A car driving on the highway produces emissions. A t-shirt made in a factory produces emissions. The tasty sandwich you ate the other day was made in a factory containing industrial machines that produce emissions. So, how do you quantify the carbon footprint of these activities? 

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The answer is emission factors (sometimes called air pollution factors). 

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Calculating how much pollution an activity or object generates is often challenging. Add to that the fact that different greenhouse gases have varying levels of warming impact on the environment, and things get even more complicated. This is where emission factors come in, as they help estimate emissions per unit. In this article, we’ll take a deep dive into emission factors, how they’re determined, why they’re important, and their limitations. 

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What are emission factors? 

Emission factors are representative values that relate an activity to the quantity of the pollutant it produces. These pollutants are typically greenhouse gases (GHGs) like carbon dioxide, methane, and nitrous oxide, which trap heat and warm the planet. 

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Emission factors are expressed as the weight of the pollutant over the weight, volume, duration, or distance of the activity, whichever is relevant. For instance, the emission factor for energy generation may be expressed as kilograms of CO2 per kilowatt-hour. 

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These numbers are used to determine the emissions a certain activity produces and are often based on average data (at comundo, we always try to use the latest relevant and highest frequency factors in our calculations). The averages are based on long-term data available for the particular source category. 

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How are emission factors calculated? 

Emission factors can be determined in several ways. Researchers study the quantitative relationship between reactants and products (stoichiometry) to develop emission factors specific to a pollutant and activity, such as the CO2 produced from burning coal. Statistical samples and mathematical modelling can also help calculate emission factors for certain products or activities. The latter method is more often used. 

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When calculating emission factors, the focus is usually on a single source category, such as CO2 emissions per litre of diesel. That said, some emission factors cover a broad range of activities typically associated with a single product. For instance, gasoline's emission factor may represent emissions from fossil fuel extraction, processing, and consumption. Such emission factors are based on the product's environmental life cycle. 

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Global warming potentials and carbon dioxide equivalents 

When discussing emission factors, it’s important to talk about carbon dioxide equivalents and global warming potential, as these two are equally important when calculating emissions. 

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Although emission factors are available for specific GHGs, emissions across GHGs are converted to carbon dioxide equivalent (CO2e) by setting carbon dioxide as the base unit. The latter’s global warming potential (GWP) is considered when calculating CO2e. The GWP has been determined based on 100 years of data (i.e., 100 years of the warming effect of a gas relative to that of carbon dioxide).

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Simply put, CO2e for a GHG is calculated by multiplying its amount by its GWP. For example, methane’s GWP is 28. Suppose an activity generates 1 ton of methane. In that case: Methane CO2e = Methane amount (in tons) x GWP = 1 x 28 = 28 tons 

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This result means that releasing one ton of methane into the atmosphere is equivalent to releasing 28 tons of CO2 in terms of global warming impact. The Intergovernmental Panel on Climate Change (IPCC) has determined the GWP for the seven main GHGs, which are the standard for calculating CO2e. 

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CO2e essentially helps calculate the total emissions produced by an activity or product. This way, you get a single number for emissions, regardless of the GHGs emitted – nice and simple! 

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The importance of emission factors

Emission factors are vital to carbon accounting, as they help calculate CO2 emissions from materials and activities in different industries. Without them, we may be unable to quantify emissions from human activities rapidly warming the environment. 

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Here are three other reasons why air pollution factors are so important:

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Carbon footprint and target setting

For any government, company, or individual, determining the carbon footprint is the precursor to setting targets for decarbonisation. 

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Imagine that you’re trying to reduce your monthly spending. To do that, you need to know how much you’re spending and on what. If you don’t know these things, you can’t effectively reduce spending. Similarly, if you don’t know how much emissions you produce, how will you reduce them? To set actionable targets, you must first know your carbon footprint and emission factors to help determine that. 

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Regulatory compliance

Emission factors have enabled governments and regulatory bodies to limit certain activities. Companies use these emission factors to calculate and report their emissions. Without emission factors, environmental regulations wouldn’t exist. The right emission factors are important for environmental, social, and governance (ESG) reporting. 

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Simplification of emissions from GHGs

Not all GHGs have the same environmental impact. For instance, methane traps much more heat than carbon dioxide. Emission factors help determine each GHG's impact, allowing companies and people to better understand it and help them make fully-informed decisions.

Uncertainty in emission factors

Emission factors, although useful in fighting global warming, are accompanied by uncertainties. These uncertainties are typically the result of sampling errors, as most organisations use empirical data to determine an emission factor for an activity. 

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More importantly, emission factors have not been standardised yet, which can result in variations and inaccuracies in emissions data. Organisations that have compiled emission factors use different methods for calculating them, often resulting in different numbers. 

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Unfortunately, such a scenario allows for manipulation and errors, especially if companies have discretion in choosing emission factors. They may select the lowest-value ones to help them satisfy regulators.

Emission factors database

Several organisations have developed their own emission factors database for different activities using scientific research or industry averages. While some cover a wide range of activities and products, some focus on specific industries, such as oil and gas. 

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Here are the major emission factor databases available for carbon accounting purposes:

• IPCC Emission Factor Database (EFDB): The IPCC maintains this database, which contains emission factors for greenhouse gases and other pollutants from various sectors and activities. It’s the most comprehensive database to date that offers granular-level emission factors for activities. 
• EPA's AP-42 Compilation of Air Pollutant Emission Factors (Stationary Sources): The United States Environmental Protection Agency (EPA) maintains its own database of emission factors. Like the IPCC database, the emissions factors cover various industries, particularly those well-known for high emissions. EPA updates this database as the quality of emission factors improves with more research and testing. 
• DEFRA Emission Factors: Published by the Department for Environment, Food and Rural Affairs (DEFRA) in the United Kingdom, this resource provides emission factors for UK and international activities and industries.
• EMEP/EEA Air Pollutant Emission Inventory Book: The European Environment Agency has created a guidebook that contains emission factors calculated through various methodologies. These emission factors are more relevant to activities in Europe.

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How to choose the emission factors for carbon accounting

Emission factors, especially those based on statistical data, are prone to inaccuracy. At the same time, they’re the next best thing where direct emission calculations and monitoring aren’t possible. So, when determining your carbon footprint, remember the following when selecting emission factors from a database. 

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Application

Emission factors can differ by application scale. An activity at a national level may require a slightly different number than the same activity done at the local level. Site-specific emission factors are used for specific places. 

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Location

Emission factors also correspond to a specific location. That’s because the impact of pollutants may be different under different conditions. So, some databases also mention the location of specific emission factors.

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Quality 

Emission factors are constantly updated using research and study and are bound to be of the highest quality. The better the quality, the more accurate your ESG data will be. 

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Unit

Emission factors in different databases may also differ by unit. It’s important to convert all numbers into a single number represented by the same unit. 

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Realising climate accountability

With a climate emergency at our doors, it’s now or never for action against global warming. Sadly, there are still gaps in emissions data worldwide, which only worsens the problem. You can use emission factors in your calculations to determine the emissions of activities. The IPCC offers some of the most relevant, accurate, and valuable emission factors for many activities in different industries. These can quantify every activity's impact and help you calculate the best possible carbon footprint. 

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At comundo, we’re all about accuracy and understanding its importance for decarbonisation. Without accurate data, climate targets can’t be correct.

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FAQs

What is an economic emission factor?

An economic emission factor is a measure used to assess the environmental impact of economic activities relative to their economic output, such as emissions per unit of GDP or unit of product manufactured.

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Who provides emission factors?

Emission factors are provided by various organisations and institutions involved in environmental research, regulation, and policymaking, including government agencies, research institutions, and non-profit organisations. The Intergovernmental Panel on Climate Change (IPCC) and the Environmental Protection Agency (EPA) are some of the leading organisations with an active database of emission factors. 

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What is an emission rate?

Emission rate refers to the rate at which pollutants are released into the environment from a specific source or activity, typically measured in units such as kilograms per hour or tons per year. The emission rate helps calculate the total emissions from a source over a timeframe.

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