COBRA Questions and Answers

1. What is the difference between the desktop version of COBRA and the Web Edition?
2. How does COBRA estimate public health impacts of changes in emissions?
3. What is the geographic scope and spatial resolution of COBRA?
4. Can COBRA be used for tribal lands?
5. Can COBRA be used outside the United States?
6. Can COBRA model health impacts of changes in indoor air quality?
7. What is the difference between COBRA and BenMAP?
8. What years can be evaluated with COBRA? Can COBRA be used to estimate impacts of a change in emissions over multiple years?
9. Where do the baseline emissions in COBRA come from?
10. Can I supply my own baseline emissions or population, incidence, or valuation data?
11. How do I estimate changes in emissions that I can use in COBRA?
12. How do I enter a scenario to assess the impact of emissions changes across the entire United States?
13. Does COBRA analyze the impacts of changes in ozone (O₃) formation?
14. What is a discount rate? Why do I need to choose one?
15. Why does COBRA display health impacts in counties for which I did not provide emissions changes?
16. Why are there “Low” and “High” values for some of the estimated health impacts (e.g., mortality, heart attacks)? Is it possible to pick just one best number from the ranges reported in COBRA?
17. What does it mean when COBRA estimates a fractional change in the incidence of health impacts?
18. How does population density affect health impacts estimates?
19. What is the currency year for COBRA dollar values?
20. How are economic values of health impacts determined? What is the Value of a Statistical Life (VSL)?
21. What is the scientific basis for COBRA’s estimates of public health impacts and their economic value?
22. How do I interpret the map of the results?
23. How can I use COBRA with EPA's AVERT tool?

1. What is the difference between the desktop version of COBRA and the Web Edition?

Although the desktop version and Web Edition of COBRA both use the same methodology to calculate outdoor air quality and health impacts from changes in air pollution emissions, the desktop version offers additional advanced features that are not included in the more streamlined Web Edition. In particular, the desktop version is preloaded with input data on emissions, population, and baseline health incidence for 2016, 2023, and 2028; the Web Edition includes data only for 2023. Similarly, the desktop version allows users to import custom input datasets, while the Web Edition does not. The Web Edition, however, does not require the user to download or install additional software, and it runs more quickly than the desktop version. Users might choose to use the desktop version if they would like to use advanced features, such as custom input data and/or use the preloaded data for 2016 or 2028. Otherwise, users may choose to use the Web Edition for data analysis relevant to 2023.

2. How does COBRA estimate public health impacts of changes in emissions?

To estimate public health impacts, COBRA takes user-supplied inputs of sector-specific changes in emissions of primary fine particulate matter (PM_2.5) and precursors of secondary PM_2.5 and ozone (O₃), including nitrogen oxides (NO_x), sulfur dioxide (SO₂), and volatile organic compounds (VOCs); and conducts multiple modeling steps to translate changes in emissions to changes in health effects. First, COBRA uses a simplified air quality model, the Source Receptor (S-R) Matrix, to estimate changes in total annual ambient concentrations of O₃ and PM_2.5, including the formation of secondary PM_2.5 from precursor pollutants. COBRA then uses a series of health impact functions, taken from the peer-reviewed epidemiological literature, to estimate how changes in outdoor air quality result in changes in the incidence of a variety of health outcomes (e.g., premature mortality, heart attacks, asthma exacerbation, lost work days). Finally, COBRA multiplies the change in incidence for each health outcome by a monetary value specific to that outcome (e.g., the average cost of going to the emergency room for asthma symptoms or the cost of a lost work day) to determine the monetized health impacts. For information on how the monetary values for each health outcome are determined, see Appendix F of the COBRA User’s Manual.

3. What is the geographic scope and spatial resolution of COBRA?

COBRA estimates county-level public health impacts for the contiguous United States. COBRA does not estimate public health impacts in Alaska, Hawaii, or the U.S. territories. The S-R Matrix used to model changes in air quality in COBRA consists of a set of county-level relationships with estimates of how changes in air pollution emissions in each source county affect air quality in all other receptor counties. The modeling used to develop the S-R Matrix is only available for the contiguous United States. For more information on the S-R Matrix, see Appendix A of the COBRA User’s Manual.

4. Can COBRA be used for tribal lands?

COBRA can be used to estimate public health impacts in tribal land in the contiguous United States, but it does not separate the results for tribal lands from those from surrounding areas. COBRA provides results for each U.S. county, but tribal lands do not always align with county boundaries. The COBRA results will include the health benefits for the entire county, including the portion that is not included in a tribal nation. EPA recommends that tribal nations run COBRA for all counties that intersect the tribal lands they are analyzing.

5. Can COBRA be used outside the United States?

COBRA cannot be used for analysis outside of the United States. It has been developed around a database containing U.S.-based health and valuation impact functions, baseline emissions inventories, population, and baseline health data; and cannot be altered to use data from other locations. The U.S. Environmental Protection Agency’s (EPA’s) Environmental Benefits Mapping and Analysis Program (BenMAP), on the other hand, can be customized for application in other countries. See Question 7 for more information on BenMAP.

6. Can COBRA model health impacts of changes in indoor air quality?

COBRA cannot be used to model health impacts of changes in indoor air quality. COBRA only analyzes the health impacts associated with changes in outdoor air quality.

7. What is the difference between COBRA and BenMAP?

The key difference between BenMAP and COBRA is that COBRA has a built-in, reduced-form air quality model, while BenMAP does not. To convert changes in air pollution to changes in air quality, a tool will need an air quality model. Because BenMAP does not have this built-in model, it requires inputs of the changes in air quality rather than emissions. BenMAP users can utilize standalone air quality models, such as the Community Multi-scale Air Quality Model (CMAQ), to determine air quality changes. Additionally, BenMAP can be used to analyze health impacts at a finer geographic resolution than COBRA, depending on the scale of the air quality data that users provide. BenMAP also includes preloaded population and baseline health incidence data forecasts for all years out to 2050. COBRA only analyzes health impacts at the county-level and baseline health incidence data for 2016, 2023, and 2028. However, the desktop edition of COBRA also has advanced features that allow the user to import inputs for other years.

Apart from these differences, COBRA and BenMAP both use the same approach to estimating health impacts, and both tools use the same default concentration-response functions and economic valuation functions for their calculations.

8. What years can be evaluated with COBRA? Can COBRA be used to estimate impacts of a change in emissions over multiple years?

Each COBRA run represents benefits from emissions changes in one year. The desktop version of COBRA currently includes default datasets that can be used to estimate public health impacts of emissions changes in years 2016, 2023, and 2028. Users also have the option to input custom datasets, including the emissions baseline, population, and health incidence, for any year (see Question 10). The Web Edition of the tool includes default datasets to analyze health impacts for the year 2023. To analyze multiple years of emissions changes, you must conduct a COBRA run for each year to be analyzed. These results can be added together.

9. Where do the baseline emissions in COBRA come from?

The emissions baselines in the desktop version of COBRA reflect emissions in 2016, 2023, and 2028. The COBRA Web Edition includes baseline emissions for 2023. The baseline emissions are taken from EPA’s Emissions Modeling Platform 2016v1, which was developed based on data from the National Emissions Inventory. These baselines account for federal and state regulations as of May 2018; and electric generating unit (EGU) emissions comply with the Cross-State Air Pollution Rule Update, the Mercury and Air Toxics Rule, and the Standards of Performance for Greenhouse Gas Emissions from New, Modified, and Reconstructed Stationary Sources. Baselines include emissions from 14 broad emissions categories (e.g., Fuel Combustion: Electric Utilities, Highway Vehicles), which are referred to as “Tier 1” by the model. Tiers 2 and 3 represent subcategories such as subsectors or specific fuel types. EPA updates the baseline emissions in COBRA approximately every two years.

10. Can I supply my own baseline emissions or population, incidence, or valuation data?

Yes. Users can supply their own baseline emissions, population, and incidence data; and health impact and valuation functions to customize the model in the Advanced Options tab in the desktop version. Chapter 3 of the COBRA User’s Manual provides more details on how to format these files for use in COBRA. EPA has developed inputs for select future years (including 2030, 2035, 2040, 2045, and 2050), which can be downloaded here. The Web Edition does not allow users to supply their own data files or health impact or valuation functions.

11. How do I estimate changes in emissions that I can use in COBRA?

Users have two options for entering annual emissions changes into COBRA: either as a percentage change from the baseline (e.g., a 15% annual increase in NO_x emissions) or as a whole number (e.g., a 1,000 ton annual decrease in PM_2.5 emissions). EPA has developed a factsheet called Source of Data for COBRA Inputs that provides information on several tools and databases that can be used to estimate emissions changes for inputs into COBRA.

12. How do I enter a scenario to assess the impact of emissions changes across the entire United States?

The desktop version of COBRA can run very slowly when users enter emissions changes across the entire United States. The Web Edition runs much faster for these types of analyses. Users are encouraged to use the Web Edition for national-level COBRA analyses. If users need to use the desktop version for a national-level scenario, there are options to expedite the analysis. Users can create a custom emissions scenario file and enter it into COBRA using the Advanced Options tab under the Select Analysis Year tab. Custom emissions scenarios are .csv files, which can be edited in Microsoft Excel. To create a custom emissions scenario file for a national-level emissions reduction scenario, users should distribute the emissions reductions to all sources in a given sector or emissions tier and proportionally based on the baseline emissions. Chapter 3 of the COBRA User’s Manual provides more details on how to format a custom emissions scenario file and how to load it into COBRA.

13. Does COBRA analyze the impacts of changes in ozone (O₃) formation?

Yes. With the release of version 5.0, COBRA now estimates the impacts of changes in O₃ formation as a result of changes in pollutants assessed by the model, specifically NO_x and VOCs. EPA has published a helpful primer on the basics of ground-level ozone formation.

14. What is a discount rate? Why do I need to choose one?

A discount rate is an economic concept that accounts for the time value of money. In other words, it accounts for the perception that money in the present is worth more than money in the future. COBRA uses a discount rate to express future economic values in present terms because not all health effects and associated economic values occur in the year of analysis. COBRA assumes changes in adult mortality and non-fatal heart attacks occur over a 20-year period. Based on OMB Circular A-4, both the desktop and web editions of COBRA include a 2 percent real discount rate by default, as well as the option to enter a custom discount rate.

15. Why does COBRA display health impacts in counties for which I did not provide emissions changes?

COBRA estimates health impacts in counties without changes in emissions because emissions in the atmosphere travel across county and state borders. The S-R Matrix approximates atmospheric dispersion of air pollutants using transfer coefficients to estimate how changes in emissions in one county affect total PM_2.5 concentrations in every other county in the continental United States. See Appendix A of the COBRA User’s Manual for more details.

16. Why are there “Low” and “High” values for some of the estimated health impacts (e.g., mortality, heart attacks)? Is it possible to pick just one best number from the ranges reported in COBRA?

The Low and High values for some health outcomes in COBRA represent important uncertainties in the estimates of the health impacts of changing air quality (e.g., results for avoided premature mortality are based on two different epidemiological studies of the impacts of PM_2.5 on mortality in the United States). The “Low” estimate represents results based on an evaluation of mortality impacts of PM_2.5 by the Harvard T.H. Chan School of Public Health (Wu et al. 2020). The “High” estimate represents results based on a study from the Environmental Health Perspectives peer-reviewed journal (Pope et al. 2019). Rather than average the results of these studies, EPA’s standard practice has been to report the estimated change in mortality separately as Low and High values. Similarly, Low and High estimates for avoided non-fatal heart attacks are based on epidemiological studies reporting different estimates of the relationship between the risk of this health impact and PM_2.5. All other health impacts in COBRA are reported as a single value.

17. What does it mean when COBRA estimates a fractional change in the incidence of health impacts?

Incidence refers to the number of new cases of a health outcome over a specified time period. The change in incidence is not necessarily a whole number because COBRA calculates small statistical risk reductions that are then aggregated over the entire population (e.g., if 150,000 people experience a 0.001% reduction in mortality risk, this would be reported as 1.5 “statistical lives saved”). This statistical life, and its associated monetary value, represent the sum of many small risk reductions and does not correspond to the loss or value of an individual life.

18. How does population density affect health impacts estimates?

Estimates of health impacts have a direct relationship with population density: areas with higher population density tend to see higher health benefits of reduced emissions than less dense areas. This is because there are more people breathing cleaner air in the areas with higher population density.

19. What is the currency year for COBRA dollar values?

COBRA’s monetary values are currently expressed in 2023 U.S. dollars (USD), which reflects the value of the U.S. dollar in year 2023.

20. How are economic values of health impacts determined? What is the Value of a Statistical Life (VSL)?

To estimate the economic value of health impacts, COBRA multiplies the estimated change in the number of cases of each health impact by the economic value per case (the “unit value”). Some unit values are based on valuation studies estimating people’s willingness-to-pay (WTP) for reducing mortality risk or avoiding various adverse health effects (e.g., asthma exacerbation, respiratory symptoms). When WTP estimates are not available, cost-of-illness (COI) estimates based on economic market valuation studies are applied (e.g., hospitalizations, emergency department visits).

The benefits of avoiding premature mortality are valued using the Value of a Statistical Life (VSL). The VSL is an estimate used in benefit-cost analyses to represent how much people are willing to pay for small reductions in their risks of dying from harmful health conditions that may be caused by environmental pollution. EPA recommends using an estimate of $7.4 million (2006 USD) updated to the year of the analysis. The value used in COBRA has been mathematically adjusted to account for income growth and inflation, and it has a value of $9.8 million at the 2% discount rate (see Question 1 for more information on discount rates and COBRA) in 2016. Each analysis year in COBRA (2016, 2023, and 2028) has different VSL values, based on projections of income growth in each year. See Appendix F in the COBRA User’s Manual for more information on the economic values for each health impact included in COBRA. In addition, EPA’s Frequently Asked Questions on Mortality Risk Valuation provides more background on the VSL.

21. What is the scientific basis for COBRA’s estimates of public health impacts and their economic value?

COBRA’s estimates reflect the current scientific thinking on the relationship between particulate matter and human health, as well as the economic valuation of these health effects. In particular, EPA draws from the PM Integrated Science Assessment. Additionally, EPA’s methodology for characterizing health impacts has been reviewed by two National Academy of Sciences panels and multiple EPA Science Advisory Boards. Because health impacts of air pollution and approaches to value these impacts are areas of active research, the selection of studies used in COBRA may evolve over time, as new evidence and studies emerge.

22. How do I interpret the map of the results?

Both the desktop version and the Web Edition of COBRA provide results in a map format. You can choose to map PM_2.5 or O₃ concentrations, change in incidence for each health endpoint, the monetary value of the change in each health endpoint, or the total monetary value of the scenario. Both the desktop version and the Web Edition of COBRA use an “equal frequency” method of mapping the results, in which the results for all counties are split into 5 groups containing the same number of data points. In many cases, COBRA results can have a few counties with very large health benefits that can be orders of magnitude larger than the benefits seen in other counties. As a result, the top group shown on the map often has a very large range. For example, depending on the scenario, the top group could include counties with results ranging from a few thousand dollars to more than one million dollars. The desktop version includes advanced features to adjust the results map, including changing how the counties are assigned to each group. The Web Edition does not include the same advanced features, but Web Edition users can click on the counties on the map to see the range of values across counties. Chapter 6 of COBRA User’s Manual has more information about how to modify the map in the desktop version.

23. How can I use COBRA with EPA's AVERT tool?

Users can run COBRA together with EPA’s AVoided Emissions and geneRation Tool (AVERT) to estimate some of the health benefits of energy efficiency and renewable energy projects. AVERT is a model that estimates the reduction in air pollution emissions and greenhouse gases associated with energy efficiency, wind, and solar projects. The outputs from AVERT include reductions in emissions of NO_x, SO₂, PM_2.5, NH₃, and VOCs at the county level. These emissions changes can easily be integrated as inputs into COBRA. Like COBRA, AVERT has both a web-based version and a desktop (Excel-based) version. In the web-based version of AVERT, users can export the results from AVERT directly into the web-based of COBRA with the click of a button after AVERT has run. In the desktop version of AVERT, users can export the results from AVERT to a text file, which can then be imported into the desktop version of COBRA on the Create Emissions Scenario screen. The web-based version of AVERT can also generate a CSV file that can be imported into the desktop version of COBRA. In both the web and desktop versions of COBRA, the emissions reductions from AVERT are imported to the Fuel Combustion: Electricity Generation tier at the county level in COBRA.