Jennifer Vanos
Texas Tech University, USA
Robert Brown
University of Guelph, Canada
When parents send their children off to school they expect them to spend their days in a safe and healthy environment. There’s plenty of evidence of how outdoor play promotes personal health and well-being, and access to a playground in an effectively designed space is a large component of being physically active. But not all playgrounds are as safe as they might appear. With canopy coverage lower than many city averages, children might be exposed to increased levels of ultraviolet radiation (UVR) that can lead to erythema (sunburn) and an elevated risk of skin cancer. In addition, surfaces that might look to the human eye to be cool and inviting can be extremely hot. These hot surfaces create two potentially dangerous conditions for children: burned skin from direct contact, and very high infrared radiation load that can increase the risk of hyperthermia.
The use of Evidence-Based Landscape Architecture (Brown and Corry, 2011) can be helpful when designing playgrounds to be safer and healthier through bioclimatic design. Thermal infrared photography makes hot surfaces visible to the human eye through false-color imaging so hot spots can be identified and ameliorated. 3D Computer modeling programs such as Google SketchUp can be used to analyze sun and shade patterns and test potential designs to determine if children will receive too much, too little, or just the right amount of solar radiation.
Children are more vulnerable to heat stress than adults under high temperature conditions (Falk and Doton, 2008), and also have a higher sensitivity to UVR exposures (e.g., Oliveira et al., 2006). Inconsistencies also exist in the literature with respect to heat-health outcomes in children as compared to adults, and what their true exposures to temperature and radiation may be (Vanos et al., 2015). Two principal reasons exist for these inconsistencies: 1) the actual (or true) exposures of temperature and radiation are unknown due to the use of sparse observations, which may cause exposure misclassifications (e.g., Kuras et al., 2015) who studied individually experienced temperatures); and 2) there are many fewer research studies on the impacts of heat in children compared to those for adults (Falk and Doton, 2008).
Extreme heat and high radiation exposure are particularly prominent in very sunny climates in Mediterranean, arid, and semi-arid climate zones. Research published this year (Vanos et al., 2016) with colleagues Ariane Middel and Ben Ruddell at Arizona State University took a first step toward improving heat and sun safety at playgrounds. With the goal of improving the comprehensiveness of observations at the “touch-scale” in urban areas (both in shade and sun), we studied two playgrounds in Gilbert, Arizona, a suburb of Phoenix. On a September day with air temperatures reaching 41.6°C, and a maximum incoming solar radiation of 874 W m-2, we found surface temperatures reaching near-boiling point levels. For example, the well-intentioned dark rubber surface on which one playground was constructed––which was soft to cushion falls––was recorded at 87.2°C in the sun at noon, less than 13°C from the boiling point of water. This same playground had a shade sail recently installed (see Figure 1), which provided a unique opportunity to study the temperatures of the same surfaces in both shade and sun. In the shade of the sail, the same rubber surface was measured at 46.7°C, and in the shade of a tree it was even lower at 42.2°C, both temperatures being much closer to the air temperature (which was used as our reference temperature for comparative purposes).
In terms of play equipment, a green, molded plastic slide with a high density polyethylene coating was measured at 71.7°C in the sun and 43.9°C under tree shade. A beige-colored slide of the same material and coating was measured at 63.9ºC in the sun and 40.6°C under the shade tree. For a point of reference, the burn threshold for the material of which the slide is made is one minute at 60°C, five seconds at 74°C and just three seconds at 77°C (ISO 13732, 2006), meaning a child’s skin does not have to contact the surface for very long to be burned. The temperatures of each piece of equipment are influenced by numerous biophysical variables, such as shade, sky view factor, solar energy, the material and type of equipment, heat capacity, admittance, conductivity, and length of exposure to sun.
Coarse scales of commonly employed satellite imagery of surface temperature are unable to discern a human’s experiences at the finest scales of exposure (i.e., the human-scale and the touch-scale of ~1cm) that relate directly to one’s thermal comfort and thus environment. To address this, our team also compared the in situ touch-scale temperatures from the NASA MODIS/ASTER[1] Airborne Simulator (MASTER) remote sensing airborne imagery at a 7m horizontal resolution obtained on a day with nearly identical weather. From this, we were able to derive temperature offsets (or errors) that indicate how much higher many of the ‘touch-scale’ surfaces temperatures were. For example, objects to sit on were 13.6°C higher with touch-scale observations as compared to that which the MASTER imagery could resolve at a 7m resolution. The research paper provides a foundation to build upon for multi-scalar modeling of fine-scale urban temperature estimates in high-use, high-contact urban areas, such as children’s playgrounds.
Another well-intentioned surface material used in recreational spaces is artificial turf. Artificial turf is commonly installed in playgrounds and sports fields and it can become very hot during sunny conditions, even in moderate climates. We took some measurements of an artificial turf field during a modestly warm summer day in Guelph, Ontario (Figure 3). The image shows the infrared image on the left and the visible light image on the right. Someone viewing only the visible light image might think that the turf on the field would be about the same temperature as the grass beside the field. But they would be very wrong. While a parent or coach might be comfortable on the sidelines standing on real grass in the shade of a tree, a child on the field would be bathed in large amounts of terrestrial radiation emitted by the 47°C artificial turf. What’s more troubling is temperatures that are founded on artificial turf during hot summer days in warmer, sunnier locations. On a clear day in West Texas with air temperatures reaching 38.5°C, our microclimate station measured an artificial turf temperature of 78°C, which according to the football coach, was hot enough to melt his players’ shoes.
Although safety in organized sports and playgrounds has improved substantially in the last few decades, the use of natural surfaces and protection from the sun with shade goes largely unaddressed. It may, however, be the most important. There are reports of children burning themselves on playground surfaces, yet we found little guidance in the Certified Playground Inspector training (organized through the National Program for Playground Safety (NPPS)), regarding the dangers of surface temperatures and sun exposures. And much of this seems obvious: solar radiation makes surfaces hotter, and UV radiation causes sun burns that could lead to skin cancer, but not everyone knows that 80% of a person’s lifetime exposure to UV radiation occurs in their first two decades of life. We are hopeful our research adds to the discussion, and provides a new evidence base that highlights the importance of bioclimatic principles and effective green space for reducing heat stress and extreme surface temperatures in recreational play spaces, particularly parks and children’s play places.
Implementing Solutions
The main solution to extreme surface temperatures is both simple and complex. The simple solution is to provide shade. Whether from natural sources such as trees or artificial sources such as shade sails, shade can dramatically improve the safety of playgrounds and reduce the burning potential for children. The more complex part is knowing what type of shading device to use, where to locate it, and when throughout the year it is most beneficial. In locations that have distinct warm and cold seasons, designers need to provide opportunities for children to absorb some solar radiation during cold seasons, but have cool and shady environments to play in during warm seasons.
Deciduous trees can be very effective design elements as they provide shade from the sun in the summer, and after they shed their leaves in the fall they allow considerable amounts of solar radiation to pass through their branches. Different species of trees have different transmissivity values both during winter and summer. In addition some species leaf out early in the spring and drop leaves early in the fall, while others get their leaves later and keep them later into the fall. Tables of data about different species of trees can be found in publications such as “Microclimatic Landscape Design” (Brown and Gillespie, 1995) and can help guide design decisions. Thermal images can also be used to reveal the microclimatic effect of different species of trees. The image in Figure 4 was taken on a cool spring day (17.4°C) in Guelph, Ontario. The unshaded ground near the top of the photo is about 30°C, the light blue and yellow area near the bottom of the photo (approximately 20°C) is in the shade of a Betula papyrifera (Paper birch) tree, while the dark blue area in the middle of the photo (approximately 16°C) is in the shade of a Acer platanoides (Norway maple) tree. The Norway maple has a much lower transmissivity than the Paper birch. The temperature cooling effect of evapotranspiration from healthy vegetation with available moisture is also a well-studied solution for decreasing air temperatures, allowing for park cooling islands to be present in a range of climate zones (e.g., Declet-Barreto et al., 2013; Vanos et al., 2012). This “oasis effect” of greenspace is commonly found in dry climates to cool near-surface air temperatures in urban areas, and when employed in conjunction with shade cover, can greatly enhance thermal comfort and improve the usability of a space.
Designers will have to keep in mind that differences in playground design will also be influenced by culture and policies. An example of contrasting playground designs in three different countries is shown in Figure 5. There is no cookie-cutter solution to playground design, but microclimate analysis should definitely be part of the design process to ensure comfort and safety during use. And when it comes to producing natural shade, not all trees are created equal, and we also know that certain climate regimes have few native species. Hence, designers can match the most appropriate species for the climate with the intended use of a space. A schoolyard closed to the public may only be used during early fall through winter, and spring, but neighborhood playgrounds are a space for children to play actively all year round. Trees that provide a heavy shade in mid-summer might be preferred in these locations, and whether or not they leaf early or late is not as important. Creating climate-zone-specific guidelines is an important goal for use in evidence-based bioclimatic urban design for a wide-range of climates.
The use of Evidence-Based Landscape Architecture will play a large role in mitigating future warming projected for many of the world’s cities through bioclimatic design (e.g., Brown et al., 2015). Results presented here from one small-scale study, along with related research in urban greenspace, have demonstrated shade as an important solution in creating “effective” greenspace and thus spaces that are more thermally conducive for comfort and activity. Although many components of the microclimate are not visible to the human eye, technology provides us with evidence of key parameters affecting the thermal environment, thus leaving little justification for the presence of recreational surfaces that are as hot as an asphalt parking lot.
[1] MODIS: Moderate Resolution Imaging Spectroradiometer; ASTER: Advanced Spaceborne Thermal Emission and Reflection Radiometer.
Dr. Jennifer Vanos is Assistant Professor of Atmospheric Science at Texas Tech University in Lubbock, Texas, USA.
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Dr. Robert Brown is Professor of Landscape Architecture at the University of Guelph in Guelph, Ontario, Canada.
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