energy retrofits increase worker health and thus productivity, "People in the U.S. spend about 90% of their time indoors. EPA studies indicate indoor levels of pollutants may be up to ten times higher than outdoor levels. LEED-certified buildings are designed to have healthier, cleaner indoor environmental quality, which means health benefits for occupants." Studies have shown over a 20-year life period, some green buildings have yielded $53 to $71 per square foot back on investment. Confirming the rentability of green building investments, further studies of the commercial real estate market have found that LEED and Energy Star certified buildings achieve significantly higher rents, sale prices and occupancy rates as well as lower capitalization rates potentially reflecting lower investment risk. Regulation and operation As a result of the increased interest in green building concepts and practices, a number of organizations have developed standards, codes and rating systems for use by government regulators, building professionals and consumers. In some cases, codes are written so local governments can adopt them as bylaws to reduce the local environmental impact of buildings. Green building rating systems such as BREEAM (United Kingdom), LEED (United States and Canada), DGNB (Germany), CASBEE (Japan), and VERDEGBCe (Spain), GRIHA (India) help consumers determine a structure's level of environmental performance. They award credits for optional building features that support green design in categories such as location and maintenance of building site, conservation of water, energy, and building materials, and occupant comfort and health. The number of credits generally determines the level of achievement. Green building codes and standards, such as the International Code Council's draft International Green Construction Code, are sets of rules created by standards development organizations that establish minimum requirements for elements of green building such as materials or heating and cooling. Some of the major building environmental assessment tools currently in use include: United States: International Green Construction Code (IGCC) Green neighborhoods and villages At the beginning of the 21st century, efforts were made to implement the principles of green building, not only for individual buildings, but also for neighborhoods and villages. The intent is to create zero energy neighborhoods and villages, which means they're going to create all the energy on their own. They will also reuse waste, implements sustainable transportation, and produce their own food. International frameworks and assessment tools IPCC Fourth Assessment Report Climate Change 2007, the Fourth Assessment Report (AR4) of the United Nations