A Disability-Adjusted Life Year (DALY) is a standardised unit of measurement used to quantify health Externalities in Life Cycle Assessment and public health research. DALYs express the burden of disease and health impacts as a loss of person-years, providing a common metric to compare different health outcomes across populations and interventions.
The DALY metric combines two components: years of life lost due to premature mortality and years lived with disability. Each component is weighted by the severity of the health state on a scale from 0 to 1, where 0 represents death and 1 represents a state of full health. A health state with a weight of 0.5, for instance, would indicate a condition that reduces quality of life by half compared to full health. The total DALY burden for a given health impact is calculated by multiplying the duration of the health effect by its severity weight.
Originally developed by the World Health Organization for the Global Burden of Disease study, DALYs have become a standard metric in Life Cycle Impact Assessment for evaluating human health impacts. They enable practitioners to aggregate diverse health effects—from respiratory diseases to injuries to chronic conditions—into a single, comparable unit that represents lost healthy life years.
An important distinction exists between DALYs and Quality-Adjusted Life Years (QALYs). Whilst both metrics weight health states on a 0 to 1 scale, DALYs measure health loss (using a negative framing) whereas QALYs measure health gain or wellbeing (using a positive framing). Additionally, QALYs may incorporate broader aspects of wellbeing beyond purely health-related quality of life. In LCA applications, DALYs are typically preferred for quantifying environmental and occupational health damages because they align with the impact-oriented perspective of environmental assessment.
