ed to worsen with climate change (Orru et al. 2014).
Across the US, human health concerns related to air quality are dominated by particulate matter and ground-level ozone (American Lung Association undated; Ward and Smith 2001; Tao et al. 2005).
Particulate matter (PM)—especially PM2.5 and PM10, describing particulates under 2.5 μm and 10 μm diameter, respectively—has been associated with multiple health effects. Those effects include cardiovascular, respiratory, and immunological problems, the latter manifesting as increased risk for pneumonia (Rappold et al. 2011; Reid et al. 2016b; Tinling et al. 2016), and neurological issues, in the form of greater risk for dementia (reviewed by Peters et al. 2019). Particulate matter has been found to penetrate the blood-brain barrier—i.e., move into the human brain—and cross the placenta in pregnant women (Bové et al. 2019). It has also been associated with possible increased risk of preterm birth (DeFranco et al. 2016; Trasande et al. 2016), low birth weight (Fleischer et al. 2014), and increased medication use in children with asthma (Gielen et al. 1997).
Ground-level ozone forms when sunlight and heat act on nitrous oxides and volatile organic compounds (Rani et al. 2011). It has been associated with asthma exacerbations, increased hospitalizations, and premature mortality (Bell et al. 2004; Zanobetti and Schwartz 2008; Di et al. 2017), as well as with preterm birth (Olsson et al. 2013). Ground-level ozone is expected to increase with warming temperatures (Melillo et al. 2014).
In Montana, ozone levels are generally rated low (<50) on the USEPA air quality index (AQI), but particulate matter levels are seasonally elevated (>100 AQI) in some areas (AirNow undated). Ten counties in Montana, for example, received grades of “F” for particulate matter in the American Lung Association State of the Air 2020 report card (American Lung Association 2020). Some local areas have elevated particulate-matter levels during winter months due to inversion layers and high woodstove usage. In eastern Montana, particulate matter is expected to increase as summer drought increases dust emissions in agricultural areas where land gets tilled for crops (Gage et al. 2016) and tillage is employed as a common summer fallow practice (Cook et al. 2014; Dawson et al. 2014). Wildfires also contribute significantly to high levels of particulate matter, as we discuss in the next subsection.
Separately, but related to air quality concerns, pollen counts in the United States are increasing with projections for doubling by 2040 (Zhang et al. 2015) due to warming temperatures with longer growing seasons (Ziska et al. 2019), and higher atmospheric carbon dioxide levels (Ziska and Caulfield 2000). In addition to worsening seasonal allergies, this increase in pollen poses risks of triggering exacerbations of asthma, which affects 9.1% and 6.8% of Montana adults and children, respectively (MTDPHHS 2013).
Wildfires and wildfire smoke
Between 1997 and 2006, 339,000 human deaths per year globally were attributed to smoke from landscape fires, which includes forest fires, peat fires, grass fires, prescribed burns, agricultural burns, and tropical deforestation burning (Johnston et al. 2012). Deaths and injuries occur to people caught in fast-moving fires, as well as to firefighters, emergency response personnel, and others assisting with fire management. Wildfires can result in large-scale, temporary evacuations, or permanent displacement following the destruction of homes or even entire towns (Insurance Information Institute undated). The 2019/2020 fires in Australia, for example, killed 33 people across the country; in New South Wales alone over 2000 homes were destroyed (BBC News 2020). Closer to home, the November 2018 Paradise Fire in California killed 85 people, displaced hundreds, and destroyed over 18,000 buildings (Vox 2019). Large fires with adverse impacts on communities throughout the western US, including Montana, are increasing as measured in numbers of acres burned (Figure 3-6) because a) climate warming p