A Quality-Adjusted Life Year (QALY) is a unit of measurement for wellbeing used in Life Cycle Assessment and cost-benefit analysis, expressed in person-years weighted by the degree of wellbeing on a scale from 0 to 1, where 1 represents a state of full wellbeing and 0 represents a state equivalent to death.
The QALY provides a comprehensive measure that combines both the quantity and quality of life. Each year of life is adjusted according to its quality, allowing analysts to compare different health states and interventions on a common scale. For example, one year lived in perfect health equals 1.0 QALY, whilst one year lived with a health condition that reduces quality of life to 70% of full wellbeing equals 0.7 QALY.
This metric plays a crucial role in sustainability assessment by enabling the quantification of human wellbeing impacts across a product's life cycle. QALYs can capture not only health-related quality of life but also other dimensions of wellbeing that affect human welfare beyond physical health, such as psychological wellbeing, social functioning, and general life satisfaction.
The QALY differs from Disability-Adjusted Life Years (DALY) in two important ways. First, QALYs use a positive sign, meaning that wellbeing loss is measured as a reduction in QALYs rather than as an increase in negative burden. This makes QALYs conceptually aligned with measuring gains in wellbeing rather than losses. Second, whilst DALYs focus specifically on health-related quality of life, QALYs consider a broader range of wellbeing aspects beyond health alone, making them more comprehensive for assessing overall human welfare impacts in Life Cycle Sustainability Assessment.
In LCA applications, QALYs help quantify the social dimension of sustainability by translating diverse impacts on human wellbeing into a single, comparable unit that can be integrated into decision-making frameworks alongside environmental and economic indicators.
