The department's Air Pollution Control Program develops multiple plans to meet federal standards for six criteria pollutants: lead, ozone, particulate matter, carbon monoxide, nitrogen dioxide and sulfur dioxide. These plans constitute one main state implementation plan (SIP).
The EPA reviews pollutant standards every five years. Sometimes, a review results in a tightened standard. When this happens, the multi-step process to draw up a new SIP begins again. It's not uncommon for work on the old and new standards to overlap. The department considers input from all stakeholders when developing a SIP.
EPA has recently approved two maintenance plans and redesignation requests that affect the St. Louis area. This means EPA has redesignated the area from nonattainment to attainment for the 2008 ozone standard and the 1997 fine particulate matter standard. The department has developed a white paper summarizing what the redesignations mean for the area. See the nonattainment area map depicting all remaining/current nonattainment areas in the state.