Sulfur dioxide air quality designations

EPA evaluates how each area of the country meets the national air quality standards 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 air quality standards by using a specified value, called a design value. In plain terms, the lower the design value, the cleaner the air. All areas of Washington, except a small area in Whatcom County, meet the national air quality standard for sulfur dioxide.

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 air quality 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 as "unclassifiable."

If an area does not meet an air quality 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 sulfur dioxide 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 sulfur dioxide. 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 air quality standard.

Air quality designation process

EPA established the current sulfur dioxide standard in 2010. EPA typically evaluates and designates areas within three years of establishing a new standard. When EPA’s 2010 designation process for sulfur dioxide 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 EPA a recommendation about 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 in attainment or unclassifiable, there are no additional requirements besides ongoing regular monitoring.

Washington area designations for 2010 sulfur dioxide standard

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

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."1

1 In Lewis and Thurston counties, Ecology modeled emissions from the TransAlta coal-fired power plant to demonstrate that the area continues to meet the air quality standard. However, EPA requested additional evaluations to support the proposed attainment designation. Due to resource constraints and the low risk of the area violating the air quality 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.”2
  • EPA designated a small area in Whatcom County, the Intalco aluminum smelter and the area around it, as “nonattainment.”3
  • EPA designated the rest of Whatcom County as “attainment/unclassifiable.”

2 We received questions about why monitoring sulfur dioxide pollution near the aluminum smelter in Malaga was necessary, since the plant curtailed its operations in 2016-2017. Regulatory requirements do not allow us to postpone monitoring and the facility may begin operating again. (Curtailment is different than closure. It means the facility maintains its operational permits.) At this time, the monitor near the facility shows that sulfur dioxide readings are below national air quality standards.

3 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, EPA designated a small area in Whatcom County, including the Intalco aluminum smelter and the area around it, as “nonattainment.” EPA designated the rest of Whatcom as “attainment/unclassifiable.”

Ecology and Northwest Clean Air Agency are working on a 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 sulfur dioxide nonattainment areas as quickly as practical, but no 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.

Sign up for email updates about our sulfur dioxide attainment planning work.

More information

Health impacts

Breathing elevated levels of sulfur dioxide causes inflammation and tightening of airways, and can trigger asthma symptoms. Sulfur dioxide 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 pre-existing breathing or health issues.

EPA established Air Quality Index (AQI) ratings for sulfur dioxide pollution to identify when 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 sulfur dioxide (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.

See more information about AQI categories

Environmental impacts

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