Also See: Current Air Quality & Smoke Monitoring page and general Air Quality page.

Webcam on June 24, 2008, when regional smoke filled Yosemite after statewide lightning strikes. Click on the image to see the current view.

Fire and smoke are as much a part of the Yosemite ecosystem as water and ice. Every year, thousands of lightning strikes occur within park boundaries, igniting vegetation made tinder-dry by Yosemite’s long, hot summers. Inevitably, some of these strikes cause fires, which in turn emit smoke.

The gases, particles (also called particulate matter) and ash that comprise smoke contain many ingredients, depending on what is being burned. Many of the gases are toxic--acrolein gas in smoke is what causes your eyes to water while carbon monoxide gas is virtually undetectable by humans and can cause suffocation in enclosed/unventilated areas. Nutrients like nitrates, phosphates and sulfates are also volatilized from burning soils and vegetation into the air while metals and other elements in burned material and soils remain as ash. It's possible to tell when something has burned really hot because none of the black organic material remains but rather only a white ash does.

The material that isn’t completely “combusted” or turned to gas and ash ends up as particles. Smoke particles come in all sizes, from large pieces of ash that quickly sink to the ground near a fire to microscopic particles barely larger than molecules that float for weeks, traveling hundreds, even thousands of miles before hitting the ground. These small or “fine” smoke particles pose the greatest health concern, especially those 2.5 micrometers in diameter or less. They also are most responsible for the haze and “smog” associated with large fires.  

In order to measure fine particles at Yosemite and quantify potential health impacts, a stationary monitor is installed at the Yosemite Valley Visitor Center, and two mobile monitors are deployed to smoke-sensitive areas when fires occur. (It's interesting to note that short-term health impacts from smoke in a rural area can be more severe than health impacts from the air in the most polluted of urban environments.)

Yosemite's fire management program aims to minimize smoke emissions.

Yosemite’s fire managers strive to protect the health and clarity of the air that park visitors breathe. Fire and smoke cannot be eliminated in Yosemite, only managed to minimize smoke emissions while preserving the fragile ecology that keeps vegetation sparse enough to prevent much larger fires. More than 30 years of fire ecology have taught fire managers that suppressing all fires only delays the inevitable, making the results more intense than they otherwise would have been. Just as dam operators must let some water spill through their dams in order to prevent floods, fire managers must let some fire and smoke occur to keep the larger conflagrations at bay, especially in the mid-elevation mixed confer forests where a “flood” of biomass accumulates every year. Overall, Yosemite’s fire management program minimizes the amount of smoke the region’s inevitable fires will emit while maximizing the resource benefits that result from those fires. Smoke impacts can be minimized by allowing fires to do most of their burning during the best “burn window,” when the weather is most like to dilute and blow smoke out of harm’s way. Yosemite collaborates with fire managers, air quality experts and meteorologists from around the state every day during the fire season (April through November).

Whether from a prescribed or wildland fire, smoke can be a common occurrence in the Sierra.

Just as rain is characteristic to the Oregon coast and humidity is to Florida, summer smoke from local and regional fires is part of the Yosemite experience. In late June of 2008, for instance, a statewide lightning event ignited wildfires throughout California; then, in late July of 2008, the Telegraph fire began in the Midpines area, burning 36,000 acres adjacent to the park. Regional smoke affected the park’s air quality for more than a week, causing both particulate matter and ozone to reaching unhealthy levels at the same time. Typically--during smaller fires that do not fill the region with smoke--ozone and fine particle levels are only weakly related or correlated. A top priority of our smoke management program is to minimize the duration and severity of smoke impacts so that such extended and severe smoke impacts do not occur.

What does the pervasive nature of summer smoke in the Sierra Nevada mean, then, for visitors? If people are healthy, they are usually not at a major risk from short-term exposures to smoke. Temporary symptoms from these short-term exposures can include burning eyes, runny nose, coughing, phlegm, chest tightness, shortness of breath and illnesses like bronchitis. Chances of being affected by particles increase with strenuous activity because exercise causes people to breathe faster and more deeply and to take more particles into their lungs. Young, active children are sensitive for the same reason. Also, those with health conditions or other smoke sensitivities should, as a precaution, check the current conditions as they relate to the Air Quality Index (AQI) based on data from local air quality monitors. This information is provided by the Current Air Quality & Smoke Monitoring webpage and at a fire information panel next to the Yosemite Valley Visitor Center. (For even more background information, see www.epa.gov/airnow for more information on these AQI guidelines.)

For those sensitive to smoke, here are some rules of thumb for minimizing smoke exposure:

• During the day: If you must excercise strenuously on smoky days, wait until the afternoon, when fine particles are usually at their lowest levels. 
• During the night: Avoid extended exposure to outdoor air in canyon bottoms like Yosemite Valley and other low-lying areas near rivers and lake basins--these areas commonly experience the highest fine-particle levels during calm, smoky periods.

Where to Learn More:

• Current Fire Activity in Yosemite (View fire maps, including of the Big Meadow Fire)
• Current Air Quality & Smoke Monitoring (View near real-time data and webcams)
• Learn more about Yosemite's air quality monitoring program. 
• View more about Yosemite's fire management program.
• Study the science of fire ecology, which addresses the origins of wildland fires and their relationship to the environment.
• Watch a short video (4 minutes) to hear a fire archeologist describe his dual job.