Fact Sheet
Agency	Slate and Local
Energy and Environment Pro yam
Co-Benefits Risk Assessment
Health Impacts Screening and Mapping Tool
Natural Gas Emissions Using COBRA
The U.S. Environmental Protection Agency's (EPA) CO-Benefits Risk
Assessment Health Impacts Screening and Mapping Tool (COBRA)
estimates and monetizes the health impacts resulting from changes in
outdoor air pollution emissions.To conduct analyses in COBRA, users must
enter data on emissions of primary fine particulate matter (PM25), sulfur
dioxide (S02), nitrogen oxides (NOx), ammonia (NH3), and volatile organic
compounds (VOCs). This fact sheet explains how to use COBRA to analyze
energy efficiency programs or policies that reduce or avoid natural gas
Background: According to the U.S. Energy Information Administration (EIA),
the residential sector consumed over 4.4 trillion cubic feet of natural gas in
2017.1 Natural gas combustion leads to significant criteria pollutant emissions,
particularly emissions of NOX, which impact public health. In 2017, EPA reported
that residential natural gas combustion led to 205,415 tons of NOx emissions.2
EPA's COBRA tool can help users assess the change in health effects and
monetized public health benefits of efficiency programs that reduce or avoid
natural gas use. This fact sheet explains how to use the desktop software
version of COBRA to assess a hypothetical program in the state of California.3
It provides step-by-step guidance on how to calculate avoided emissions
from natural gas reductions and then enter these data into COBRA to
estimate and monetize the public health benefits from improved outdoor
air quality.4 For more detailed information on using COBRA, see the COBRA
User's Manual.
Calculating Emission Changes:
An Example Scenario: To illustrate
how to use COBRA to analyze natural
gas efficiency program, we present
an example analysis based on a
hypothetical residential natural gas
efficiency program in California that
led to a savings of 60 million therms5
of natural gas in 2017. To understand
the health impacts of this program,
you must first use emissions factors
for residential natural gas to calculate
avoided emissions of PM2 5, S02,
NOx, NH3, and VOCs.The emissions factors for residential natural gas listed
in Table 1 are from the EPA reference document AP-42 Compilation of Air
Emissions Factors, which is a comprehensive source of air emissions factors
from multiple sectors and emissions sources in the U.S.6 Emission factors are
reported in units of pounds (lbs.) of pollution per million cubic feet (MMcf) of
natural gas.
To use these emissions factors, you must convert natural gas savings from
units of therms to MMcf using the conversion factor 1 therm = 96.4 cubic feet
of natural gas. Therefore, in the hypothetical California example:
60 million therms x 96.4 cfper therm + 7 million = 5,786 MMcf of natural gas
Table 1. Residential Natural Gas
Emission Factors
Source: U.S. EPA.7
Emissions Factor
Primary PM25
March 2021 -430F21008

Next, multiply the MMcf of natural gas saved by each of the emissions factors
to determine the emissions of each pollutant avoided through natural gas
efficiency in California.
5,786 MMcf of natural gas x 0.43 lbs. PM2; per MMcf + 2000 lbs. per ton = 7.24 tons PM2;
Since the COBRA tool accepts emissions input in units of short tons, you
next need to convert the units from lbs. to short tons of emissions using
the conversion factor 2,000 lbs. = 1 short ton.The following calculation
demonstrates how to estimate reductions in primary PM25 emissions:
lbs. PM2 5 per MMcf+2000 lbs. per ton = 7.24 tons PM2;
Table 2 shows the results of these calculations for all pollutants in this
example using the emission factors from Table 1.
Table 2. California Example Results:
State-level Avoided Emissions from
Natural Gas Efficiency
Avoided Emissions
(short tons
Primary PM25
Estimating Health Impacts with COBRA
~ Open the COBRA software and click on the "Select
Analysis Year" tab. Choose 2016 from the drop-
down menu since you are assessing the impact of
emissions reductions in 2017 and this is the closest
baseline year available.8 Click"Apply Analysis Year Data."
Next, open the"Create Emissions Scenario"tab.
Select	Click the box next to California under"Select
Location	Location"since this is the location of the gas
efficiency program. You have the option to select
individual counties when you have data on emissions changes at that level.
For this example, you only need to select the state of California. COBRA will
distribute the emissions changes across counties within the state based on
the proportion of baseline emissions in each county.
.	Under"Select Emissions Tier" clickon the + sign
HO Emfssions next to"Fuel Combustion: Other"to expand the
UO jjer	next tier level. Click the + sign next to "Residential
Other"and then select "Natural Gas"to indicate that
the emissions changes stem from changes in household natural gas use.
EUnder"Modify Emissions"enterthe emissions
changes for each pollutant as shown above in
Table 2. Select the "reduce by" and "tons" radio
buttons, then click"Apply Changes."COBRA allows
users to assess the cumulative impact of emissions changes from programs
in multiple locations or different emissions tiers. To enter changes from
additional programs, simply repeat Steps 1-2 and click"Apply Changes"each
time. For this example, you can proceed directly to Step 3.
After you have entered the emissions reduction
Discount inputs, open the"Execute Run"tab. Your last step
paje	before running the tool is to select a discount rate
to use in the analysis. 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 EPA's Guidelines for Preparing Economic Analyses,
the desktop version of EPA recommends using two default real discount
rates: 3 percent and 7 percent.9
When you have selected a discount rate, click"Run using above option."
COBRA will then estimate changes in total annual ambient concentrations
of PM25, including primary PM25 emissions and the formation of secondary
PM25 from precursor pollutants, such as S02, NOx, NH3 and VOCs. 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.

~ Once the model has finished running, COBRA will
automatically open the "View Health Effects and
Valuation Results" tab with a table of nationwide
results. Even though this example involves
emissions changes only in the state of California, COBRA calculates health
benefits in all counties in the contiguous United States due to the transport
of outdoor air pollutants between counties and states. You can filter the table
to show results for a particular state or county. Click on the"Map"tab to view
the results in map form. Both the table and the map provide county-level
changes in air quality (total annual average PM25 concentration in ng/m3),
incidence of each health endpoint, and associated economic values.
Table 3 summarizes the results for this example by health endpoint. Based
on this analysis, a natural gas efficiency program in California that saved
60 million therms of gas in 2017 would provide approximately $12.8 million to
$28.9 million in nationwide health benefits due to improved outdoor air quality.
1.	U.S. EIA. 2020. Natural Gas Consumption by End Use.
cons sum dcu nus a.htm
2.	U.S. EPA. 2017. National Emissions Inventory,
3.	EPA also offers the COBRA Web Edition that users can run within their internet browser.
Although the Web Edition has streamlined features and a slightly different layout, users can
still follow the basic steps outlined in this factsheet. Access the COBRA Web Edition at www.
4.	COBRA assesses the air quality and health impacts from changes in total outdoor PM25
concentrations (annual average |jg/m3). It does not assess any impacts related to changes in
indoor air quality that may also occur.
5.	A therm is a unit of heat energy equivalent to 96.4 cubic feet of natural gas.
6.	U.S. Environmental Protection Agency. AP-42: Compilation of Air Emissions Factors, https://
7.	U.S. EPA. 2021. 2017 National Emissions Inventory: January 2021 Updated Release,Technical
Support Document. Available:
8.	The COBRA Web Edition only provides baseline data for the year 2023.
9.	The COBRA Web Edition also includes an option for users to entera custom discount rate.
Table 3. Nationwide Public Health Benefits of Natural Gas Efficiency in
California in 2017,3% discount rate
Health Endpoint
(cases, annua l)t
Monetary Value
($2017, annual)
Infant Mortality
Nonfatal heart attacks*
0.07 - 0.64
All Respiratory Hospital
All Cardiovascular
Hospital Admissions
Acute Bronchitis
Upper Respiratory
Lower Respiratory
Emergency Room Visits,
Minor Restricted Activity
Work Loss Days
Asthma Exacerbation

$12,829,959 - $28,879,663
* Avoided mortality and nonfatal heart attacks are presented as a range to
represent important uncertainties in the estimates of the health impacts of
changing air quality. For example, results for avoided premature mortality
are based on two different epidemiological studies of the impacts of PM25 on
mortality in the United States.
t Incidence refers to the number of new cases of a health endpoint over a
specified period of time. The change in incidence is not necessarily a whole
number because COBRA calculates statistical risk reductions which are then
aggregated over the population.