Smoggy Days Alerts Methodology

Air pollution from oil and gas facilities can have a significant impact on public health – even in areas far from oil and gas production. Air pollution from oil and gas facilities travels far downwind, contributing to elevated ozone levels throughout the county. The purpose of Oil and Gas Smoggy Day Alerts is to educate and mobilize community members living in areas with significant ozone caused by oil and gas air pollution when ozone levels in their area are high.

Air pollution from the oil and gas industry causes ozone smog, which harms public health. The oil and gas industry dumps more than 9 million tons of methane and other pollutants, like volatile organic compounds (VOCs) into our air each year. These pollutants contribute to the formation of ozone smog pollution, blanketing the U.S. in the warmer months. VOCs and methane vented and leaked from the oil and gas supply chain, and nitrogen oxides (NOx) formed by gas flaring and engines at natural gas facilities react together in the presence of sunlight to form ozone smog. Methane pollution from oil and gas facilities also worsens climate change, resulting in hotter weather and stagnant air. These warmer conditions in turn worsen ozone smog levels. When inhaled, ozone smog can impair lung function, trigger asthma attacks, and aggravate conditions of people with bronchitis and emphysema, in some cases leading to premature death.

We identified 752 counties in 24 states that have significant ozone attributable to oil and gas air pollution. This is primarily based on data from Fann et al,1 a study by EPA scientists that modeled how summer season average daily 8 hour maximum ozone concentrations were impacted by oil and gas air pollution. We considered a county significantly impacted if oil and gas air pollution raised the 8-hour average ozone level on the highest ozone day of the summer season by at least 3 ppb. 737 counties met this criteria. In addition, we added 14 oil and gas counties with significant winter-time ozone problems, because the chemistry of winter-time ozone was not modeled in Fann et al.2 We also included Kern county in California, due to feedback from partner groups that suggested that emissions in California are underestimated in the national inventory.

The map below shows the geographical scope of counties with significant ozone attributable to oil and gas air pollution. However, not all of these counties have ozone monitors. The smoggy day alert project relies on actual observed ozone levels, so alerts can only be sent for counties with ozone monitors. Of the 752 counties identified, only 148 counties in 22 states have ozone monitors. (See list of counties here)

When users sign up to receive alerts, they must first select the geographic area of interest. The geographical area will be either a county or a metropolitan area (core based statistical area) as defined by the U.S. Census Bureau.3 We use the metropolitan area designation when one or more counties in the area meet the inclusion threshold described above. If a user selects a metropolitan area in which not all counties meet the threshold, the tool only looks at ozone monitors in counties above the threshold. For example, the Pittsburgh metropolitan area includes 7 counties, but only 5 have significant ozone from oil and gas, so the tool will only send an alert to users who select the Pittsburgh area if ozone readings in one or more of those 5 counties are elevated.4

The user also chooses the ozone level threshold for their alerts. The threshold levels are set to Air Quality Index (AQI) levels – ozone concentrations of 55-70 ppb (yellow AQI), 71-85 ppb (orange AQI), or  over 85 ppb (red AQI). See scale below:

Ozone Level Ozone Parts Per Billion (ppb) Who Needs to Be Concerned
Green (good) Less than 55 It’s a great day to be active outside.
Yellow (moderate) 55-70 Some people who may be unusually sensitive to ozone.
Orange (unhealthy for sensitive groups) 71-85 Sensitive groups include people with lung diseases such as asthma, older adults, children and teenagers, and people who are active outdoors.
Red (unhealthy) Greater than 85 Everyone

Each day, the alert tool checks data from each ozone monitor in an identified county, and if the 8-hour ozone level exceeds their selected alert level, it sends an alert email.

  • 1Fann, Neal. “Assessing Human Health PM2.5 and Ozone Impacts from U.S. Oil and Natural Gas Sector Emissions in 2025.” Environ. Sci. Technol. 2018, 52, 15, 8095-8103.
  • 2The atmospheric model used in the study only considered the traditional processes of summer-time ozone creation. These fourteen counties are: Teller, Clear Creek, and Park Colorado; Williams, Dunn, and McKenzie North Dakota; and Natrona, Uinta, Campbell, Weston, Fremont, Sheridan, Big Horn, and Sublette Wyoming.
  • 3
  • 4One exception to this is Albuquerque, NM. Sandoval county meets the criteria for inclusion and is part of this metropolitan area. However, the city of Albuquerque is in Bernalillo county, which does not meet the inclusion criteria, so we use the county designation instead of the metropolitan area for this county to describe this area.