In recent years, the significance of climate-driven wildfires in contributing to air pollution, particularly on particulate matter (PM) levels, has increased at national and global levels. (1) This escalation can be attributed, in part, to climate change, which has intensified the duration, frequency, and magnitude of such events. (2) In addition, a history of fire suppression practices in North America is likely causing wildfires to become larger and more severe. (3)

In the US, over 60,000 wildfires burn an average of 2.8 million hectares of land every year. (4) Due to climate change-driven drought, extreme heat, and reduced snowpack, recent wildfire season lengths are expanding dramatically. (5) In 2020, over 28 million people, approximately 70% of the population in California (CA) experienced more than 100 days of unhealthy air quality as specified by the US Environmental Protection Agency Air Quality Index values above 100 from elevated ambient particulate matter with an aerodynamic diameter of less than 2.5 μm (PM_2.5) and ozone. (6) During the 2020 wildfires in CA, daily PM_2.5 levels often reached 350–500 ug/m3, significantly higher than the 24 h average limit of 35 ug/m3, which is specified by the National Ambient Air Quality Standards (NAAQS). (7)

The impacts of wildfires, however, are not limited to the US. Worldwide, 2.2 billion people were exposed to ≥1 day of substantial wildfire pollution per year in 2010–2019, with the average person having almost 10 days of exposure per year. (8) The same analysis determined that, globally from 2000 to 2019, the population weighted average WFPM_2.5exposure was 2.5 ug/m3. It is significant to mention that, since WFPM is dominated by nanoscale PM, which has less mass compared to micron-scale particulates, a 2.5 ug/m3 add on is translated to millions of particles per volume of air with different and unique chemical composition. Therefore, relying on PM mass concentrations may underestimate the associations between WFPM and health outcomes given the nanoscale nature of such particles. Furthermore, wildfire air pollution poses a significant health threat, particularly to socially vulnerable Americans living in environmental justice communities already burdened with compromised air quality. Davies et al., 2018, identified 29 million Americans at risk for extreme wildfires, of which 12 million belonging to Black, Hispanic, or Native American communities face approximately 50% greater vulnerability to wildfires compared to other census tracts. (9)...

Figure 1. HYSPLIT backward trajectories calculated at 500 and 1000 m above ground level for air mass arriving at the Rutgers Piscataway Campus (Lat: 40.5240, Long: −74.4684; depicted on the figure with a star) on June 7th, 2023 1800 EDT. Red dots represent active forest fire sites on June 7th according to the Canadian Wildland Fire Information system (https://cwfis.cfs.nrcan.gc.ca/home; consulted 12/01/2023).

Figure 2. One hour average ambient concentrations for (a) PM_2.5; and (b) CO, O3, and NOx. PM_2.5and O3 data were obtained from the US EPA Photochemical Assessment Monitoring Station near Rutgers University and CO and NOx from the EPA Newark site. Shaded gray areas indicate the sampling phases with real-time and integrated measurements at Rutgers Piscataway Campus.

Figure 3. Time-integrated ambient particle mass concentration as a function of PM aerodynamic size fraction from four sampling phases: start phase (Phase A): June 6 19:50–June 7 14:00; peak phase (Phase B): June 7 15:00–June 7 19:06; postpeak phase (Phase C): June 7 20:12–June 8 12:02; end phase (Phase D): June 8 15:22–June 9 13:28. Error bars show 95% confidence interval.

Figure 4. Percent mass concentration of size-fractionated PM-bound PAHs collected in the peak phase (Phase B). Detected methylated phenanthrenes are shown aggregated in (sum symbol capital sigma)Σ-C1-Phe and (sum symbol capital sigma)Σ-DMP.

Figure 5. Size-fractionated mass of deposited wildfire PM in the head, tracheobronchial (TB), pulmonary (P), and total region of the human respiratory tract derived by MPPD for a 72 h exposure to concentrations measured during the peak phase (Phase B).

As wildfires increasingly contribute to air pollution and air quality, affecting the health and well-being of millions, it is imperative that these events no longer be considered exemptions under EPA regulatory standards. Historically, such natural events were seen as rare and unpredictable, leading to their exemption from daily and annual air quality evaluations. However, with climate change enhancing the frequency and intensity of wildfires, these are no longer sporadic events but recurrent ones, necessitating a shift in regulatory paradigms. In summary, the magnitude, size distribution, and chemical composition of WFPM in a major densely populated metropolitan area warrant further studies to better understand the impact on human health.