text: Passive daytime radiative cooling surfaces can mitigate extreme heat from the urban heat island effect which occurs in over 450 cities worldwide, where it can be as much as  hotter in urban areas in comparison to surrounding rural areas. On an average hot summer day, the roofs of buildings can be  hotter than the surrounding air, warming air temperatures further through convection. Well-insulated dark rooftops are significantly hotter than all other urban surfaces, including asphalt pavements, further expanding air conditioning demand (which further accelerates global warming and urban heat island through the release of waste heat into the ambient air) and increasing risks of heat-related disease and fatal health effects.
  PDRCs can be applied to building roofs and urban shelters to significantly lower surface temperatures with zero energy consumption by reflecting heat out of the urban environment and into outer space. The primary obstacle of PDRC implementation in urban areas is the glare that may be caused through the reflectance of visible light onto surrounding buildings. Colored PDRC surfaces may mitigate glare issues, such as Zhai et al. "Super-white paints with commercial high-index (n~1.9) retroreflective spheres," as per Mandal et al., or the use of retroreflective materials (RRM) may also mitigate glare, although further research and development is needed. Surrounding buildings without PDRC application may weaken the cooling power of PDRCs.
  Even when installed on roofs in highly dense urban areas, broadband radiative cooling panels have been shown to lower surface temperatures at the sidewalk level. A study by Khan et al. published in 2022 assessed the effects of PDRC surfaces in winter, including for both non-modulated and modulated PDRCs, in the Kolkata metropolitan area. A non-modulated PDRC with a reflectance of 0.95 and emissivity of 0.93 decreased ground surface temperatures by nearly  and with an average daytime reduction of .
  While in summer the cooling effects of broadband non-modulated PDRCs may be desirable, they could present an uncomfortable "overcooling" effect for city populations in winter and thus increase energy use for heating. This can be mitigated by broadband modulated PDRCs, which they found could increase daily ambient urban temperatures by  in winter. While in the tropical metropolitan area of Kolkata, for instance, "overcooling" is unlikely, elsewhere it could impact the willingness to apply PDRCs in urban spaces. Therefore, modulated PDRCs may be preferred in cities with warm summers and cold winters for controlled cooling, while non-modulated PDRCs may be more beneficial for cities with hot summers and moderate winters. The authors expected "low-cost optically modulated passive systems" to be commercially available soon.
  In a study on urban bus shelters, it was found that most shelters fail at providing thermal comfort for commuters, noting that, on average, a tree could provide  more cooling. Other methods to cool shelters often resort to air conditioning or other energy intensive measures that can crowd commuters in an enclosed space for cooling. Urban shelters with PDRC roofing can significantly reduce temperatures with zero added costs or energy input, while adding "a non-reciprocal mid-infrared cover" can increase benefits by reducing incoming atmospheric radiation as well as reflecting radiation from surrounding buildings, as per Mokharti et al.
  For outdoor urban space cooling, it is recommended that PDRC implementation in urban areas primarily focus on increasing albedo so long as heat emissivity can be maintained at the standard of 90%, as per Anand et al. This can rapidly and significantly lower temperatures while reducing energy demand and costs for cooling in urban environments.