results in cumulative, negative heat impacts on people. In a study that looked at large cities at sea level, human mortality increased when extremely hot nights followed hot days (Xu and Tong 2017). While a different scenario than faced by Montanans, increased human mortality has the potential to become a concern here given projected increased extreme temperatures.
- Winter minimum temperature.—Winter (December, January, February) minimum temperatures indicate an increasing trend (i.e., winter progressively getting warmer) at only two of 38 stations, both in southwestern Montana, and four stations indicate decreasing trends in winter minimum temperature from 1990-2019. Over the longer period of 1970-2019 (50 yr), 11 out of 35 stations show winters warming while no stations indicate decreasing winter temperature. Thus, the time span for analysis influences the trend, as do the months included in a season. For example, when winter is expanded to include November and March in a different study (not published), 13 out of 15 weather stations show winters getting warmer.
Increasing temperature trends over the last 30 yr at individual weather stations are not limited to the larger cities in Montana where heat islands might exist. Small rural towns are also vulnerable to increasing heat and, based on limited healthcare facilities and other factors (see Section 4), may include citizens who are already vulnerable to extreme conditions. The number of summer days with maximum temperatures over 90oF (32oC) is projected to increase 11-33 days by mid century (Whitlock et al. 2017).
Degraded air quality, or air pollution, refers to myriad substances, both human-made and naturally occurring, including tobacco smoke, carbon dioxide, nitrous oxides, sulfur dioxide, particulate matter, ground-level ozone, and volatile organic compounds. Outdoor air pollution, some forms of which are also global warming gases, is estimated to contribute to 64,200 premature deaths in the US annually (Watts et al. 2019). Outdoor air pollution is expected 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 usag