Air quality in Boise

What is the air quality in Boise, Idaho?

In 2019, Boise averaged an annual PM2.5 concentration of 6.9 μg/m³, thus meeting both the US federal target (≤ 12 μg/m³) and the more stringent World Health Organization (WHO) target (≤ 10 μg/m³).

Despite generally healthy air, Boise still experiences an average of 2.8 unhealthy PM2.5 days annually, where air quality reaches levels considered “unhealthy for sensitive groups” or worse.¹ Seasonal forest fires, winter wood burning, and vehicle exhaust are largely to blame for elevated PM2.5 levels in Boise.

In August 2020, 500 wildfires in Idaho, Eastern Oregon, and Northern California were responsible for causing Boise pollution levels to be classified as “unhealthy for sensitive groups” for several days.²

PM2.5 is the greatest threat to the Boise’s air quality due to its prevalence and impact on human health. When inhaled, PM2.5 can embed deep in the lungs and enter the bloodstream. Breathing PM2.5 has been linked to numerous respiratory and cardiac health concerns. The growing population in Boise and the Treasure Valley, coupled with increasingly severe and prolonged wildfire seasons in the western U.S. and Canada, suggest that Boise will continue to face a number of high PM2.5 pollution days annually.

Between 2016 and 2018, Boise averaged 6.7 unhealthy ozone days per year, roughly double the federal target of no more than 3.2 unhealthy ozone days annually.

Ozone (O₃), a key component of smog, is created when nitrogen oxides (NO₂) and volatile organic compounds (VOCs) – both found in vehicle emissions – chemically react in sunlight. Once formed, ozone can remain in the air for long periods of time as a result of limited pollution dispersion conditions, such as stagnant air from Boise’s valley location. Los Angeles and Mexico City can experience similar effects, owing to regional mountainous and high summer temperatures. Both cities are well known for their struggles with ozone.

Ozone is primarily a summer problem for Boise, as temperatures exceeding 84 degrees are necessary for ozone formation. Boise is located in arid Ada County, which only receives 13 inches of rain and 16 inches of snow each year.³ There are 50 days of the year on average when the temperature in the county rises over 90 degrees, thus posing increased risk for ozone formation. Higher temperatures often result in higher ozone levels, as heat causes the ozone formation process to speed up.

Why is Boise air quality so bad?

The air quality index (AQI) in Boise for PM2.5 pollution is typically rated “good” and often meets the more stringent World Health Organization air quality standard of 10 μg/m³ or less. But while air quality in Boise is generally rated “good,” the city can experience elevated ozone levels in the summer and elevated PM2.5 levels in the fall and winter. During snowy periods, it’s not uncommon that Boise records PM2.5 levels above 10 μg/m³.⁴ In January and February of 2019, Boise averaged PM2.5 levels above both the federal and WHO standard, with concentrations of 12.8 μg/m³ and 17.1 μg/m³ respectively.

Sources of poor air quality in Idaho identified by the U.S. Department of Agriculture include:⁵

• vehicular emissions
• residential wood burning
• wildfires
• dust
• animal-feeding operations
• crop burning

Boise’s air quality is also strongly impacted by its climate and geography. Located in southwest Idaho about 41 miles east from Oregon and 110 miles north of Nevada, Boise is the largest metropolitan area in Idaho. The city’s location within Treasure Valley near the foothills of the Rockies (to the northeast) and the Owyhee Mountains (to the southwest) puts the population at risk for air quality issues arising from:

• temperature inversions
• seasonal forest fires
• winter wood burning

Boise’s climate can be described as semi-arid continental, with four distinct seasons. The summer months are hot and dry while the winters are very cold (despite being moderated by the Rockies, which form a barrier from colder Canadian air). Precipitation is most common from October to May. During the summer months of June through September, rain is infrequent.

Boise is prone to air quality stagnation events as a result of its location in the Treasure Valley basin. Particulate levels here are generally worst between December and February, when winter wood burning and temperature inversions combine to create polluted air that is unable to escape the U-shaped valley.

What is Boise air quality like during a typical inversion?

Boise is particularly susceptible to temperature inversions because of the topography and weather patterns of the Treasure Valley.⁶ Inversions occur when cool air becomes trapped under a warmer air layer above, trapping pollutants close to the ground. The longer the temperature inversion lasts, the more air pollution accumulates and the worse Boise air quality becomes.

The Treasure Valley area is more prone to temperature inversions as a result of the surrounding mountains and cooler winter weather.⁷ Inversions happen in Boise throughout the year but are particularly likely to occur between November and February when the winter sun sits low in the sky and provides less warmth to the Earth’s surface, blanketing the valley in fog and other stratus cloud formations. The denser air that creates these stratus formations helps trap air pollutants low on the valley floor, which can greatly impact Boise air pollution levels.

While the air quality in Boise is generally in the “good” range, it is not unusual in the winter months for the air quality to degrade significantly, causing frequent “moderate” air quality alerts.⁸ Temperature inversions during these months can last anywhere from a single day to weeks at a time. City-wide bans on outdoor burning during these events help to minimize the amount of additional pollution being released into the air.

Emissions from vehicles and industry are the most common cause of the poor air quality in Boise during temperature inversions. While some emissions drift into the area from eastern Oregon, the bulk of the air pollutants that get trapped during inversions originate in the Treasure Valley itself. Local officials have long recommended that residents of Boise and the rest of the Treasure Valley area drive less during the winter months to try and reduce tailpipe emissions. In 2019, Valley Regional Transit began purchasing electric buses in an effort to reduce emissions from mass transportation.

Where is the smoke coming from in Boise, Idaho?

In 2020, widespread forest fires burned across neighboring Oregon and Washington as well as further south in California. Smoke particles from those fires spread across the country, reaching as far as Pennsylvania, New York, and New England.⁹ According to European satellite data, some particles were carried up to 5,000 miles from their sources, some even crossing the Atlantic and reaching Northern Europe.¹⁰

Boise witnessed its share of nearby fires as well, including:¹¹

• the 48,000-acre Indian Creek Fire outside of Juntura, Oregon
• the 97,000-acre Woodhead Fire to the north in Idaho
• the Grouse Fire east of the city, affecting 4,000 acres

Wildfires aren’t the only source of smoke affecting Boise. Wood stoves and fireplaces are popular options for home heating in Idaho given the abundance of wood for fuel. The U.S Environmental Protection Agency found that Idaho was the eighth-highest polluter from wood burning stoves per capita among all fifty states.¹²

Wood-burning is commonly used for home heating in Boise, but it is a significant source of particulate matter pollution. Just like wildfire smoke, wood-burning fireplaces and stoves can increase PM2.5 and PM10 levels in the home.

When will the air pollution in Boise clear?

Wind, rain, and changing temperature conditions help to improve Boise’s air quality by dispersing stagnant air.^13,14

Rain can be helpful in reducing some pollution by coagulating fine particles and increasing the weight of larger particulate matter pollution like PM10, causing them to fall more quickly. Substantial, sustained rainfall is more helpful in reducing smoke and air pollutants, as gentle rain may not be strong enough to have any significant impact. Microscopic particles may not be impacted by rain, as they will behave like a gas and not be captured in rain droplets. Those particles will require wind to help with their dispersal.¹⁵

In an effort to improve air quality in the long term, the city of Boise is working to reduce air pollution through legislation and public information campaigns. For example, Boise encourages residents to reduce vehicle idling while waiting in lines to help reduce vehicle emissions.¹⁶

Air pollution has been linked to increased health risks associated with the COVID-19 coronavirus. In an effort to reduce pollution and the health consequences experienced by some from COVID-19, Idaho's departments of Environmental Quality, Lands, and Health and Welfare expanded their request for the public to avoid non-essential burning in order to help reduce smoke inhalation risk in the Treasure Valley.¹⁷

The Idaho Department of Environmental Quality further seeks to improve state air quality by promoting:¹⁸

• reduced lawn mowing (especially on poor air quality days)
• fewer employee commutes
• improved fuel economy in fleet vehicles.

+ Article resources

[1] American Lung Association. (2020). Napa - State of the Air.
[2] Eggers Bri. (2020, August 24). Where is the fire smoke in the Treasure Valley, southern Idaho coming from? KTVB.
[3] BestPlaces.net. (n.d.). Ada County, Idaho.
[4] Green M, et al. (2015). Effects of snow cover and atmospheric stability on winter PM2.5 concentrations in western U.S. valleys. DOI: 10.1175/JAMC-D-14-0191.1
[5] Idaho Department of Environmental Quality. (n.d.). Agriculture and air quality in Idaho.
[6] Idaho Department of Environmental Quality. (2019). Ada County/Treasure Valley air quality plans & reports.
[7] Public News Service. (2019). Idahoans can fight bad air during winter inversions.
[8] Moeller K. (2017, December 11). Is there anything in the forecast to shake up the inversion gripping Boise? Idaho Statesman.
[9] Dwyer D. (2020, September 15). Smoke from the wildfires on the west coast is hanging over New England. Boston Globe.
[10] European Commission. (2020, September 16). CAMS monitors smoke release from devastating US wildfires.
[11] KTVB7. (2020, September 4). Here are all the wildfires burning in Idaho, eastern Oregon.
[12] Associated Press. (2015, March 9). Idaho among top polluters from wood burning stoves.
[13] Moeller K. (2018, November 16). What’s causing the haze in Boise, and when will it clear out? Idaho Statesman.
[14] Blanchard N. (2020, September 13). Smoke made Boise air ‘very unhealthy’ Sunday, and it won’t clear for several days. Idaho Statesman.
[15] Wing J. (2020, September 14). Air quality scientists say wind — not rain — will clear out smokey skies. KNKX.
[16] City of Boise. (2020). Air quality.
[17] Carlson B. (2020, March 31). Idaho asks public to refrain from nonessential burning amid COVID-19 concerns. Capital Press.