What are Impact Categories in LCA?

Impact categories translate emissions and resource consumption into environmental effects. Raw inventory data lists kilograms of CO₂, nitrogen oxides, and phosphates. Impact categories convert these into climate change, acidification, and eutrophication.

Why Multiple Categories Matter#

Carbon footprints measure climate change. That’s one environmental concern among many. A product might have low carbon emissions but high water consumption. Another might avoid toxicity but deplete scarce resources.

Single-metric assessment misses trade-offs. Using recycled materials might reduce resource depletion but increase local air pollution from reprocessing. Impact categories reveal these relationships.

Core Impact Categories#

Most LCA studies assess similar categories. The specific list depends on your chosen impact assessment method, but common categories include:

Climate Change#

Greenhouse gas emissions create atmospheric warming. Different gases have different warming potentials over time. Methane traps more heat than CO₂ over 20 years but breaks down faster.

Climate change impact gets expressed as CO₂ equivalents. All greenhouse gases convert to a common reference based on their Global Warming Potential. This allows adding up contributions from different emission types.

Ozone Depletion#

Certain chemicals damage stratospheric ozone. Chlorofluorocarbons and halons were the main culprits. The Montreal Protocol reduced these emissions substantially.

Ozone depletion remains relevant for products using refrigerants or foam blowing agents. Modern alternatives have lower ozone depletion potential but may have high climate change impact.

Acidification#

Sulphur dioxide, nitrogen oxides, and ammonia create acid deposition. This damages forests, acidifies lakes, and corrodes buildings.

Emissions get characterised as H⁺ equivalents – their potential to generate hydrogen ions. Sulphur dioxide dominates acidification from most industrial processes.

Eutrophication#

Excess nutrients stimulate algae growth in water bodies. This depletes oxygen and damages aquatic ecosystems. Nitrogen and phosphorus compounds drive eutrophication.

The category splits into freshwater and marine eutrophication. Phosphorus limits freshwater systems. Nitrogen limits marine environments. The distinction matters for impact assessment.

Photochemical Ozone Formation#

Ground-level ozone harms human health and vegetation. Unlike beneficial stratospheric ozone, tropospheric ozone causes respiratory problems and crop damage.

Nitrogen oxides and volatile organic compounds react in sunlight to form ozone. Summer smog demonstrates this impact. The category measures contributions to ozone formation potential.

Particulate Matter Formation#

Fine particles damage respiratory health. PM2.5 and PM10 particles penetrate lungs and enter bloodstreams. Direct emissions and secondary formation from precursors both contribute.

Health impacts vary by particle size and composition. Impact assessment accounts for these differences through characterisation factors.

Human Toxicity#

Chemical releases can poison humans through various exposure routes. Heavy metals, persistent organic pollutants, and other toxic substances accumulate in food chains or contaminate drinking water.

Toxicity assessment faces uncertainty. Many chemicals lack complete toxicity data. Exposure pathways vary by location. Environmental fate models predict where chemicals end up and who gets exposed.

Ecotoxicity#

Aquatic and terrestrial toxicity categories assess harm to ecosystems. Different species have different sensitivities. Ecosystems respond in complex ways to chemical stress.

Freshwater, marine, and terrestrial ecotoxicity get assessed separately. Each environment has different exposure conditions and vulnerable species.

Water Scarcity#

Water consumption matters most in water-stressed regions. Using water in Scotland differs from using it in Arizona. Some impact methods account for regional scarcity through weighting factors.

Water use and water consumption differ. Cooling water might be withdrawn and returned. Irrigation water gets consumed through evaporation and crop uptake. Consumption impacts matter more.

Resource Depletion#

Extracting minerals and fossil fuels depletes reserves. Some resources face near-term scarcity. Others remain abundant but require more energy to extract from poorer ores.

Resource depletion gets assessed by comparing use rates to reserve estimates. The category includes both abiotic depletion (minerals, fossil fuels) and sometimes biotic depletion (biomass, fish stocks).

Land Use#

Converting natural ecosystems to human use destroys habitat and ecosystem functions. Agriculture, infrastructure, and resource extraction all require land.

Impact assessment examines land occupation (area and time) and land transformation (quality change). A forest converted to cropland has different impacts than existing cropland.

Selecting Categories#

Your study goal determines which categories to assess. Climate-focused policies might emphasise greenhouse gases. Water-stressed regions prioritise water scarcity. Comprehensive assessments cover all relevant categories.

Product Category Rules for EPDs specify required impact categories. This ensures consistency across similar products. Studies for different purposes might assess additional categories.

Characterisation Methods#

Different models calculate impact scores. CML, ReCiPe, TRACI, and other methods use different characterisation factors and models.

Method choice affects results. Climate change is relatively standardised. Toxicity differs substantially between methods due to model uncertainty.

ISO 14044 requires documenting your chosen method and justifying the choice. Method selection can’t be arbitrary – it should suit your geographical scope and impact focus.

Limitations#

Impact categories don’t cover everything. Noise, radiation, and biodiversity impacts often fall outside standard methods. Some categories lack robust characterisation factors.

Categories assess potential impacts, not actual damage. Whether potential eutrophication causes real ecosystem harm depends on local conditions. LCA provides relative comparison, not absolute prediction.

Weighting different categories involves value judgements. Is climate change more important than water scarcity? The answer depends on priorities that sit outside scientific assessment.