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Short-Term 1-Hour and 24-Hour NAAQS Requirements

DEQ's Clean Air Act State Implementation Plan, approved by EPA, requires DEQ to ensure that the National Ambient Air Quality Standards are protected. DEQ has a responsibility to issue permits that are National Ambient Air Quality Standards compliant and determined current methods used to ensure the short-term NAAQS compliance are inadequate in its current permitting process. 

As of June 9, 2021, DEQ has implemented the short-term NAAQS modeling requirement for new facilities seeking to obtain a Simple or Standard Air Contaminant Discharge Permit. DEQ will expand this requirement to existing sources with Simple, Standard, and Title V permits. We have categorized sources into three groups for implementation. 

  • New Sources: DEQ requires all new Simple and Standard sources to demonstrate short-term National Ambient Air Quality Standards compliance before obtaining a permit. This new requirement also applies to new sources who have pending applications as of June 9, 2021.

  • Existing Sources Called-in by Cleaner Air Oregon Program: Existing sources already called into the CAO program have the option to perform the short-term NAAQS modeling concurrently with their CAO risk assessment or wait until there is a demonstration request from DEQ. By conducting modeling now, facilities can avoid being subject to additional modeling fees.

  • Existing Sources: DEQ will call in existing facilities to conduct the short-term NAAQS modeling based on a prioritization process that it will develop with input from stakeholders. DEQ will call in individual sources with guidance, timelines and resources based on the prioritization of existing facilities through this process.

DEQ is in the process of developing short-term significant emission thresholds as a screening modeling tool to ensure that the short-term NAAQS are protected. DEQ created short-term NAAQS Guidance and Modeling Procedures to use until the short-term NAAQS SET is established.


The Clean Air Act requires EPA to set National Ambient Air Quality Standards for six criteria air pollutants, which can be harmful to public health and the environment. The Clean Air Act identifies two types of national ambient air quality standards. Primary standards provide public health protection, including protecting the health of sensitive populations such as asthmatics, children, and the elderly. Secondary standards provide public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings. These requirements are outlined in Oregon Administrative Rules 340-202.

In 2010, EPA established 1-hour NAAQS for both NO2 and SO2 for the first time. In 2006, EPA lowered the primary and secondary 24-hour PM2.5 standards. This shift in focus from the long-term annual effects of these pollutants to short-term timeframes is because of 'strong scientific evidence indicating that short-term exposures to NO2 [and SO2] can worsen asthma symptoms in people with the disease.' Some studies indicate that as little as 5-10 minutes of exposure to SO2 can exacerbate asthma symptoms.​

​Oregon's current air quality permitting program has historically only required computer modeling to determine compliance with the NAAQS if New Source Review is triggered (See OAR 340-224-0010), though the authority to require modeling at any time is included in current rule. NSR is a permitting process created by the US Congress in 1977 as part of a series of amendments to the Clean Air Act. The NSR process requires industry to undergo pre-construction review for environmental controls if they propose either building new facilities or any modifications to existing facilities that would create a "significant increase" of a regulated pollutant. Significant increases, called Significant Emission Rates, are based on thresholds for increases of annual emissions, not short-term emissions. EPA defined SERs in 1980, well before any short-term NAAQS were set.​​

​The methodology used to develop the annual SERs did not take into account hourly variations in emissions or hourly meteorological conditions. These two factors, varying emissions coupled with varying meteorology, become important considerations when modeling sources with high intermittent emissions at 1-hour averaging times.

Facility emissions that are below the SERs may still cause short-term NAAQS exceedances. In-house analyses at DEQ have demonstrated, through the modeling of hypothetical facilities, that a 1-hour standard can be exceeded while annual average emissions are below the SER.

Both the basis of the SERs and the in-house modeling demonstrations lead DEQ to conclude the current practice of using the SERs to screen out sources from NAAQS compliance modeling may not sufficiently demonstrate that the NO2 and SO2 1-hour NAAQS, and the 24-hour PM2.5 NAAQS are protected. ​

Additional Resources


Need assistance?

  • For questions on future NAAQS modeling requirements, please contact your DEQ Air Quality Regional Manager.
  • For questions on your current air quality permit, please contact your Regional Permit Writer.
  • For questions on short-term NAAQS modeling, please contact Air Quality Technical Services Modelers Kristen Martin or Phil Allen.

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