Focusing on Vision Through an Environmental Lens

Our eyes are our window to the world, but for many people the view becomes dim or even darkens entirely due to visual impairment. Although the full impact of the environment on sight is unknown and significant gaps remain in our understanding of vision disorders, many reports have shown that low vision and blindness can be directly or indirectly related to environmental exposures. Vision is described in terms of visual acuity and field of vision. Visual acuity is a measure of how well an individual sees compared with someone with normal sight—for example, a person with 20/60 vision must be within 20 feet of an object to see it as clearly as a normal-sighted person at 60 feet—and a normal field of vision is 160 to 170 degrees. The World Health Organization (WHO) estimates that approximately 124 million people have low vision, which it defines as visual acuity between 20/60 and 20/400 with the best possible correction or a visual field of 20 degrees or less. Another 37 million people meet the WHO definition for blindness, which is visual acuity that cannot be corrected to better than 20/400 or a visual field of 10 degrees or less. An analysis published in the April 2004 Archives of Ophthalmology by the Eye Disease Prevalence Research Group, a consortium representing several institutions, indicates that low vision or blindness affects 3.3 million Americans over age 40 and predicts that this figure may be as high as 5.5 million by 2020. (In this study, low vision was defined as visual acuity between 20/40 and 20/200 with best correction, while blindness was defined as visual acuity of 20/200 or worse with best correction.) According to the WHO, the rising trend is likely to be seen globally as well. An expanding population explains some of the increase, but more critically, the fastest-growing population sector comprises people older than 50. Worldwide, more than 80% of people who are blind are 50 or older, although they represent only 19% of the world’s population. Gender is also significantly associated with visual impairment. Women represent two-thirds of those with blinding eye disease, even after controlling for women’s longer life spans. Risk also varies by race, ethnicity, and world region. Socioeconomic development often predicts regional prevalence of a disorder. Of the approximately 1.4 million children with blindness in the world, about 75% live in high-poverty areas in Asia and Africa. Among children in high- and middle-income countries, optic nerve defects, other neurological problems, and retinopathy of prematurity (a consequence of incomplete eye development) are the most common causes of blindness. Developing nations are disproportionately affected due in large part to the burdens associated with poverty: lack of clean water and sanitation, limited or nonexistent health care, and malnutrition. Among children in low-income countries, vision problems arise mostly from complications of measles or rubella, nutritional deficiency, improper or inadequate treatment, and eye infections in the first days of life. In Tibet, an area with one of the highest prevalences of cataract, a lack of vitamin A is compounded by exposure to high-altitude ultraviolet (UV) light, soot and pathogens from indoor burning of coal and yak dung, and a dusty, windy environment. As a result, 10.9% of the total Tibetan population suffers visual impairment.