The Cross-State Air Pollution Rule (CSAPR) was adopted in August 2011 as a replacement to the Clean Air Interstate Rule (CAIR). It establishes new SO2 and NOx markets that affect 28 states in the Eastern half of the US. The rule is viewed as more stringent than CAIR because it includes restrictions on regional trading and was to take effect less than six months after it was adopted. The CSAPR SO2 market is split into two groups, with generators in the 16 group 1 states unable to trade with those in the seven group 2 states. Utilities, industry advocacy groups and state and local governments challenged the rule in the DC Circuit Court of Appeals, which on December 30, 2011, put implementation of CSAPR on hold while the judges consider the legal arguments against it. If the rule is upheld, it is unlikely to take effect until 2013 or 2014.