How many people need glasses depends on the definition of “need”. For many years the World Health Organisation (WHO) and the International Agency for the Prevention of Blindness (IAPB) have endorsed relatively low estimates of the number of people who need but don’t have glasses to see in the distance: fewer than 200 million globally. That is because their criterion for need was a visual acuity of 20/60 (6/18) i.e. if you had visual acuity of 20/60 or better in at least one eye, the WHO said you did not need glasses. However, someone with acuity of only 20/60 will fail the driving sight test in countries such as the UK; according to the UK government, such a person cannot drive safely without glasses. Most researchers would consider 20/60 too poor for a child in school to function in the classroom. (To be as fair as possible to the WHO/IAPB position, it is not so much that they said someone with visual acuity of only 20/60 does not need glasses, rather that they simply failed to say anything much at all about ordinary people who just need a pair of glasses; all the research papers and policy announcements and campaigns focused on the minority of people with the most severe cases of refractive error.)
More recently, the optical industry – especially leading suppliers of lenses for glasses such as Carl Zeiss Vision and Essilor – have endorsed much larger numbers for the population in need: about 2.5 billion globally. This is based on a definition of need that is more in accordance with the expectation of eye care professionals that most people should achieve “20/20 vision”. Influential researchers and public health officials are beginning to accept these larger numbers.
As I have discussed previously, the need for glasses for myopia depends on the functional context: the visual acuity needed to drive safely is less than that required to read the blackboard; the research simply has not been done to say what distance visual acuity (or other aspects of visual function such as field of view) are needed to perform different kinds of work or to have a fulfilling social life. The unmet need for glasses also depends on geographical context: the prevalance of myopia (defined as refractive error worse than -0.5 D) in sub-Saharan Africa is less than 10%, compared with about a quarter of the population in Latin America and about half in East Asia. The trend everywhere appears to be for an increasing proportion of the population to be myopic, starting in childhood and progressing especially during the teenage years.
The data do not allow for precise estimates and we have to make assumptions about what standard of acuity is “good enough”. However, it seems reasonable to state that about a billion people in the developing world, including more than 100 million children, need glasses for myopia – and few of them have access to conventional eye care services. These numbers are based on the assumptions that all school-aged children need to be able to read the blackboard in class (visual acuity of 20/25, or 6/7.5) and that all adults need to see clearly enough to be able to drive safely (20/40, or 6/12). Of course not everyone can afford a car, especially in the developing world, but this seems to be a reasonable criterion for “good enough” for a variety of work and social activities until we develop a better understanding of need in different populations. Future research should consider both objective need (how clearly must someone see to avoid typical hazards in the roads or to read text on a blackboard?) and perceived, subjective need (how clearly does someone in a given culture feel they need to see in a given social situation, weighed against the potential financial and aesthetic costs of wearing even the most affordable and fashionable glasses?). In addition to the billion people who need glasses to see in the distance, another billion or so presbyopic adults need glasses for reading.
In conclusion, a reasonable choice of assumptions leads to huge numbers: about 2 billion people need glasses and don’t have them. This lack of access to refractive vision correction is one the largest health problems in the world today. Yet the work of most international charities in the field of eye health is still focused on eye disease and the most severe cases of refractive error, not the much larger numbers of people who simply need a pair of glasses. The next question I will address is: Why?
This is the third in a series of articles by Chris Wray on the problem of lack of access to glasses in the developing world.