What are System Boundaries in LCA?

System boundaries define what’s included in your assessment and what’s excluded. Every LCA needs clear boundaries because no study can track everything.

Defining the System#

The boundary line separates your product system from the rest of the economy and environment. Inside the boundary, you track all processes and flows. Outside, you assume no responsibility for impacts.

A boundary decision affects results. Include transport and your impacts increase. Exclude end-of-life disposal and you miss waste management burdens. These choices change the environmental profile.

Geographical Boundaries#

Location affects impact factors. Electricity from coal-heavy grids creates more climate impact than renewable-dominated grids. Transport distances vary by production location. Waste management infrastructure differs by region.

LCA specifies geographical scope. A global product might use weighted averages across production sites. A regional assessment uses local data. The choice depends on your goal and data availability.

Temporal Boundaries#

Time matters. Current electricity grids differ from projected future grids. Manufacturing technology improves. Environmental regulations change. These temporal aspects need explicit boundary decisions.

Prospective LCA uses future scenarios. A product launching in 2030 should use projected 2030 electricity mixes, not 2024 data. Retrospective LCA uses historical data for past performance.

Technology changes within product lifetimes create uncertainty. A vehicle manufactured today uses today’s electricity for production but tomorrow’s electricity for charging. The boundary needs to account for this.

Technological Boundaries#

Technology boundaries specify which processes to include. Primary production from raw materials requires more upstream processes than production from recycled content.

Background systems sit outside direct control. Steel production needs electricity. The electricity generation process sits in background. LCA includes these upstream systems but may use generic data.

Foreground systems represent processes under study. Your manufacturing operations use primary data. Background systems use secondary database values. This distinction affects data quality and results.

Capital Goods#

Infrastructure and equipment create impacts during construction. Should you include factory construction emissions in your product LCA?

ISO standards allow excluding capital goods if their contribution is negligible. A factory producing millions of units over 20 years contributes minimal impact per unit. A small production run might need inclusion.

The boundary decision needs justification. Calculate capital goods impacts and show they’re below your significance threshold, or include them explicitly.

Cut-off Criteria#

Tracking every input becomes impractical. Insignificant flows might be excluded using cut-off criteria. These criteria specify thresholds for exclusion based on mass, energy, or environmental relevance.

A 1% cut-off means processes contributing less than 1% of mass or energy can be excluded. This sounds reasonable but creates problems. Multiple small exclusions accumulate. Toxic substances might be trace amounts by mass but significant by impact.

Cut-off rules need documentation. State your criteria and justify them. Show that excluded flows don’t significantly affect results.

Multifunctional Processes#

System boundaries face problems when processes generate multiple products. A refinery produces multiple fuels. Cows provide milk and meat. Electricity cogeneration produces power and heat.

You can expand boundaries using system expansion. Track what each co-product displaces in the market. Subtract avoided impacts from the system total.

Alternatively, allocate impacts across products using physical or economic relationships. Mass allocation splits by weight. Economic allocation uses relative market values. The choice affects product-specific results.

End-of-Life Boundaries#

Does end-of-life sit inside or outside boundaries? The answer depends on your research question.

Cradle-to-grave includes disposal. You account for landfill emissions, incineration impacts, or recycling processes. This captures the complete life cycle.

Cradle-to-gate stops at factory gate. End-of-life sits outside boundaries. This suits manufacturer-focused studies where disposal scenarios remain unknown.

Recycling complicates boundaries. Material recovery avoids virgin production impacts. Should you credit the product providing recyclate or the product using it? Boundary conventions address this, but choices affect results.

Documenting Boundaries#

ISO 14040 requires clear boundary documentation. State what’s included and excluded. Justify exclusions. Specify geographical, temporal, and technological scope.

Transparency matters. Users need to understand what your study covers. Hidden exclusions mislead. Explicit boundaries enable informed interpretation.

Boundaries enable comparison only when matched. Don’t compare one product’s cradle-to-gate assessment with another’s cradle-to-grave results. The difference might reflect boundaries rather than actual performance.

Practical Boundary Setting#

Perfect boundaries are impossible. Resources, data availability, and time constrain scope. The goal is defensible boundaries that serve your purpose.

Start broad, then refine. Initial screening identifies significant processes. Detailed study can focus on important areas. Iterative boundary setting improves results while managing resources.

Sensitivity analysis tests boundary robustness. Include or exclude questionable processes and observe result changes. Stable conclusions survive boundary variations. Sensitive results need careful interpretation.