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Why is Portland air quality bad today?

Portland air quality has achieved a mean annual air quality index (AQI) rating of “good.” For the last two decades, Portland has achieved air pollution standards set by the Environmental Protection Agency (EPA) for all six criteria pollutants measured in real-time: PM, PM10, ozone, carbon monoxide (CO), nitrogen dioxide (NO₂), and sulphur dioxide (SO₂).

Despite this success, Portland’s air pollution levels have been climbing since The concerning trend coincides with a growing population and number of vehicles on the road, numerous EPA regulatory rollbacks, warming global temperatures, and more frequent and severe wildfires.

In the most recent 3-year monitoring period spanning to , Portland averaged roughly 3 unhealthy pollution days a year.¹ Due to the frequency of these events, the American Lung Association (ALA) graded Portland a “C” for hour PM and hour ozone in their State of the Air report. This is a slip from Portland’s “A” rating in the to monitoring period, which experienced less than 1 unhealthy day a year.

In Portland, each season brings unique pollution challenges. Fine particle pollution tends to rise in the winter months with increased household wood burning and pollution-trapping temperature inversions. Meanwhile, ozone, a secondary pollutant, only becomes problematic in the summer, when abundant sunshine and heat create ideal conditions for its formation.

Monitor Portland air pollution data at the top of this page to stay aware of pollution spikes and health advisories. The “main pollutant” indicates which of the six criteria pollutants is present at the riskiest levels (usually PM or ozone). When Portland PM levels are exceedingly high, there is usually a fire burning nearby or domestic wood burning during a cool air inversion. Ozone tends to indicate warmer temperatures and increased vehicular traffic, such as during rush hour, though individual sources may vary.

Why is there air pollution in Portland?

Portland’s unhealthy air pollution is a combination of PM and ozone pollution, two of the most prevalent and dangerous pollutants in the US. While both pollutants share some of the same sources, such as motor vehicles and fossil fuel combustion at industrial complexes, power plants, and factories, they are also unique.

Ozone, a secondary pollutant, occurs in the atmosphere when high temperatures (over 84°F) and sunlight (UV radiation) causes pollutants, nitrogen oxides (NO_x), and volatile organic compounds (VOCs) to react. Precursor pollutants are largely emitted by vehicles and smoke stacks; however, they can also travel hundreds of miles from neighboring cities when carried by wind. Portland, for example, can be adversely affected by Seattle air quality, Bend air quality, or Eugene air quality.

Portland’s ozone season runs from May through September, when the city experiences an average of 51 days over 80 degrees.² As temperatures continue to rise as a result of climate change, it will be increasingly difficult to manage ozone levels. Limiting the emissions of pollution sources, such as switching to cleaner energy, transitioning to electric vehicles, and cooperating with regional towns and cities, offer an opportunity for reducing ozone despite warming temperatures creating more ideal conditions for its formation.

Portland’s PM pollution tends to be attributable to vehicular emissions, wildfires in the late summer and early fall, and domestic wood burning in the winter months.

The Mosier Creek Fire, which burned east of Hood River in the town of Mosier in August , is an example of a wildfire that caused “unhealthy” PM pollution levels in Portland.³

More often, PM spikes are the result of domestic wood burning and cool air inversions in the winter. This is the case for Portland’s most polluted months in November and December. Cool air inversions, or temperature inversions, occur when warm air caps colder air below, preventing the normal dispersion of air pollution in the atmosphere. Snow cover and sparse light contribute to these conditions, creating freezing cold surface air. Cool air inversions persist, accumulating surface-level air pollution until temperature and weather change allow sunlight to warm the surface air more than the air above.

How does Portland Oregon reduce air pollution?

Portland has made significant gains in improving air quality over the last two decades. The progress has been the result of a combination of regulations on a local, state, and federal level which have increasingly shifted to cleaner and more efficient uses of energy, and emission limits. There is still more to be done.

In , Portland’s most polluted months were November and December (respectively), with PM concentrations of μg/m³ and μg/m³ (three times the average PM concentration of summer months). Particle pollution spikes during these months have caused Portland to rank among the top 25 cities with the most short-term particulate pollution, according to the ALA’s report.

High PM levels during these months are the result of domestic woodburning and temperature inversions, which create a pollution-trapping effect. These seasonal particle pollution fluctuations cause Portland to average more PM pollution than New York City, the most populous metropolitan area in the US (which experiences an annual average of 7 μg/m³).

A Multnomah County ordinance passed in January seeks to reduce the impact of wood burning in the winter by enforcing a ban based on Portland’s live air quality conditions. Wood burning is limited to times of “green” or “good” US AQI levels.⁴ Exceptions have been made, however, for those with permits, or if wood burning is for cooking, heat, and light during a power outage, or otherwise serves as a household's only source of heat.

When will air quality in Portland improve?

Ozone and PM pollution contribute equally to Portland air pollution, with unhealthy pollution days attributed to ozone and unhealthy pollution days attributed to PM pollution. Improvements to Portland air quality must therefore strive to manage both PM and ozone precursors.

Emissions from motor vehicles are a leading source of both ozone and particle pollution (PM) in Portland. Targeting emission reductions here can reduce prevalence of both pollutants.

During the COVID pandemic, lockdown measures shuttered non-essential businesses and greatly reduced non-essential travel. In doing so, Portland had a rare glimpse into the impact reduced traffic emissions could have on the city’s air quality. A study conducted by the Oregon Department of Environmental Quality found that COVID restrictions led to 40 percent less traffic on the I-5, a 60 percent drop in nitrogen oxides( an ozone precursor pollutant), and a 25 percent drop in black carbon (PM soot).⁵

Sustaining these reductions in the long term is likely possible by shifting at least half of currently registered motor vehicles to electric vehicles or other hybrid low emission alternatives. The State of Oregon currently seeks to add million zero emission vehicles by and hopes that all vehicles will be zero-emission by ⁶ Such changes will result in drastic improvements to Portland’s air quality.

Where is pollution in Portland coming from?

Air pollution in Portland comes from a range of sources, including a combination of mobile sources (motor vehicles), stationary sources (smoke stacks from power plants and factories), and area sources (agricultural areas and wood burning fireplaces).

In much of the US, mobile sources represent the largest single emission source. This emission source is particularly culpable for Portland’s ozone challenges.

Stationary emission sources in Portland include Portland General Electric, Oregon’s only coal-fired power plant, pulp and paper mills, and natural gas-fired plants.⁷ Most of Portland’s stationary emission sources are concentrated on the northern end of the city.

Racist redlining practices dating back to the s have had a lasting impact on Portland’s neighborhood demographics and the distribution of air pollution in the city. High-polluting city features such as airports, major roadways, factories, industrial complexes, and power plants have historically been placed near these disadvantaged communities, further driving down the value of the homes in these neighborhoods while also burdening them with higher amounts of air pollution.

A study led by Portland State University found that 42 percent of historically redlined neighborhoods are less than 1 mile from the city’s top 10 polluters.⁸ As a result of the lingering impact of redlining, 38 percent of Portland’s non-white residents live in a 2-mile radius of these emission sources. Not only do these communities bear a disproportionate amount of air pollution, they also suffer more adverse and acute health effects as a result.

Non-white Americans who have historically occupied lower-income neighborhooods have 50 percent higher infant mortality and low birth weights than whites as well as higher rates of diabetes, high blood pressure, physical disabilities, stroke, diabetes, heart disease, and cancer deaths. These negative effects, brought on in part by commercial and government operations, must be addressed as an environmental justice issue. All communities deserve the right to healthy, clean air.

+ Article Resources

[1] American Lung Association. (). State of the air –
[2] Current Results. (). Portland temperatures: Averages by month.
[3] Raineri J. (, August 13). Air quality advisory in place for parts of the Gorge from Mosier Creek Fire. KGW.
[4] Multnomah County Environmental Health Services. (). Winter wood burning restrictions.
[5] Williams K. (, May 1). Amid the coronavirus shutdown, some types of air pollution down by as much as 60 percent in Portland. The Oregonian/OregonLive.
[6] City of Portland Oregon. (). ENN - Portland Electric Vehicle Strategy.
[7] Learn S. (, January 10). Power plants Oregon's largest single sources of global warming emissions, new EPA inventory shows. The Oregonian.
[8] Profita C. (, April 29). Study: More people of color live near Portland’s biggest air polluters. Oregon Public Broadcasting (OPB).