Millions of Americans live in air quality "blind spots." This study maps the counties left in the dark and reveals how social and geographic disparities shape who gets protected from pollution.
An air quality monitoring station - Study: Identifying air quality monitoring deserts in the United States. Image Credit: Chemival / Shutterstock
In a recent study published in the Proceedings of the National Academy of Sciences, researchers evaluated counties in the United States (US) lacking air quality monitoring stations.
Increased exposure to pollutants or low air quality has been linked to adverse health effects, including lower cognitive ability, an earlier onset of cognitive decline, an increased incidence of cognitive conditions, a higher risk of premature death, and the development or exacerbation of cardiovascular and respiratory conditions.
Air quality measures are typically provided by environmental organizations or government agencies and are contingent upon the monitoring of pollutants across the area of interest. While air quality data are widely accessible, it is unclear whether social and place-based disparities exist regarding the presence of air quality monitoring sites and their implications for unmonitored sites.
However, the study emphasizes that gaps in monitoring coverage are not just technical gaps, but reflect deeper issues of environmental justice and social inequity, as many unmonitored areas are rural or home to historically marginalized communities.
Understanding whether differences exist in the location of monitoring sites will be essential to determining the risk of air pollution exposure and its consequences for the nation. Despite increasing awareness and calls to action, limited information exists on which areas are monitored and which remain unmonitored. The lack of monitoring infrastructure in certain areas can lead to an underestimation of local pollution and greater invisibility of the people most affected.
Additionally, the authors note that recent federal initiatives, such as President Biden’s 2021 Executive Order 14008 and new EPA funding programs, aim to address these gaps, but challenges remain.
The study and findings
In the present study, researchers analyzed US counties lacking air quality monitoring stations. The Environmental Protection Agency’s AirData monitoring sites directory was accessed to examine the presence or absence of air quality monitoring sites. This dataset comprises information on over 20,000 air quality monitoring sites that were active at some point between 1957 and 2024.
The team explored the number of active and closed monitoring sites and air quality monitoring policy changes since 1970. They examined the county-level absence of air quality monitoring sites; counties without a single monitoring station were classified as air quality monitoring deserts. To enrich their analysis, the researchers linked EPA data with demographic and socioeconomic data from the American Community Survey and urban-rural classifications from the Department of Agriculture’s Rural-Urban Continuum Codes. Next, air quality monitoring deserts were assessed in the southern United States, a region with the lowest air quality monitoring coverage.
Furthermore, the researchers examined whether socioeconomic and demographic characteristics were associated with the likelihood of a county being designated as an air quality monitoring desert. Logistic regression models were used, with the outcome of classifying the county as a monitoring desert. An additional model was fit considering the population size.
Findings
Overall, 20,815 air quality monitoring sites have been active in the US at some point since 1957. Of these, only 4,821 remained open. The number of monitoring sites increased exponentially during the first 25 years, followed by a linear trend up to 2024. Further, there were state-level disparities in the percentage of air quality monitoring deserts. For example, states in the Northeast and West had lower percentages of monitoring deserts.
By contrast, states in the mountain regions and the Midwest had higher percentages of air quality monitoring deserts. Monitoring deserts may reflect non-adjacency to metropolitan areas, low population density, or small population size. Less than 20% of counties in Washington, Arizona, Oregon, California, and Massachusetts were monitoring deserts.
All counties in Delaware and Connecticut had a monitoring site. Conversely, at least 80% of counties in Arkansas, Kansas, Nebraska, and South Dakota were air quality monitoring deserts. This corrects the previous omission and now matches the journal’s specific state list. Overall, 59% of US counties were monitoring deserts, and 66% of the southern US counties were monitoring deserts. Non-metropolitan counties were more likely to be a monitoring desert.
Furthermore, counties with a higher proportion of residents over 25 years old without a high school diploma and those with a higher proportion of residents working in forestry, agriculture, mining, or fishing had higher odds of being designated as air quality monitoring deserts. Increased poverty rates were associated with a greater likelihood of a county being classified as a monitoring desert. Conversely, higher levels of working-age adults were associated with lower odds of a county being a monitoring desert.
Further, the odds of a county being a monitoring desert increased as the Black or African American population increased. In contrast, a higher Hispanic population was associated with lower odds of a county being a monitoring desert. In the model considering population size, the associations of African American or Black populations and non-metropolitan areas were sustained, while other associations weakened.
Notably, the concentration of Hispanic populations was now associated with greater odds of a county being a monitoring desert. Moreover, the association between the percentage of the population working in agriculture, fishing, mining, and forestry was reversed. Overall, this model indicated that the odds of a county being a monitoring desert were low with a larger population size.
Regionally, monitoring deserts were found to be especially concentrated in the Midwest and South, with a notable cluster in the "Southern Black Belt"—a region with a historically high proportion of African American residents and persistent social disadvantage.
The study also notes that while air quality estimates are readily available on smartphones and weather platforms, these estimates often rely on distant monitoring stations and may not reflect actual local air quality in monitoring deserts. This uncertainty could lead to an underestimation of pollution risks for millions of people.
Additionally, the researchers highlighted that many air quality monitoring sites are aging, with the median age of open sites over 26 years, raising further concerns about the reliability of the existing infrastructure.
The authors point out that these monitoring gaps may also limit the effectiveness of public health and environmental policies, as data-driven decisions are only as reliable as the underlying measurements.
Conclusions
The study found that over half of the counties in the US lacked air quality monitoring sites. Monitoring sites were present in areas with larger populations, consistent with primary standards, while monitoring deserts were concentrated in areas with indirect exposure, consistent with secondary standards.
Crucially, the paper argues that these systematic monitoring gaps may leave entire communities "invisible" to research and policy, limiting our understanding of environmental exposures and health disparities in these regions.
The authors call for an equitable expansion and modernization of air quality monitoring infrastructure throughout the US, especially in under-monitored rural and disadvantaged areas, to ensure more accurate data and better inform health and environmental policy.
They also note that without comprehensive monitoring, any associations between air quality and health outcomes may be less reliable for unmonitored regions, further reinforcing inequities.
The study concludes by emphasizing the need for collaboration among federal, state, and local governments to achieve universal, equitable air quality monitoring coverage and to address both the technical and social dimensions of environmental health.