Cataracts form when proteins inside the lens of your eye break down and clump together, creating cloudy patches that scatter light instead of letting it pass through. This process happens to nearly everyone eventually. By age 80, roughly 31% of people have cataracts, and the number climbs from there. But aging isn’t the only cause. Diabetes, medications, eye injuries, UV exposure, smoking, and even genetics can all trigger or accelerate the process.
What Happens Inside the Lens
Your eye’s lens is made almost entirely of water and specialized proteins called crystallins. These proteins are arranged with extraordinary precision, packed tightly enough to stay transparent while bending light onto the retina. Unlike most cells in your body, the lens has no blood supply and almost no ability to replace damaged proteins. The crystallins you’re born with are largely the ones you keep for life.
Over decades, these proteins accumulate damage from UV light, oxidation, and normal chemical wear. The damage causes them to partially unfold, exposing sticky regions that are normally tucked inside. These exposed regions latch onto neighboring proteins, forming larger and larger clumps. Eventually, the clumps grow big enough to scatter incoming light instead of transmitting it cleanly. That scattering is what makes the lens look cloudy and your vision look hazy, washed out, or blurred.
The process is slow. Protein damage builds up over years or decades before it reaches the point where you notice any change in your vision. This is why cataracts are overwhelmingly a condition of older adults, though the underlying damage starts much earlier.
Age: The Biggest Risk Factor
Age-related cataracts account for the vast majority of cases. In a large U.S. study, cataract prevalence was about 14% among people in their 60s, 25% in their 70s, and 31% among those 80 and older. These numbers likely undercount mild cases that haven’t yet caused symptoms. The protein damage that drives cataracts is cumulative and, to some degree, inevitable. How quickly it progresses depends on everything else on this list.
UV Light and Sun Exposure
Ultraviolet radiation directly damages lens proteins. Research from the National Eye Institute shows that UV light triggers a specific type of chemical damage called glycation, the same kind of protein modification seen in cataracts and oxidative stress. UVA rays, which penetrate deeper into the body than UVB, are particularly concerning because they can reach the lens more easily.
The damage works through a chain reaction. UV light activates certain amino acid byproducts already present in the lens, which then modify surrounding proteins in ways that promote clumping. This is why years of sun exposure without eye protection increases cataract risk, and why cataracts are more common in populations living closer to the equator.
Diabetes and High Blood Sugar
People with diabetes develop cataracts earlier and more frequently than the general population. The mechanism is distinct from normal aging. When blood sugar is high, excess glucose floods into the lens and gets converted into a sugar alcohol called sorbitol. Sorbitol doesn’t easily leave the lens, so it accumulates and draws water in through osmosis, causing the lens to swell.
That swelling alone can distort vision, but the deeper problem is what happens to the lens’s antioxidant defenses. The chemical reactions that produce sorbitol use up the same molecules the lens needs to neutralize free radicals. With those defenses depleted, oxidative damage to crystallin proteins accelerates. The result is a faster path to the same protein clumping that causes age-related cataracts, just on a compressed timeline. Poorly controlled blood sugar makes this worse.
Steroid Medications
Long-term use of corticosteroids is one of the most well-documented medication-related causes of cataracts. These drugs, prescribed for conditions like asthma, autoimmune diseases, and certain cancers, can cause a specific type of cataract that forms at the back of the lens.
The risk depends heavily on dose, duration, and how the medication is taken. Oral steroids carry the highest risk, followed by topical eye drops and then inhaled forms. In studies of children treated with oral steroids for kidney disease, about 18% developed cataracts after an average of four years of treatment. Among adults with multiple myeloma on steroid-containing regimens, 36% developed cataracts within six treatment cycles. Even inhaled corticosteroids used for COPD showed nearly 40% cataract prevalence at higher doses after more than a year of use, though low doses showed no significant risk.
The risk rises with cumulative exposure. Treatments lasting longer than six months and higher total doses are the strongest predictors.
Smoking and Alcohol
Smoking significantly raises your risk of cataracts. A large meta-analysis pooling data from studies worldwide found that people who have ever smoked are about 40 to 60% more likely to develop age-related cataracts than people who never smoked. The strongest link is with nuclear cataracts, the type that forms in the center of the lens, where smokers had roughly 66% higher odds. Current smokers face higher risk than former smokers, suggesting that quitting offers some protection.
The mechanism is straightforward: cigarette smoke introduces a heavy load of free radicals into the body, overwhelming the lens’s limited antioxidant defenses and speeding up the same protein damage that drives age-related cataracts. Heavy alcohol consumption has also been identified as a risk factor through similar oxidative pathways, though the evidence is less robust than for smoking.
Eye Injuries
A hard blow to the eye or a penetrating injury can cause a cataract at any age. If a sharp object like glass or metal pierces the eye and reaches the lens, a cataract can form within minutes to hours. When the lens capsule (its thin outer membrane) is breached, fluid rushes in and the proteins rapidly lose their organized structure.
Blunt trauma, like a ball or fist striking the eye, works differently. The lens capsule may stay intact, but the shockwave disrupts the protein arrangement inside. These concussion cataracts often develop slowly, sometimes appearing months or even years after the original injury. The delay can make it hard to connect the cataract to the event that caused it.
Radiation Exposure
Ionizing radiation, the type used in medical imaging and cancer treatment, can damage the lens. For decades, the assumed safe threshold was a cumulative dose of about 2 gray (a unit of absorbed radiation). But a 20-year study of U.S. radiologic technologists found that cataracts developed at doses well below that level, suggesting the true threshold is substantially lower than previously thought. This is primarily a concern for people who receive radiation therapy near the head or who have prolonged occupational exposure.
Genetics and Cataracts at Birth
Some people are born with cataracts or develop them in childhood. Genetic mutations are the leading cause of cataracts present at birth (congenital cataracts), particularly when both eyes are affected. The most common inheritance pattern is autosomal dominant, meaning a child needs only one copy of the gene from one parent to be affected. This accounts for about 44% of familial cases. The mutations typically affect the same crystallin proteins involved in age-related cataracts, or the structural and membrane proteins that keep the lens organized.
Several chromosomal conditions also carry cataract risk. Down syndrome (an extra copy of chromosome 21) and Patau syndrome (an extra copy of chromosome 13) both feature cataracts among their clinical signs. Rarer genetic syndromes like Lowe syndrome, Nance-Horan syndrome, and myotonic dystrophy type 1 include cataracts as a hallmark feature.
Infections during pregnancy can also cause congenital cataracts. Rubella is the most common infectious cause worldwide, though cytomegalovirus, herpes simplex, and toxoplasmosis can also damage the developing lens.
Diet and Protective Factors
Your diet may influence how quickly lens proteins break down. Lutein and zeaxanthin, two pigments found in leafy greens, eggs, and corn, accumulate in eye tissue and act as both antioxidants and filters for damaging light wavelengths. In the Melbourne Visual Impairment Project, people with the highest dietary intake of these pigments had roughly 40% lower odds of developing nuclear cataracts compared to those with the lowest intake. The protective effect was specific to nuclear cataracts and didn’t extend to other types.
The top dietary intake group consumed just over 1 milligram of lutein and zeaxanthin per day, an amount easily achieved by eating a serving of spinach, kale, or broccoli several times a week. Wearing UV-blocking sunglasses, not smoking, and managing blood sugar if you have diabetes are the most actionable steps for slowing the protein damage that leads to cataracts.