Comparing the cumulative NOx emissions of the National Map dataset to National Emissions Inventory reporting on industrial emissions in 2020, we found that boilers produced 6 percent of all industrial NOx pollution. That is more than the reported NOx emissions from other major source categories, including coal-fired power plants, cement production facilities, and petroleum refineries. NOx is a dangerous pollutant and potent greenhouse gas that forms ozone in the atmosphere and contributes to cardiovascular and respiratory disease. Given the diverse number of emissions sources operating at U.S. industrial facilities, it is striking that boilers contribute a full 6 percent share of that pollution.

California exhibits the largest number of boilers in the U.S., with 1,386 boilers. Minnesota has 951 boilers, Illinois has 1,237 boilers, Massachusetts has 921 boilers, and Texas 802 boilers. The data indicate a widespread reliance on process heat across diverse industrial landscapes.

Our analysis finds that a quarter of all industrial boilers are located in nonattainment areas where ozone pollution consistently exceeds the National Ambient Air Quality Standards (NAAQS) 8-hour ozone limit, based on the 2015 standard set by EPA. Combustion boilers emit major precursors that form ozone in the atmosphere, including NOx and volatile organic compounds (VOCs). The boilers in these nonattainment areas are thereby currently worsening local ozone pollution, but electrifying those units would eliminate NOx and VOC emissions at the source; in nonattainment areas with especially high concentrations of industrial boilers, widespread electrification could meaningfully help bring ambient ozone down to within legal limits.

Among the set of highest-emitting NOx boilers (top 1 percent nationally), we found that two companies—Westrock LLC (21 reported units) and International Paper (16 reported units)—operate 30 percent of the highest-emitting boilers in the U.S. Looking more broadly at the entire boiler population, 3,523 companies operate boilers across the country. The top 10 companies with the highest number of boilers collectively operate 839 boilers and are primarily within the food manufacturing (44 percent) and paper manufacturing (27 percent) industries.

Annual NOx emissions were reported for 93 percent of the sources listed in the dataset. In evaluating those emissions, we found that the histogram was heavily right-skewed, meaning that high-emissions units are pulling the average upwards, while the majority of data points are clustered around lower values (median of 1.0 ton and average of 15.3 tons). The standard deviation was also high (68.0 tons), indicating a wide spread in the data. The top 1 percent highest-polluting sources reported values above 341.2 tons, meaning that 152 boilers reported NOx emissions higher than this value.

When analyzing the highest emitting NOx boilers in the country (top 1 percent), we found that 66 percent of the reported units were located in federally recognized disadvantaged communities (DACs), with the highest concentration in Louisiana. In turn, a majority of these reported units (68 percent) were associated with paper manufacturing, with unit capacities typically over 100 MMBtu/hr.

Alarmingly, over 40 percent of industrial boilers nationwide are situated within federally recognized DACs. Our analysis reveals another consistent and concerning trend: Industrial boilers (reported units) located within those communities exhibit significantly higher average emissions across all major pollutant types compared to those in non-DACs. Nationally, the average annual boiler NOx emissions in DACs are nearly double the emissions elsewhere—20.8 tons vs. 10.8 tons. We see similarly elevated disparities for other harmful pollutants, including fine particulate matter, volatile organic compounds, mercury, lead, cadmium, formaldehyde, and hydrochloric acid. This finding underscores the disproportionate pollution burden these communities face and highlights the urgent need for targeted mitigation strategies in these areas to remedy the environmental injustices that industrial boiler emissions exacerbate.

In total, we found 16 states where over half of their reported units were operating in disadvantaged communities, as identified by the Biden EPA based on several environmental and social indicators. These reported units are primarily associated with the chemical and food manufacturing sectors, and have unit capacities between 10-100 MMBtu/hr or over 100 MMBtu/hr.  The states with the highest concentration of reported units operating in DACs are in the Southern U.S., which includes Oklahoma (81 percent of reported units), Arkansas (75 percent of reported units), and South Carolina (69 percent of reported units).