State Implementation Plans 101

Missouri’s State Implementation Plan (SIP) is the state’s federally approved plan to protect air quality in the state by attaining and maintaining compliance with the National Ambient Air Quality Standard (NAAQS). The Code of Federal Regulations (CFR) identifies Missouri’s federally approved SIP at 40 CFR 52.1320, where it is broken down into three categories. These categories include, (1) EPA-approved state regulations; (2) EPA-approved source specific permits, consent judgements and orders; and (3) EPA-approved non-regulatory/quasi-regulatory SIP provisions. Each category includes numerous EPA approved SIP revisions. All of the elements in these three categories collectively make up Missouri’s SIP.

The first two categories are the foundation of the SIP as they provide the enforceable emission limits, permitting requirements, work practice standards and the monitoring and reporting requirements for emission sources in the state. The third category of SIP provisions are demonstrations of how the enforceable requirements, included in the first two categories, will assist the area in achieving federal Clean Air Act requirements.

Several recent revisions to Missouri’s SIP address criteria air pollutants for which the EPA established a NAAQS including ozone, particulate matter [coarse (PM10) and fine (PM2.5)], nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO) and  lead. When the EPA revises any NAAQS, the air program is responsible for developing:  boundary designation recommendations; an infrastructure SIP for the revised NAAQS; plans for nonattainment areas or areas determined to cause or contribute to a nonattainment area; and redesignation requests andmaintenance plans for areas previously designated nonattainment, but now demonstrate compliance.

For all SIP actions, state plans, boundary designations and redesignation requests, the Missouri Air Conservation Commission, whose primary duty is to carry out the Missouri Air Conservation Law (Chapter 643, Revised Statutes of Missouri), must review and adopt these actions prior to submittal to the EPA. In addition, most SIP actions require a public comment period and a public hearing prior to adoption. Both the public hearing and the request to adopt are held at commission meetings, which are open to the public.

Types of SIPs

Every time a NAAQS is revised, the state is required to submit a SIP.

  • Attainment/unclassifiable areas: The state is required to submit an infrastructure SIP that demonstrates the state’s authority to implement and enforce the standard and demonstrate that compliance with the standard will continue to be maintained.

  • Nonattainment areas: The state is required to submit several different documents as part of the SIP for these areas under each standard in which the area has been designated nonattainment. The scope of the different SIP submittals required for each nonattainment area depends on the severity of the nonattainment area and the pollutant for which the area has been designated nonattainment. 
    • Nonattainment areas are usually required to submit an attainment demonstration SIP.  This portion of the SIP lays out all of the control strategies required to demonstrate the nonattainment area will meet the new standard by a certain date, known as the attainment date.
    • Nonattainment area plans also typically require submission of a comprehensive emission inventory, a demonstration that reasonably available control measures have been implemented and a demonstration that reasonable further progress goals will be achieved. Finally, once an area that has been designated as a non-attainment area has monitored pollutant levels in compliance with the standard for an extended period, then the area can submit a maintenance plan and be redesignated to maintenance area. The maintenance plan must demonstrate how the area will continue to maintain compliance with the standard for 10 years after attainment has been reached.

SIP elements are revised from time to time and the more recent revisions to SIPs are listed on this website.

Boundary Designations 101

The Clean Air Act requires the EPA to review the National Ambient Air Quality Standards (NAAQS) for each of the six criteria pollutants every five years. 

Per the Clean Air Act, states are allowed to submit boundary designation recommendations to the EPA for consideration in the final boundary designation. The EPA may release boundary designation guidance following the finalization of a NAAQS that includes certain criteria to be considered in the boundary designation process.  The state may use this guidance in developing their recommendations to the EPA. 

If … then …
the EPA revises the NAAQS for any criteria pollutant after one of these five year reviews, the EPA is required to establish boundary designations for all areas of the country.
States submit their recommendations to the EPA within one year after a new NAAQS is finalized , the EPA reviews the states’ recommendations and issues a 120-day letter informing the state whether the EPA is planning to accept or modify the state recommendation. 
the EPA issues a 120-day letter to the state, the state may respond to the 120-day letter in order to offer additional data, justification and reasoning for the EPA to consider prior to the final boundary designations. 


After this process, the EPA finalizes the boundary designations for each area in the country into one of three categories.

  1. An attainment designation signifies that the air quality in an area is in compliance with the NAAQS. 
  2. A nonattainment designation signifies that either the air quality in an area exceeds the level of the NAAQS or that the emissions in the area contribute to a nearby area that is exceeding the level of the NAAQS. 
  3. An unclassifiable designation signifies that insufficient data exists to make a determination as to whether an area is attaining or not attaining the NAAQS.