of votes. Reduced thermal sensitivity was noticed in winter and spring at the first vote, resulting in greater prediction bias (underestimation), which was attenuated at higher temperatures and during longer exposure times. An initial overshooting at the first vote towards cool response occurred at moderate temperatures in summer, increasing bias (overestimation), which was also attenuated with increasing temperature and time of exposure. Acclimatization to a heatwave in summer lead to enhanced heat tolerance just after it, with acclimatization loss in the subsequent session.

Introduction

The paper is concerned with the time of residency factor when conducting outdoor comfort studies. Short-term acclimatization in terms of space residency plays an important role on the perceived thermal sensation, with a longer exposure of a person to the thermal environment leading to more accurate perceptions of it. Analogously, long-term acclimatization as influenced by seasonal aspects can affect thermal expectancy factors as regards outdoor spaces. Somewhere in between lies the time frame around sudden changes in atmospheric conditions, as in the case of heatwaves, which is also investigated in this paper.

Skin temperature strongly affects how one perceives the thermal environment. Höppe [1] shows through computer modelling of the skin temperature that in the cold at least 3 h would be needed for steady state to occur in the heat exchange between skin and air temperature; in warm conditions, steady state is reached more quickly but nevertheless only after approximately half an hour.

According to the physiological concept of Alliesthesia [2] “a given stimulus will arouse either pleasure or displeasure according to the internal state of the stimulated subject”, thus stepping from thermal homogeneity to transient outdoor conditions should create immediate responses that would then diminish with time of exposure. When a subject experiences a thermally static environment for longer periods, “with no opportunity for the body to interpret the ‘usefulness’ of a stimulus for thermoregulation”, there is a greater chance that he will more effectively experience thermal pleasure or displeasure under sudden transient conditions. De Dear [3] pointed to the relevance of the alliesthesia concept to the planning of transitional spaces and tested this hypothesis in a climate-chamber study with participants exposed to step-up and step-down temperature [4]. Results described the more immediate effect in step-up than in step-down changes on reported thermal sensation; the authors suggested the accuracy in thermally perceiving a given thermal environment to be closely related to cutaneous thermoreceptors.

In this context, the residency time question or the time needed for short-term acclimatization gains in importance because interviewing people with a short residency time can create bias in interpreting the survey results. As pointed out by Wu and Mahdavi [5] “A disregard of thermal evaluation processes pertaining to transitional states may result in inappropriate temperature settings, inefficient thermal controls, and poor thermal comfort conditions”. Similarly, if the thermal processes involved in transitioning from indoors to the outdoor environment area are not accounted for, it might result in an inadequate interpretation of the thermal perception reported by pedestrians. This will put an unnecessary burden on urban planners to seek for more intricate solutions from climate-responsive urban design.

As for seasonal, long-term acclimatization, the adaptive comfort concept [6] is based on changes in thermal preference over different seasons, with increased tolerance towards heat in summer and towards cold in winter. In a field study in Israel, Pearlmutter et al. [7] observed how expectations to seasonal changes in weather conditions affect the seasonal acclimatization factor.

There will be a great need for Europeans to adapt to heatwave conditions, which will likely increase in frequency due to global climate change. As pointed out by Koppe et al. [8], effective interventions, measures and policies to protect the health of vulnerable Europeans will thus need to be developed and evaluated. However, studies of human acclimatization effects to heatwaves are still sparse. Lam et al. [9] comment that most of such studies use modelling approaches and report on how Australians reacted in terms of thermal perception comfort during a heatwave and after it. Results showed that park visitors felt significantly hotter and wore less clothing for the same ranges of the Universal Thermal Climate Index (UTCI) during heatwave than under non-heatwave conditions.

In this paper we explore short- and long-term acclimatization effects from subjective responses to outdoor conditions of participants who took part of a controlled field experiment over three seasons in a temperate climate (Karlsruhe, Germany). As in the summer sessions a heatwave occurred, acclimatization effects in its aftermath are also analysed in the paper. The analysis was based on the evaluation of prediction bias from the non-steady state thermal index UTCI for subgroups of thermal votes, given by the Dynamic Thermal Sensation, which can be derived from UTCI calculations and which allow a more direct comparison to reported thermal sensation (as in a seven-point thermal scale). For the purpose of testing short-term acclimatization effects, DTS results were thought to better represent the varying, dynamic thermal responses of the respondents. Rationale for doing so is given by Höppe [1].

Section snippets

Methods

The study was carried out in and outside a climate chamber located at the Karlsruhe Institute of Technology. The ‘Laboratory for Occupant Behaviour, Satisfaction, Thermal Comfort and Environmental Research’ (LOBSTER) climate chamber is composed of two adjacent 24 m2 offices (Fig. 1). It was designed as a semi-controllable environment with operable windows where two office spaces, each provided with two workstations would closely resemble a conventional work environment. Windows have

Indoor conditions prior to outdoor exposure

As a result of the tight control of thermal conditions indoors, PMV conditions were kept within the limits of a Category B thermal environment, which prescribes according to ISO 7730 [11] for a given indoor space PMV ranging −0.5 to +0.5 in the absence of significant temperature gradients, thermal asymmetries and draughts. In the test chamber, thermal conditions stabilized within that range just about bordering the thermal conditions of a ‘Category A’ indoor space, which specifies PMV within

Discussion

Results obtained point to the importance of two processes of adaptation (long-term and short-term) which could be at play when we interpret changes in reported thermal sensation over the 30 min time frame surveyed during each session and from seasonal changes in this respect. The concept of thermal adaptation includes the way people interact with the environment and their behavior and is influenced by contextual factors and past thermal history, which can act upon their expectations and thermal

Conclusion

In this study, we compare TSV against predicted thermal sensation (DTS) with the non-steady state index UTCI. Results suggest that the longer exposure time will reduce prediction errors and therefore lead to more consistent thermal votes' estimates. A longer exposure proved to be advantageous in the three seasons evaluated, which could be regarded as a necessary condition for validating thermal votes in outdoor comfort surveys.

From previous research on transient indoor conditions [4] which have

Acknowledgements

We acknowledge the Brazilian research funding agency CAPES, the European Union 7th Framework Programme (FP7/2007–2013) grant agreement no. PIRG08-GA-2010-277061 and the Fachgebiet Bauphysik & Technischer Ausbau/Karlsruher Institut für Technologie (fbta/KIT).

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