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Explained: Environmental impact categories
Explained: Environmental impact categories

Learn about environmental impact categories and their units.

Emily Lalonde avatar
Written by Emily Lalonde
Updated over a week ago

Environmental impact categories are essential tools in Life Cycle Assessment (LCA), quantifying the effects of activities on the environment. Standards like Environmental Footprint Method (EF), CML, EN15804+A2, and TRACI (and many others!) outline these categories to ensure consistency and reliability in environmental assessments. This article details the units prescribed by these standards, providing insight into their significance and application. Understanding these units enhances your ability to interpret LCA results and make informed sustainability decisions. This article covers the following questions:

  • What are environmental impact categories?

  • What do the main environmental impact categories represent?

Feel like you're missing information? This article builds upon the following articles. Check them out if you want to learn more:


What are environmental impact categories?

Environmental impact categories represent different ways in which we affect the environment around us - like global warming, ozone depletion, and resource depletion. These categories help assess the environmental performance of products throughout their life cycle. Environmental impact categories enable consistent comparison and analysis by quantifying impacts in standardized units, such as kilogram CO2 equivalent (kg CO2-eq) for the Global Warming Potential impact category. Knowing these categories offers a comprehensive understanding of environmental impacts, guiding efforts to reduce negative effects and improve sustainability practices.


What do the main environmental impact categories represent?

Here, we describe the seven impact categories defined by CML, TRACI, EN15804+A2, and the EF Method. However, there are over 50 impact categories set by different standards. If you can’t find the impact category you’re curious about, try reading up on the standard that prescribes it.

Global Warming Potential (GWP)

Units

  • The unit is kg CO2-eq for all four standards.

Description

  • GWP accounts for the total emissions from greenhouse gases such as CO2 (carbon dioxide), CH4 (methane), and N2O (nitrous oxide). The amount of CH4 and N2O is converted into CO2 equivalents by utilizing the characterization factors (36.75 for CH4 and 298 for N2O respectively).

  • GWP emissions occur from activities like coal combustion, production processes, landfilling, and fertilizer use in soil.

  • A 100-year time horizon is usually used, and the units are referred to as CO2 equivalents.

  • Other LCA standards refer to the GWP impact category as 'Climate Change'.

  • Figure 1 helps conceptualize this impact category further.

  • Learn more about this impact category here.

    Figure 1: The GWP of tap water compared to the production of a laptop.

Ozone Depletion Potential (ODP)

Units

  • The unit is kg CFC-11-eq for all four standards.

Description

  • The ozone layer is an atmospheric layer that protects us from the sun’s harmful radiation.

  • Halogens such as chlorine, bromine, and fluorine (Cl, Br, F) are highly reactive and are responsible for ozone depletion. Consequently, the units are described in terms of Chlorofluorocarbons (CFCs).

  • The characterization factors for each gas are predetermined and used during calculation. For example, 1 kg of methyl bromide is equivalent to 0.57 kg of CFC - similar to the approach taken for greenhouse gases in the Global Warming section above.

  • Learn more about this impact category here.

Acidification Potential (AP)

Units

  • CML & TRACI: kg SO2-eq

  • EN15804+A2 & EF: mol H+ eq

Description

  • The acidification impact category quantifies the amount that each gas contributes to acid rain, a cause of acidity (measured in pH) change in soil.

  • Gases like NH3 (ammonia), SO2 (sulfur dioxide), and NO2 (nitrogen dioxide) contribute to acid rain. Consequently, the impact category unit is expressed in kilograms sulfur dioxide equivalents (kg SO2-eq). The acidification potential of other gases is converted into SO2-eq by using a characterization faction - like 1 kg of NH3 is equal to 1.6 kg of SO2.

  • Learn more about this impact category here.

Note - Differing AP units: Although the AP unit is different for the EF and EN15804 standards, the result is the same. In these standards, the change in the pH is measured through mol H+ ions.

Eutrophication Potential (EP)

Units

  • CML: kg PO4-eq

  • TRACI: kg N-eq

  • EN15804+A2 and EF:

    • Terrestrial: mol N-eq

    • Marine: kg N-eq

    • Freshwater: kg P-eq

Description

  • Aquatic eutrophication refers to the nutrient enrichment of aquatic environments which impacts water quality by promoting excessive plant growth. In the summer this often manifests as algae blooms in lakes due to nutrient influx.

  • Nutrient inputs, particularly phosphorus (P) and nitrogen (N), drive the proliferation of plankton and algae in water bodies. Therefore, measurements typically represent amounts of P, N, or their compound forms, such as phosphate (PO4).

  • EN15804 and EF distinguish between three levels of eutrophication (terrestial, marine, freshater), while the CML and TRACI methods group them.

  • Learn more about this impact category here.

Photochemical Ozone Creation Potential (POCP) / Smog Formation

Units

  • CML: kg C2H4-eq (C2H4: ethylene)

  • TRACI: kg O3-eq (O3: ozone)

  • EN15804+A2 and EF: kg NMVOC (NMVOC: non-methane volatile organic compounds)

Description

  • In the lower air layers of Earth's atmosphere, volatile organic substances (except methane) and NOx (nitrogen oxides) can undergo photochemical oxidation, leading to smog.

  • The potential for smog formation depends on the amount of reactants present in the air or the products formed from these reactions. Therefore, smog potential is measured in units that reflect these factors (ozone forms from the reaction between oxygen radicals and O2).

  • Learn more about this impact category here.

Abiotic Depletion Potential (ADP) / Fossil Fuel Depletion

Units

  • CML: kg Sb-eq or MJ (Sb: Antimony)

  • TRACI and EF: MJ

  • EN15804+A2: kg Sb-eq

Description

  • Resource consumption is an important sustainability indicator, especially when the resources used are scarce.

  • This category can be measured in two ways: mineral consumption (kg SB-eq), and fossil fuel consumption (MJ) for energy generation.

  • Antimony (Sb), a rare earth element, is used to quantify mineral loss due to human activities. For energy generated from fossil reserves, megajoules (MJ) are the standard unit of measurement.

  • Figure 2 helps conceptualize this impact category further.

  • Learn more about this impact category here.

Figure 2: The ADP or fossil fuel depletion of wood pellet production compared to sodium ion battery production.

Water Depletion Potential (WDP)

Units

  • CML: n/a

  • TRACI: n/a

  • EN15804+A2 and EF: m3 world eq. deprived

Description

  • This method measures the potential reduction of available water for human or ecosystem use. It uses the AWARE (Available WAter REmaining) method, which calculates the water remaining per unit of surface area in a given watershed, relative to the world average, after meeting human and aquatic ecosystem demands. The resulting Characterization Factor (CF) ranges from 0.1 to 100 and is meant to be multiplied by the local water consumption inventory.

  • Learn more about this impact category here.

Caution - WDP: Be cautious when using this impact indicator as the characterization factors may be inaccurate or not representative for certain regions and activities.

Toxicity

The harmful effects of pollutants are observed on humans and the ecological systems surrounding us. Toxicity is used to characterize pollutants' effects on humans and ecosystems. Therefore, this category is divided into two types:

  • Human toxicity

  • Ecotoxicity

Human toxicity

Unit

  • EF, TRACI, and EN15804: CTUh (Comparative Toxic Unit for humans)

  • CML: kg 1,4-DCB-eq (1,4 dichlorobenzene eq)

Description

  • Due to a lack of more accurate information, the characterization factor for human toxicity impacts (also known as human toxicity potential) is expressed in comparative toxic units (CTUh), which are the estimated increases in morbidity in the total human population per unit mass of a chemical emitted. This is done by either assuming equal weighting between cancer and non-cancer or by using the USEtox model - developed by UNEP-SETAC Life Cycle Initiative - which classifies the pollutants into carcinogenic and non-carcinogenic to bring out more granularity in the analysis.

  • TRACI brings out the granularity in the measurement with more sub-categories, such as carcinogenic and non-carcinogenic pollutants in the human toxicity category.

Ecotoxicity

Unit

  • EF, TRACI, and EN 15804: CTUe (Comparative Toxic Unit for ecosystems)

  • CML: kg 1,4-DCB-eq

Description

  • The potentially affected proportion of species (PAF), integrated over time and volume, is an estimate of the potential harmful effects of chemicals in freshwater ecosystems.

  • Figure 3 helps conceptualize this impact category further.

Figure 3: The ecotoxicity of tap water compared to benzene production.


Next steps

By familiarizing yourself with environmental impact categories and their units prescribed by standards like EF, CML, EN15804+A2, and TRACI, you can better understand and interpret LCA results. This knowledge forms a foundation for accurate sustainability assessments, aiding in identifying and mitigating environmental impacts. Whether preparing for third-party verification or securing internal LCA knowledge, understanding these categories and their units is an important part of your sustainability efforts.

This article was written in close collaboration with Pratik Glokar:

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