The WHO and World Bank (2011) estimate that over a billion people (or 15% of the global population) have a disability, although this estimate is known to have limitations. Disability prevalence is influenced by factors that vary across countries including trends in health conditions, environmental factors and other variables such as road traffic crashes, natural disasters, conflict, diet, and substance abuse. People who have a low income, are out of work, or have low educational qualifications are at an increased risk of disability. Low and middle income countries have higher disability prevalence than high income countries.
Research has shown that the prevalence of specific disabilities, such as hearing loss, decreases exponentially as income increases. Most people who are blind or visually impaired live in Asia and age-standardised prevalence of blindness is highest in Africa and Asia.
Conflict generates injuries and trauma that can lead to disabilities. This may be exacerbated by delays in accessing care and rehabilitation for the injured. Violence and conflict account for 1.4% of all years lived with disability.
Disability prevalence is known to be increasing due to a global increase in chronic health conditions. Such conditions are estimated to account for 66.5% of all years lived with disability in low-income and middle income countries. The global increase in diabetes, cardiovascular diseases, mental disorders, cancer, and respiratory illnesses will have a profound effect on disability. Large increases in non-communicable disease related disabilities are projected for developing regions caused by population ageing, reduction in infectious conditions, lower fertility, and changing lifestyles related to tobacco, alcohol, diet, and physical activity. Despite a fall in high risk life style choices in high income contexts, prevalence of obesity and diabetes have been increasing, which may increase the disability burden, including the prevalence of dementia. In low and middle income contexts, obesity, stroke, and heart disorders are increasing, and may cause an upward trend in the incidence and prevalence of dementia.
The global prevalence of diabetes is 422 million and has quadrupled since the 1980s. Diabetes prevalence is rising faster in low and middle income countries than in high income countries. Diabetic retinopathy causes blindness, and diabetes can also lead to limb amputation and other long-term consequences.
The global prevalence of dementia is 46.8 million people. This number is expected to increase to 131.5 million by 2050. Much of the increase will disproportionately affect lower and middle income countries, where, in 2015, 58% of all people with dementia lived. It is estimated that this figure will rise to 63% in 2030 and 68% in 2050 due to rising life expectancy.
An increase in mental illness and substance use disorders is known to have driven recent increases in years lived with disability. In 2010, on a global level, mental and substance use disorders accounted for 7·4% of all disability-adjusted life years and 22.9% of all years lived with disability, making it the leading cause of YLDs worldwide.
Age is linked to increasing difficulties in functioning; as populations age, the prevalence of disability will increase. Globally, disability prevalence is estimated to be 12% for working age adults and 39% among the elderly. Evidence from Haiti suggests that disability prevalence was relatively low up until the age of 40, and increased dramatically for older people. Age was found to be a more significant driver of disability than the earthquake of 2010.
Globally, women have a higher prevalence of disability than men. For some disabilities, such as dementia, the gender specific prevalence is not expected to vary over time, with population ageing driving projected increases. Women are known to have a higher prevalence for other types of disability, such as blindness (56%), severe visual impairment (55%), and mild visual impairment (54%). For other disabilities the reverse is observed. For example, more men are affected by disabling hearing loss (56%).