In "green building", four "lifetime" phases of a building are described, namely material sources, construction, in use, and deconstruction (life-cycle assessment). Terms carbon zero and negative carbon buildings refer to the net greenhouse gas emissions over these four phases. Questions therefore arise as to whether certain structures are truly environmentally friendly. For example, raw materials must be extracted from the earth, transported and then manufactured into building materials and transported again to be sold and finally transported to the build site. A lot of fossil fuels may be used during each of these stages.