visible light is maybe linked to macular degeneration. However, significant daily exposure to bright light may be necessary for children to avoid myopia (nearsightedness). Short-term over-exposure can cause snow blindness, which is analogous to sunburn of the cornea, or can cause solar retinopathy, which is long-lasting retinal damage and vision impairment from sungazing. Frequent exposure to the sun can cause yellow non-cancerous bumps on the middle part of the sclera of the eye, called pingueculae. It is most common in younger people, mainly those who spend a lot of their time outdoors and do not protect their eyes from UV rays. To decrease the risk of developing pingueculae, it may be wise to wear sunglasses when outdoors, even on overcast days. Folate degradation Blood levels of folate, a nutrient vital for fetal development, can be degraded by UV radiation, raising concerns about sun exposure for pregnant women. Lifespan and fertility can be adversely affected for individuals born during peaks of the 11-year solar cycle, possibly because of UV-related folate deficiency during gestation. Safe level of sun exposure According to a 2007 study submitted by the University of Ottawa to the US Department of Health and Human Services, there is not enough information to determine a safe level of sun exposure that imposes minimal risk of skin cancer. In addition, there is not yet conclusive evidence on which components of ultraviolet radiation (UVA, UVB, UVC) are actually carcinogenic. UVC is almost completely absorbed by the atmosphere and does not reach the surface in any appreciable quantity. As a result, only the broad-spectrum combination (UVA, UVB, UVC) known as "ultraviolet radiation" is listed as a carcinogen; the components are only "likely to become" known carcinogens. Solar radiation (sunlight) and sunlamps are listed as carcinogens because they contain ultraviolet radiation. Lifetime sun exposure There are currently no recommendations on a safe level of total lifetime sun exposure. According to epidemiologist Robyn Lucas at Australian National University, analysis of lifespan versus disease shows that far more lives worldwide could be lost to diseases caused by lack of sunlight than to those caused by too much, and it is inappropriate to recommend total avoidance of sunlight. Over thousands of years, in many climate zones, genetic selection has helped indigenous human populations adapt toward skin pigmentation levels that provide a healthy level of UV exposure. This largely explains the tendency toward darker-skinned populations in the sunniest