Qualité de l’air à Washington

Why is the air quality so bad in Washington?

Generally, air quality in Washington state is considered healthy. Episodes of unhealthy air quality are usually the result of short-term PM2.5 and/or ozone pollution.

PM2.5 describes airborne particles from a wide variety of sources (even natural ones) that measure 2.5 micrometers or smaller. Wind-blown dust, soot and ash from wildfires, chemicals from industrial complexes, and the burning of fossil fuels all constitute common origins of PM2.5.

PM2.5 is lumped from a variety of chemical compositions into a single category based on size for its ability to slip past the respiratory system’s defenses and cause a wide range of short- and long-term health effects, from coughing, shortness of breath, and chest pain to chronic diseases that contribute to early death.

Washington’s PM2.5 spikes are often attributable to forms of incomplete combustion, such as winter wood burning and wildfires. The latter is the reason for year-to-year fluctuations.

Only 54 percent of Washington cities met WHO targets for annual PM2.5 exposure in 2017. This number rose to 78.5 percent in 2018 and 98 percent in 2019. These year-over-year improvements in Washington air quality correspond to a decline in the number of fires and acres burned during those years. Nationally, 71,500 fires burned 10 million acres in 2017, while 58,100 fires burned 8.8 million acres in 2018, and only 50,500 fires burned 4.7 million acres in 2019.¹

2020 has been a record-breaking year for fires nationally as well as within the state, causing more than a week of “unhealthy” or worse air quality in numerous Washington cities such as Seattle, Tacoma, Everett, Bellevue, and Kent.

Wildfires are expected to become more frequent and severe with the planet’s warming climate, as are Washington’s smoky conditions.² Steep mountains, strong winds, longer summer seasons, and drier conditions contribute to Washington’s wildfire problem, and have illuminated a clear trend of what is likely to come. Such exacerbated seasonal conditions have already resulted in historic wildfire seasons in recent years, notably in 2020 and 2017 before that.

Beyond the influence of wildfires, seasonal winter wood burning is cause for greatly elevated PM2.5 levels in the months from November to February. An estimated 63 percent of particle pollution in the Puget Sound area in the winter is attributable to wood stoves and fireplaces (compared to 16 percent of emissions from motor vehicles).³ During the winter months, urban cities in Washington commonly experience PM2.5 levels 3 to 5 times that of the levels experienced during the summer.

While winter emissions are typically higher than summer emissions, adverse weather conditions, such as temperature inversions, can prevent polluted air from dispersing, further heightening air pollution levels. In Washington, temperature inversions are common in the winter, particularly when snow covers the ground. Temperature inversions describe conditions in which cold surface level air becomes trapped and stagnates beneath a warmer, denser air layer above. Without a means for dispersing beyond the warm air layer, emissions accumulate in the lower atmosphere. Only changes in weather, such as with direct sunlight, strong gusts of wind, or the seasons, can reverse these conditions.

Ground-level ozone, commonly referred to as “smog,” is another air pollutant of concern in Washington state. Ozone is a highly corrosive gas formed in the atmosphere from ambient “primary” pollutants reacting in sunlight. Since abundant sunlight and heat (at temperatures 85 degrees or higher) are required for its formation, ozone occurs at heightened levels during the summer, worsening around the afternoon and typically dispersing in the evening and early morning.

Climate change is similarly exacerbating wildfires and ozone. As temperatures rise, the ozone season becomes prolonged. Higher temperatures also accelerate ozone formation, making it even more prevalent on the hottest days.

While most areas in Washington meet federal attainment levels for ozone, the Seattle-Tacoma area does not and has not since 2016.⁴ Ozone has been on the sharp rise there, likely as a result of warming temperatures and increased vehicular traffic (the number one source of the ozone precursor pollutant, nitrogen dioxide).

A transition to a greater share of fuel-efficient, low-emission vehicles offers an opportunity to combat this ozone trend and additionally reduce PM2.5 levels. Moves toward cleaner energy and a reduced reliance on fossil fuels offers another significant opportunity for further reducing Washington air pollution levels.

What are the most polluted cities in Washington state?

According to the American Lung Association 2020 “State of the Air” report that compares regional air quality across the United States, Yakima county and Okanogan county ranked 9th and 14th respectively for worst 24-hour PM2.5 pollution. Short-term PM2.5 spikes are common in locations across the Pacific northwest as a result of frequent seasonal wildfires.

In 2019, a relatively mild wildfire year, Washington’s most polluted cities for annual PM2.5 pollution were Sunnyside (11 μg/m³), Yakima (10.4 μg/m³), Toppenish (10 μg/m³), Mineral (9.8 μg/m³), and Yelm (9.8 μg/m³).

This ranking tends to differ by year, based on various factors (such as location of wildfires). From 2018 to 2019, however, Colville, Twisp, and Yakima have been in the top ten for most polluted Washington cities.

Air pollution levels often have a correlation with the prevalence of asthma in city populations as a key contributing factor. Washington’s west coast, and particularly southern west coast, have some of the highest asthma rates in the state. Counties with the highest asthma rates include:⁵

● Adult asthma: Asotin, Cowlitz, Ferry, Kitsap, Pacific, Thurston, and Wahkiakum.

● Youth asthma: Spokane, Columbia, Garfield, Asotin, Lewis, Wahkiakum, and Grays Harbor

Breathing polluted air can increase visits to the emergency room, lead to hospitalizations, and sometimes cause premature death. It is possible, however, to breathe clean air even in polluted outdoor environments. To do so, check live air quality measurements in your city, follow recommended health precautions, and make your home a clean air haven.

What causes air pollution in Washington state?

The majority of Washington’s unhealthy air quality levels are attributable to fine particulate matter, or PM2.5. Outdoor sources for PM2.5 include dust from construction sites and agricultural activity, smoke from wildfires and winter wood burning, and chemicals like black carbon from ships, trains, vehicles, and factory emissions.

Washington’s highest monthly PM2.5 levels are largely attributable to wood stoves and fireplaces which make up 63 percent of winter particulates. Washington’s highest daily PM2.5 levels on the other hand are attributable to seasonal wildfires.

Cool air inversions, which are more common in the months from November to February, can elevate measured air quality levels in Washington by stagnating air and preventing the normal dispersion of air pollution. While not directly ‘causing’ air pollution, these weather conditions allow emissions to accumulate, resulting in increased air pollution levels.

As with most US cities, ozone pollution in Washington comes primarily to vehicular emissions, including both heavy duty diesel-powered trucks and personal cars. Other sources include power plants and other industry, and sources of combustion, such as wildfires and wood burning.

Use Washington’s air quality map to discover local sources of emissions and understand how wind interacts with pollutants to cause far reaching air pollution.

When will Washington air quality improve?

From a decades-long perspective, air quality in Washington has greatly improved by measures of annual PM2.5 and short-term ozone. Despite improvements in short-term PM2.5 pollution for nearly 15 years, this measure is on the rise, with levels from 2017 and 2018 roughly matching levels from 2000 and 2001.

Further air quality improvements against a backdrop of worsening climate change will require additional action, such as further reducing the prevalence of ozone precursor pollutants by transitioning to cleaner more fuel-efficient vehicles and managing the frequency and severity of wildfires through prescribed fires.

How to prevent air pollution in Washington state?

There are a number of actions that can be taken on a state and personal level to reduce Washington air pollution levels. Ultimately, collective effort between the two is the greatest mobilizer to effect change.

Governments are a powerful force in managing state-wide emissions. Legislative and government funded strategies include:

● Promote the availability and attractiveness of public transport.

● Incentivize or legislate the transition from old wood stoves to cleaner burning appliances.

● Implementing burn bans to reduce the risk of wildfires and seasonal wood burning.

● Reduce diesel dependence through more fuel-efficient vehicles and cleaner fuels.

● Transition the state fleet of vehicles to electric or hybrid alternatives.

● Use prescribed fires to clear forest debris and reduce the risk of severe wildfires.

Reducing personal emissions is a way of taking matters into one’s own hands for the betterment of our local environment and planet at large. The most significant actions that can be taken on a personal level include:

● Reduce domestic wood burning and instead seek out alternatives for home heating, such as natural gas and propane.

● Drive less, use more modes of public transport, and opt for cleaner cars and fuels.

● Avoid lighting outdoor fires and BBQs, and comply with burn bans. When starting a fire outdoors, follow safety protocol (more

than 90 percent of wildfires are human-caused).

+ Article Resources

[1] Hoover K, et al. (2020, September 1). Wildfire statistics. Congressional Research Service (CRS).
[2] Hill A. (2020, September 16). Why U.S. wildfires will only get worse. Council on Foreign Relations.
[3] Puget Sound Clean Air Agency. (2020). Criteria air pollutants.
[4] American Lung Association. (2020). State of the air – 2020.
[5] Washington State Department of Health. (2013). Outdoor air pollution and asthma in Washington state.