Smoke in the Sound: The underestimated source of pollution in Puget Sound

By Jordan Muhs

If you’ve experienced living in the Pacific Northwest, it’s likely you’ve noticed a foggy layer low to the ground on clear and chilly days (see image below). Upon first sight, one might pass it off as fog, or moisture, that is hanging close to the ground. Given these particular weather conditions, it’s more likely that what you’re witnessing is actually a mixture for and particle pollution. Particulate matter derived from industrial and transportation pollution—and smoke—can get trapped close to the surface of the planet during cold weather inversions, which can have cascading impacts on local climate and public health. Here we will explore the impacts of pollution from wood smoke in the Pacific Northwest and draw connections to its affects on public health, as well as both global and local climates. 

Seattle on a smokey day
Figure 1: Fog and pollution trapped in an inversion. Credit: My Northwest.

The Reality of Smoke Pollution in the Pacific Northwest

It is a common misconception that the smoke created from burning wood in our fireplaces and/or from wildfire is safe to breathe because it is a naturally derived pollution source. The reality is quite the opposite; chronic exposure to wood smoke can lead to serious health effects and even death. Problematic air pollution created by smoke can be exacerbated by local weather conditions. For example, a cold weather inversion in late November of 2015 resulted in small particulate, or PM2.5, levels 5-10 times the daily average (see Figure 2) throughout the Pacific Northwest (Puget Sound Clean Air Agency 2016). To understand how such exposure may affect human health, we’ll delve into the possible health implications of smoke exposure and explore the weather conditions that make such pollution problematic during the winter.  

Puget Sound
Figure 2: 24-hour PM2.5 Average for Puget Sound Region during an inversion  (Seattle, Tacoma, and Lynnwood). Credit:

Health Impacts of Exposure to Smoke Pollution

Although wood smoke is often thought to be safe as a “natural” fuel source, it happens to be a common source of air pollution in Washington State and other places around the world. Smoke from wildfires in the summer, and wood burning stoves and furnaces in the fall and winter are common drivers to poor air quality. Smoke is the product of combustion; a mixture of fine particles and gases (some that are carcinogenic), which are often invisible to the naked eye. Chronic smoke exposure can lead increased risk for acute heart issues and cardiac arrest associated to exposure to fine particle pollution from wildfires. Other serious health effects of smoke exposure include persistent cough, shortness of breath, severe headache, and/or trouble speaking or seeing. While it’s important to recognize the health implications from smoke exposure, it’s also important to understand the weather conditions that play a role in perpetuating poor air quality conditions from smoke pollution.

Contributing Weather Conditions for Smoke Pollution

According to King 5 meteorologist Benjamin Dery, the weather conditions that perpetuate poor air quality from smoke are a combination of high pressure, absence of precipitation, light winds, and/or temperature inversions. Clear skies and a lack of wind or precipitation are representative characteristics of a high-pressure weather system, so under these conditions little motion in the atmosphere limits dispersal of pollutants. High-pressure systems can cause a layer of cold air to stay close to the Earth’s surface, trapped under a warmer layer of air, resulting in a temperature inversion. Inversions in the winter are particularly bad because a warm layer of air traps wood smoke and other pollutants lower to the ground, rather than allowing them to dissipate into the upper atmosphere. Under these weather circumstances, it is not uncommon for burn bans to be implemented by the Department of Ecology. Pierce and Snohomish counties both took this course of action this last November, as air pollutant levels soared during a temperature inversion. Over a three-day period from November 27th to the 29th, 2015 the average 24-hour PM2.5 concentration nearly tripled from 17 ug/m3 to over 40 ug/m3. In the following three days, PM2.5 levels fell to just under 10 ug/m3, representing a four-fold increase in PM2.5 concentrations over Thanksgiving weekend (Puget Sound Clean Air Agency PM2.5 Nephelometer).

In the Northwest, the time of year is a key factor in determining the source of smoke pollution. In winter, there are more wood stoves being used, so wood smoke is the primary source of pollution. Dr. Sarah Doherty, Senior Research Scientist at the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington, explained that during winter, “almost half of the particulate pollution in the air near the surface in Washington is from wood smoke”. In contrast to pollution in winter, summer pollution is more likely to be derived from smoke blown in from forest fires. In the summer of 2015, the greater Seattle area experienced an abnormal amount of smoke pollution as easterly winds carried smoke from forest fires in the Olympic Mountains into the region. Typically, the summer winds in Seattle blow north, west, or south, but there are circumstances that allow for these predominant wind patterns to deviate from the norm (Dr. Sarah Doherty 2016).

Pollution Impacts on Climate

All particle pollution–including smoke--contains black carbon, which acts as a strong absorbent of solar radiation, inducing several different effects on climate. The direct effect occurs when particulate matter absorbs incoming and outgoing radiation, contributing to warming of the atmosphere and dimming at the surface. If the particulate matter falls on snow or ice it can create an albedo effect where black carbon deposited on the surface decreases reflectivity back into the atmosphere, which in turn results in more accelerated absorption of light and further melting. Black carbon can also alter the distribution and properties of clouds affecting their lifetime, stability, and precipitation formation. While the direct and albedo effects of black carbon are widely understood to lead to climate warming, the effects on its interaction with clouds contains more uncertainty, and the net climate influence is not well understood (EPA 2012).

The climate effects of black carbon emissions are most pronounced in the Northern Hemisphere. In the Arctic region, it’s very likely that deposition of black carbon on to snow and ice is contributing to rapid melting. The mid-latitudes of the N. Hemisphere (including the majority of the U.S.) likely experiences earlier springtime melt and reduced snow cover in some regions because of BC deposition. In equatorial regions, it’s expected that particle pollution will induce changes in precipitation patterns. Figure 3 gives reference to this phenomenon. 

black carbon emissions
Figure 3: Effects of black carbon on global climate.

Climate Change Effects on Pollution

As climate change continues to take its toll in the Northwest, it is expected that more stagnation events will occur throughout the region. Stagnation events are referred to as days when there is no precipitation in the air to dissipate particulate pollution, and/or days when there is little or no wind to mix air and disperse pollution. These types of events would worsen the effects of smoke pollution by allowing pollution to concentrate, as there would be no wind or rain to disperse pollution. By the end of the 21st century, it is projected that impacts of stagnation events over the western U.S. will be quite pronounced, particularly induced by days with weak low-level winds (Horton et al. 2014). The implications of this model demonstrate that anthropogenic climate change is likely going to alter the level of pollution management needed to meet future air quality standards, which further complicates the issue of tackling global climate change. Less precipitation during the warm summer months also correlates with more intense and frequent fires in the eastern Cascades and drier parts of the state, which could exacerbate smoke pollution levels during the summer months.

In Conclusion

It is very apparent that the common notion regarding wood smoke as a “safe” pollution source is misleading and false. We’ve explored the negative impact on the human body sustained by those exposed to smoke pollution and discussed the local weather conditions that can exacerbate, or reduce, pollution levels at the ground level. The effects of particle pollution—including smoke—on both local and global climates was also discussed, followed by the effect global climate change may play on future pollution conditions. The Puget Sound is just one small region within a planet that is experiencing the ever-present effects of climate change and pollution over its entirety. As the climate continues to shift, pollution from smoke may become more of a serious problem in the Northwest and other places around the globe, proving a need to reduce pollution everywhere.


JordanJordan Muhs grew up between the Columbia Basin and Ford, Washington. He’s now in his final quarter in the Environmental Studies program at UW Bothell. “I’m concerned about the health impacts of air quality issues because as terrestrial animals, we need clear air to sustain life.”