Passive climate control is associated with the sustainable-building movement, and there has been considerable interest in this area over the last several years with energy costs rising and more attention being paid to environmental issues. Sustainable buildings are designed and constructed in ways that achieve energy savings and a tolerable internal climate. They are located on sites that offer a good microclimate and are designed to regulate heat gain and loss (for example, through passive solar design) and to regulate and improve air circulation. When designing a sustainable building, important issues to consider are location (for example, the building's orientation relative to prevailing winds and the sun), natural shading (trees, hills, other buildings), shelter (from tropical storms), and structural materials (open, lightweight walls in humid tropical areas, thick walls in arid tropical areas).