Disease burden is the impact of a health problem as measured by financial cost, mortality, morbidity, or other indicators. It is often quantified in terms of quality-adjusted life years (QALYs) or disability-adjusted life years (DALYs), both of which quantify the number of years lost due to disease (YLDs). One DALY can be thought of as one year of healthy life lost, and the overall disease burden can be thought of as a measure of the gap between current health status and the ideal health status (where the individual lives to old age free from disease and disability). According to an article published in The Lancet in June 2015 low back pain and major depressive disorder were among the top ten causes of YLDs and were the cause of more health loss than diabetes, chronic obstructive pulmonary disease, and asthma combined. The study based on data from 188 countries, considered to be the largest and most detailed analysis to quantify levels, patterns, and trends in ill health and disability, concluded that "the proportion of disability-adjusted life years due to YLDs increased globally from 21·1% in 1990 to 31·2% in 2013." The environmental burden of disease is defined as the number of DALYs that can be attributed to environmental factors. These measures allow for comparison of disease burdens, and have also been used to forecast the possible impacts of health interventions. By 2014 DALYs per head were "40% higher in low-income and middle-income regions."