Wildfire and prescribed burning impacts on air quality in the United States

Abstract

Air quality impacts from wildfires have been dramatic in recent years, with millions of people exposed to elevated and sometimes hazardous fine particulate matter (PM _2.5 ) concentrations for extended periods. Fires emit particulate matter (PM) and gaseous compounds that can negatively impact human health and reduce visibility. While the overall trend in U.S. air quality has been improving for decades, largely due to implementation of the Clean Air Act, seasonal wildfires threaten to undo this in some regions of the United States. Our understanding of the health effects of smoke is growing with regard to respiratory and cardiovascular consequences and mortality. The costs of these health outcomes can exceed the billions already spent on wildfire suppression. In this critical review, we examine each of the processes that influence wildland fires and the effects of fires, including the natural role of wildland fire, forest management, ignitions, emissions, transport, chemistry, and human health impacts. We highlight key data gaps and examine the complexity and scope and scale of fire occurrence, estimated emissions, and resulting effects on regional air quality across the United States. The goal is to clarify which areas are well understood and which need more study. We conclude with a set of recommendations for future research.

Implications: In the recent decade the area of wildfires in the United States has increased dramatically and the resulting smoke has exposed millions of people to unhealthy air quality. In this critical review we examine the key factors and impacts from fires including natural role of wildland fire, forest management, ignitions, emissions, transport, chemistry and human health.

Figure 1.

(A) (top) Observed smoke on September 4, 2017. (Top) NASA Worldview (https://worldview.earthdata.nasa.gov/) image showing fire hotspot detections from the VIIRS and MODIS satellite instruments, along with visible satellite imagery from the VIIRS instrument between 1200–1400 local time. Bright white areas are clouds; grayer areas are smoke. (B) (Bottom) 24-hour average PM_2.5, shown as the corresponding Air Quality Index (AQI) level category colors, based on surface PM sensors collected in the EPA’s AirNow system (https://www.airnow.gov/).

Figure 2.

Progression of fires throughout the year using 2017 MODIS hotspot fire detections. Source: U.S. Forest Service.

Figure 3.

Total U.S. wildfire area burned (ha) and federal suppression costs for 1985–2018 scaled to constant (2016) U.S. dollars. Trends for both wildfire area burned and suppression indicate about a four-fold increase over a 30-year period. Source: National Interagency Fire Center (NIFC) (2019).

Figure 4.

Satellite detections of the Kincade fire in northern California on October 27, 2019 by Geostationary Orbiting Environmental Satellite (GOES) and the polar-orbiting Visible Infrared Imaging Radiometer Suite (VIIRS). Hotspot detections by each are shown at the center points of the sensor pixels (yellow squares: GOES-16; red circles: VIIRS). Black outline: final fire perimeter. The VIIRS detections provide a higher resolution detection (~375 m), but only during overpasses. The geostationary GOES-16 provides a continuous observation but at a lower resolution (~2 km). The size of squares and circles is illustrative and not related to hotspot detection strength or size. Data sources: GOES and VIIRS detections based on NOAA Hazard Mapping System–collected detections; perimeter based on GeoMac data. Image source: U.S. Forest Service.

Figure 5.

Camp wildfire, northern California, November 8, 2018. A NOAA HMS smoke plume at 12:30:00 PST. Colors are qualitative representation of smoke intensity (green: light, yellow: medium, red: heavy). (b) Visible satellite imagery from GOES-16 overlaid with surface measurements of 1-hr average PM_2.5 concentrations at 13:02:00 PST. Colors for the PM_2.5 data are associated with the AQI scale (see Figure 1). The right figure is from the NOAA Aerosol Watch program (https://www.star.nesdis.noaa.gov/smcd/spb/aq/AerosolWatch/).

Figure 6.

(Top) Box and whisker plots of all daily PM_2.5 concentrations by year for air quality monitors in California. The numbers at the top of the panel show the total number of monitor-days above the daily PM_2.5 standard (35 μg/m³). Colored horizontal lines show the six AQI cut points: Good, <12 μg/m³; Moderate, <35.4 μg/m³; Unhealthy for Sensitive Groups, <55.4 μg/m³; Unhealthy, <150.4 μg/m³; Very unhealthy, <250.4 μg/m³; Hazardous, >250 μg/m³ (see Figure 1 for color key). (Bottom) Annual area burned (left y-axis) and percentage of all monitor-days that exceeded the daily PM_2.5 standard (right y-axis). All PM_2.5 data from the EPA AQS system are included (regulatory and non-regulatory). Sources: Burned area for each state is from NIFC, and PM_2.5 data are from the EPA AQS database.

Figure 7.

As in Figure 6, but for Washington.

Figure 8.

Comparison of the annual average hectares burned for each state in the continental U.S. (2006–2016) with the number of particulate matter emission factor observations for each state in the SERA database. Source for hectares burned: U.S. Environmental Protection Agency (U.S. EPA) (2019a).