First Street: “Since the middle of the last century, the United States has witnessed significant changes in air quality, driven by industrialization, technological advancements, regulatory measures, and public awareness. The most important of these interventions was the Clean Air Act of 1963, which served as the first federal legislation addressing air quality concerns. While air quality has significantly improved from the first half of the 20th century, there are growing concerns that climate change impacts are undoing some of the progress the US has made through regulations and policy. This trend has been referred to as a “climate penalty”, whereby changing environmental conditions are inducing negative effects which erase some of the progress made through federal and local regulatory policies. Changes in environmental conditions, such as extreme heat, drought, and wildfires, are contributing to the increase in the levels of specific pollutants (O3 and PM2.5, specifically), often to concentrations above safe levels in much of the country. Even with significant cuts in anthropogenic emissions over the past decades, some research has projected that PM2.5 pollution will increase by as much as 50% over the next 3 decades in the Western United States. The “climate penalty” is not felt equally across the country, with some of the most dramatic effects seen in recent upticks in PM2.5 from wildfire smoke in the West, and growing pockets of extreme Ozone exposure.  Historic station data from the EPA demonstrates that between 2000 and 2021, in the West, the number of Orange Days has increased by 477%, Red Days by 459%, Purple Days by 318%, and Maroon Days by 381%. In California alone, the average number of Green Days seen across the state has decreased from 136 to 93 (-32%), and the average number of Yellow Days has decreased from 200 to 146 (-27%). Subsequently, the average number of Orange Days has increased from 15 to 55 (+267%), Red Days increased from 10 to 16 (+60%), Purple Days increased from 1 to 17 (+1,600%), and Maroon Days from 3 to 38 (+1,167%). Using the newly created First Street – Air Quality Model (FS-AQM), this report finds continued exposure as well as climate-related increases in poor air quality due to the growing incidence of wildfires, extreme heat events, and their negative interactions with other environmental and anthropogenic conditions. The results show that there are approximately 14.3 million properties (~10% of all properties) in the US that are estimated to have a week or more (7+ days) of unhealthy air quality days solely from PM2.5 in the current climate conditions. Of those, almost 5.7 million properties (~4%) may experience two or more weeks (14+ days) annually of smoke driven unhealthy air quality days. Some of the most at risk areas include large population centers such as Seattle, San Francisco, and Sacramento. It is important to understand the nature of this hazard, its link to climate, and the growing impact it will have on areas across the US into the future, including the growing negative effects on human health, labor force productivity, and even migration patterns.”