Environmental impacts of reflective materials: Is high albedo a ‘silver bullet’ for mitigating urban heat island? Jiachuan Yang, Zhi-Hua Wang n , Kamil E. Kaloush School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ 85287, USA article info Article history: Received 4 July 2014 Received in revised form 19 February 2015 Accepted 10 March 2015 Keywords: Building energy efficiency Cool roofs Reflective materials Regional hydroclimate Thermal comfort Urban heat island mitigation Urban sustainability abstract Studies on urban heat island (UHI) have been more than a century after the phenomenon was first discovered in the early 1800s. UHI emerges as the source of many urban environmental problems and exacerbates the living environment in cities. Under the challenges of increasing urbanization and future climate changes, there is a pressing need for sustainable adaptation/mitigation strategies for UHI effects, one popular option being the use of reflective materials. While it is introduced as an effective method to reduce temperature and energy consumption in cities, its impacts on environmental sustainability and large-scale non-local effect are inadequately explored. This paper provides a synthetic overview of potential environmental impacts of reflective materials at a variety of scales, ranging from energy load on a single building to regional hydroclimate. The review shows that mitigation potential of reflective materials depends on a set of factors, including building characteristics, urban environment, meteor- ological and geographical conditions, to name a few. Precaution needs to be exercised by city planners and policy makers for large-scale deployment of reflective materials before their environmental impacts, especially on regional hydroclimates, are better understood. In general, it is recommended that optimal strategy for UHI needs to be determined on a city-by-city basis, rather than adopting a “one-solution- fits-all” strategy. & 2015 Elsevier Ltd. All rights reserved. Contents 1. Introduction ........................................................................................................ 830 2. Effect of reflective materials on environmental temperatures ................................................................ 831 2.1. Surface temperature ........................................................................................... 831 2.2. Air temperature ............................................................................................... 833 2.3. Urban heat island intensity ...................................................................................... 834 3. Impact of reflective materials on building energy consumption ............................................................... 835 4. Environmental impact at large: city and regional hydroclimates .............................................................. 836 5. Thermal comfort and health risk consideration ............................................................................ 837 6. Impact of reflective materials on air quality .............................................................................. 837 7. Discussion ......................................................................................................... 838 8. Conclusion ......................................................................................................... 839 Acknowledgment ....................................................................................................... 840 References ............................................................................................................. 840 1. Introduction The urban heat island (UHI) effect, higher temperatures in urban areas compared to surrounding rural areas, is a well-known phenomenon that has been documented in hundreds of cities worldwide [1,2]. UHI intensity scales with size and population density of cities, with an expanding city experiencing continuously Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2015.03.092 1364-0321/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ1 480 727 2933; fax: þ1 480 965 0577. E-mail address: zhwang@asu.edu (Z.-H. Wang). Renewable and Sustainable Energy Reviews 47 (2015) 830–843