Underused or abandoned urban industrial or commercial sites. In the United States, it specifically refers to those sites that are perceived to be contaminated, making them difficult to be re-used or redeveloped. (Changes in site ownership, remediation technology, and local environmental laws can affect the perception of contamination). One unintended consequence of environmental cleanup laws in the U.S., was the development of more brownfields, as property owners liable for expensive cleanups abandoned properties to develop new ones elsewhere (in undeveloped space, aka "greenfields").