No “Serious” Upgrade: EPA Decision Keeps Maricopa in the “Moderate” Lane

By Patrick J. Paul, John Habib, and Sukhmani K. Singh

On March 23, 2026, in a decision consequential to regulated Maricopa County businesses and those looking to relocate or expand, the U.S. Environmental Protection Agency (EPA) declined to reclassify the Phoenix-Mesa ozone nonattainment area, including Maricopa County, from “moderate” to “serious” under the 2015 ozone National Ambient Air Quality Standards (NAAQS). As a result, businesses avoid a meaningful step-up in regulatory burden, and the current compliance framework remains intact.

Under the Clean Air Act, an area’s nonattainment classification directly drives permitting thresholds, emissions controls, and offset requirements. A move to “serious” would have significantly tightened the screws — lowering major source thresholds to 50 tons per year of regulated pollutants, increasing offset ratios, and expanding Reasonably Available Control Technology obligations — all while adding complexity to permitting and modeling. For many companies, that translates to higher costs, longer timelines, and more friction on growth. The decision is also important to Arizona and Maricopa County as new businesses may be more inclined to move to the County and existing businesses may be more willing to expand with greater regulatory predictability.

In a press release on the decision, noting the impact on its ability to attract and support business growth, Maricopa County observed that EPA agreed that emissions from outside the United States are inflating ground-level ozone levels in the region, and if not for that uncontrollable pollution, the County otherwise would have met 2015 NAAQS for ozone. In a separate press release, EPA touted the importance of cooperative federalism, while noting that its decision would ensure that manufacturers and citizens would not be punished for emissions not within their control.

EPA’s decision reflects an increasingly important reality for the Phoenix region, namely that a meaningful portion of ozone levels is influenced by factors beyond local control. International emissions transport, wildfire activity, and background conditions continue to play a significant role in air quality conditions. Regional agencies — including the Maricopa Association of Governments and the Arizona Department of Environmental Quality — have spent years building the technical record to support that conclusion.

EPA listened as it relied on a Clean Air Act Section 179B demonstration showing the region would have met the ozone standard “but for” international emissions. In plain terms, the data supports what many in the regulated community have long argued: local sources — already heavily controlled — are not the primary drivers of exceedances.

There is also a broader signal here. This marks a notable use of EPA’s Section 179B authority following changes to prior agency guidance, suggesting a potentially more flexible approach to areas impacted by cross-border or uncontrollable emissions. For regions like Maricopa County, that distinction is not academic — it is central to how aggressively federal requirements are applied.

For businesses, the takeaway is simple: the regulatory environment remains stable — but not static. Maricopa County remains in Clean Air Act nonattainment status, and existing obligations for emissions controls, offsets, and permitting also remain fully in place. Importantly, however, the risk of future reclassification has not disappeared. If ozone levels do not improve, the Clean Air Act remains as a hammer for more stringent regulation. 

In the meantime, this decision creates a window of relative certainty. Companies should take advantage of it by:

Critically, EPA’s decision avoids a regulatory escalation that would have imposed real costs on Arizona businesses. But it is not a free pass. Maricopa County remains on a compliance clock, and the next chapter will depend on whether progress continues — and whether EPA continues to recognize the limits of local control.