Universal agreement on the definition of blindness has not been reached although in 1954 the World Council for the Welfare of the Blind urged the acceptance of the following definition: (a) total absence of sight; (b) visual acuity not exceeding 10/200 or 3/60 in the better eye with correcting lenses; (c) serious limitation in the field of vision (generally not greater than 20 degrees). The Council recognized that many persons with sight in the better eye equal to 20/200 (or 6/60) are still seriously handicapped visually and it strongly urged that whenever possible the definition of blindness be expanded to include all those with this degree of visual loss. In 1972 the World Health Organization proposed a uniform definition of blindness as visual acuity of less than 3/60 in the better eye, with best possible correction. A distinction is made between economic blindness which impedes the capacity to work and social blindness which limits educational possibilities and the ability to take care of personal needs and maintain social relations.
Causes of blindness can roughly be divided two broad categories: accidents and disease. Accidents are likely to affect all age groups. Loss of vision caused by disease may be linked to degenerative changes usually occurring late in life (for example glaucoma, diabetic retinopathy and vascular disease) particularly in industrialized countries. In less developed countries, infections and malnutrition (for example; vitamin A deficiency are the most important causes of loss of vision and they tend to manifest themselves during childhood or throughout the entire lifespan). The four leading diseases are xerophthalmia, cataracts, onchocerciasis (river blindness) and trachoma. At least two-thirds of the cases of blindness in the world could have been prevented or could be cured by the application of existing knowledge.
About 28 million people in the world are blind, that is unable to count fingers at 3 metres distance, and 42 million have suffered such severe visual impairment that they cannot count fingers at 6 metres; of these 42 million, an estimated 80% are in the developing countries. Of these, it is estimated that 30 million need not be blind. About 18 million of these suffer from cataracts. Around 10 million could have been prevented from going blind simply by public education about trachoma, xerophthalmia and leprosy.
The Southeast Asia region accounts for almost 50% of the world's blind. Of these, India alone has nine million. A total of about 500,000 pre-school age children develop corneal xerophthalmia due to malnutrition every year in Bangladesh, India, Indonesia and the Philippines. The incidence of milder forms of xerophthalmia is probably ten times higher. China has about 500,000 blind lepers, whose blindness could have been prevented by eyedrops.
A comparison of blindness rates between developing countries and highly industrialized, developed countries shows that the rates are consistently higher, often ten to twenty times in the former. An average representative blindness rate for a developed country with good medical facilities, including eye-health care, is approximately 0.1 - 0.2%, using the internationally accepted definition of blindness. The corresponding figure in many developing countries in Africa or Asia is 1 - 2% or even higher. The explanation for such pronounced differences is complex, depending both on the causes of blindness prevailing, and the preventive and curative measures that are being undertaken to combat blindness.
Two US studies in 1997 reported heavy cigarette smoking as a major cause of one form of progressive blindness; age-related macular degeneration. The condition, which afflicts an estimated 1.7 million people in the US, begins with a partial breakdown of an insulating layer between the retina and the blood vessels behind it and leads to blind spots developing in a person's vision. The researchers found that smokers had twice the chance of developing the condition than non smokers.