Sulfur dioxide air quality designations

EPA evaluates how each area of the country meets the national ambient air quality standards (NAAQS) for sulfur dioxide (SO2) and other air pollution through a process called “area designations” or “air quality designations.”

EPA and states determine whether an area meets the standards by using a specified value, called a design value. In plain terms, the lower the design value, the cleaner the air. Except for a small area in Whatcom county around an aluminum smelter, the rest of Washington meets national air quality standards for SO2

Federal air quality classifications

Under the federal Clean Air Act, if the air quality in a geographic area meets or is cleaner than the national standard, it is called an attainment area; areas that don't meet the national standard are called nonattainment areas. In some cases, EPA is not able to determine an area's status after evaluating the available information. Those areas are designated "unclassifiable."

If an area does not meet a standard, then the state is required to create and follow a state implementation plan to bring that area back into attainment.

EPA and states determine whether an area meets this standard by using a specified value, called a Design Value. For the current 2010 health-based SO2 standard, the Design Value is based on a three-year average of the fourth-highest, daily maximum 1-hour concentration recorded at a monitoring site. That three-year average number must be less than or equal to 75 parts per billion (ppb) of SO2. In plain terms, the lower the Design Value, the cleaner the air.

Only design values calculated based on the regulatory monitors that meet Ecology’s and EPA’s data quality system requirements are used to determine if an area is in violation of the standard.

Air quality designation process

EPA established the current SO2 standard in 2010. EPA typically evaluates and designates areas within three years of establishing a new standard. When the EPA’s 2010 designation process for SO2 was delayed, a federal court mandated deadlines to complete the process.

The area designation process involves collecting relevant air quality data such as emissions, monitoring values, and modeling. States may provide a recommendation to EPA on how each area should be designated. EPA considers that recommendation and public comments to make a final determination. If an area is designated as being in nonattainment, the state must meet federally mandated deadlines for fixing the air quality issues. For areas designated as attainment or unclassifiable there are no additional requirements besides ongoing regular monitoring.

Washington area designations for 2010 SONAAQS

EPA designated Washington areas in two steps: The majority of the state was designated in 2017, and the remaining areas were designated in 2020. The 2017 designation round was based on available emissions, monitoring, and modeling data. For the 2020 designation round, EPA required us to establish new SO2 monitoring sites near facilities that emit more than 2,000 tons of SO2 a year and collect data for three years (2017-2019). As a result, Ecology began monitoring SO2 concentrations around two aluminum smelters, one located in Whatcom County near the city of Ferndale, and the other located in Chelan County in Malaga, near the border with Douglas County.

In December 2017, EPA designated 36 of Washington state's 39 counties:

  • EPA agreed with our recommendation for 34 counties and designated these counties as being in "attainment/unclassifiable."
  • EPA designated two counties, Lewis and Thurston, as "unclassifiable," because of limited information. Ecology had recommended that these counties be classified as "attainment."*

*In Lewis and Thurston counties, Ecology modeled emissions from the TransAlta coal-fired power plant to demonstrate that the area continues to meet the standard. However, the EPA requested additional evaluations to support the proposed attainment designation. Due to resource constraints and the low risk of the area violating the standard, we chose to concur with EPA’s decision to designate the area as "unclassifiable.

In December 2020, EPA designated the remaining three counties:

  • EPA designated Douglas and Chelan counties as “attainment/unclassifiable.”*
  • EPA designated a small area in Whatcom County, comprising the Intalco Aluminum Smelter and the area around it, as “nonattainment.”**
  • EPA designated the rest of Whatcom as “attainment/unclassifiable.”

* We received questions about why monitoring SO2 pollution near the aluminum smelter in Malaga was necessary, since the plant curtailed its operations starting in 2016-2017. Regulatory requirements do not allow us to postpone monitoring, and the facility may begin operating again (curtailment is different than closure and means the facility maintains its operational permits). At this time, the monitor near the facility shows that SO2 readings are well below the NAAQS.

** The nonattainment area is bounded by lines connecting the following UTM Coordinates (zone 10): Northwest Corner: 519671 5412272; Northeast Corner: 524091 5412261; Southwest Corner: 519671 5409010; Southeast Corner: 524111 5409044.

Alcoa Intalco nonattainment area

In December 2020, the EPA designated a small area in Whatcom County, comprising the Intalco Aluminum Smelter and the area around it, as “nonattainment.” EPA designated the rest of Whatcom as “attainment/unclassifiable.”

Ecology, in partnership with the Northwest Clean Air Agency (NWCAA), will work on developing an attainment plan to bring the area back into attainment. The plan must include enforceable emissions reductions and a technical demonstration that the reductions are sufficient to attain and maintain the standard. The Federal Clean Air Act requires that states clean up the air in the SO2 nonattainment areas as expeditiously as practicable, but not later than March, 2026.  

The attainment plan is due to EPA in October, 2022. The public will have an opportunity to review and comment on the plan before we submit it to EPA.

To learn more about our SO2 attainment planning efforts SO2 in Washington, sign up to Ecology’s SO2 attainment email list.

Additional information

Public health impacts

Breathing elevated levels of SO2 causes inflammation and tightening of airways, and can trigger asthma symptoms. SO2 typically dissipates very quickly, but people living or working close to the plant may have been exposed to short-term levels that pose a risk to those with preexisting breathing or health issues.

EPA established Air Quality Index (AQI) ratings for SO2 pollution to identify when the air quality becomes unhealthy. The area near the Intalco aluminum smelter in Ferndale is the only area in Washington that did not meet federal air quality standards for SO2 (also known as nonattainment). The chart below shows the number of days that air quality monitors near the Intalco facility recorded readings in each AQI category. 

A chart of the potential health effects of different levels of SO2.

For a text version of this chart, click here

Environmental impacts

Sulfur dioxide decreases visibility in the form of haze and contributes to acid rain.