Cataracts form when proteins inside the lens of your eye break down and clump together, creating cloudy patches that block light from reaching the back of your eye. Over 100 million people worldwide had cataracts in 2021, nearly 138% more than in 1990, and that number is projected to keep rising. While aging is the most common trigger, cataracts can also result from UV exposure, diabetes, medications, eye injuries, genetics, and prior eye surgery.
How Proteins Break Down Inside the Lens
Your eye’s lens is made of tightly organized, transparent proteins called crystallins. One type in particular acts as a kind of molecular bodyguard: it catches other proteins that start to unfold or lose their shape and wraps them into stable clusters before they can clump into visible masses. This protective protein works by grabbing onto unstable proteins early in the breakdown process, using a flexible outer region to keep everything dissolved in the watery interior of the lens.
Over time, this protective system wears out. The bodyguard proteins themselves become damaged through years of oxidation and chemical changes, losing their ability to prevent clumping. Once that protection fails, damaged proteins begin aggregating into larger and larger masses that scatter light instead of letting it pass through. That scattering is what you experience as cloudy or blurred vision. Because the lens has no blood supply and doesn’t shed old cells the way skin does, these damaged proteins accumulate over a lifetime with no way to clear them out.
Aging: The Most Common Cause
Most cataracts are age-related, developing gradually after age 40 and becoming visually significant by the 60s or 70s. The same chemical modifications that degrade lens proteins, including oxidation, loss of water solubility, and structural changes to amino acids, simply accumulate year after year. The lens also grows throughout life, adding new layers of fiber cells on top of old ones, which compresses the center of the lens and makes it denser and more yellow over time. This is why early cataracts often cause colors to look washed out or slightly brownish before noticeable blurriness sets in.
UV Light and Sun Exposure
Ultraviolet radiation directly damages the protective proteins in your lens. Both UV-B light (the type that causes sunburns) and UV-A light (which penetrates deeper into the eye) contribute. UV-B wavelengths between 280 and 308 nanometers gradually strip lens proteins of their ability to prevent clumping. UV-A light at 365 nanometers causes its own damage by chemically altering the protein’s structure, breaking off pieces of its protective outer region and triggering modifications that change how the protein behaves.
These UV-driven changes don’t just harm individual proteins. They can also affect the structural scaffolding inside lens cells, weakening the architecture that keeps the lens clear. Research published in Investigative Ophthalmology & Visual Science showed that after UV-A exposure, the lens’s genetic instructions for making protective proteins temporarily degraded, taking more than 24 hours to restart production. People who spend significant time outdoors without UV-blocking sunglasses face a measurably higher risk of developing cataracts earlier in life.
Diabetes and High Blood Sugar
Elevated blood sugar creates a distinct pathway to cataracts. When glucose levels in the blood rise, excess sugar enters the lens. Normally the lens processes glucose through its standard energy pathway, but when sugar floods in, the lens activates a backup route that converts glucose into a sugar alcohol called sorbitol. Unlike glucose, sorbitol gets trapped inside the lens and can’t easily escape.
As sorbitol and another sugar (fructose) build up, they pull water into the lens through osmotic pressure, the same force that makes a dried sponge swell when placed in water. The incoming water causes lens fibers to swell, distorting the carefully organized cell structure that makes the lens transparent. The result is opacification, sometimes developing years earlier than typical age-related cataracts. People with diabetes are two to five times more likely to develop cataracts, and poorly controlled blood sugar accelerates the timeline significantly.
Steroid Medications
Corticosteroids, whether taken as eye drops, oral pills, or inhaled for asthma, can cause a specific type of cataract that typically forms at the back of the lens. The risk is directly tied to dosage. Research from the American Academy of Ophthalmology found that for patients using steroid eye drops, those taking more than three drops daily had a cataract incidence rate 16 times higher than those using three drops or fewer. At two drops or fewer per day, the rate dropped to effectively zero.
This dose-dependent relationship means that short courses or low doses carry much less risk than long-term, high-dose steroid use. If you take steroids regularly for conditions like asthma, arthritis, or autoimmune diseases, periodic eye exams can catch steroid-related lens changes early.
Eye Injuries
A hard blow to the eye, a puncture wound, or any trauma that disrupts the lens capsule (the thin membrane surrounding the lens) can cause a traumatic cataract. When the capsule ruptures, fluid from inside the eye rushes into the lens, hydrating the normally compact fibers and causing them to swell and turn opaque. This can happen within minutes to hours of the injury.
Blunt trauma without capsule rupture can also trigger cataracts, though these develop more slowly, sometimes appearing weeks or even years after the original injury. A characteristic flower-petal pattern on the lens is a hallmark of blunt-force cataract formation. Even seemingly minor injuries can set this process in motion, which is why protective eyewear matters during sports and high-risk activities.
Prior Eye Surgery
Certain eye surgeries, particularly vitrectomy (a procedure used to treat retinal problems), substantially increase your risk of developing cataracts afterward. A large UK registry study of over 2,000 eyes found that about 40% needed cataract surgery following vitrectomy. The progression is often rapid: 50% required cataract surgery within one year, rising to 70% by two years and 85% by five years.
The reasons include disruption of the lens’s environment during surgery, increased oxygen exposure to the lens (which accelerates protein damage), and inflammation. If you’ve had retinal surgery, cataract development is something to plan for rather than be surprised by.
Genetics and Cataracts at Birth
Not all cataracts are acquired. About 1 in 5 cases of congenital cataracts (those present at birth) have a family history of the condition, and recent research suggests genetic causes account for the majority of cases where both eyes are affected. Cataracts can also appear as part of chromosomal conditions like Down syndrome.
Infections during pregnancy are another cause of congenital cataracts. The infections most strongly linked to this risk include rubella (German measles), toxoplasmosis (a parasitic infection), cytomegalovirus, chickenpox, and herpes simplex virus. Rubella was historically the most significant cause, though widespread vaccination has made it rare in many countries. These infections can interfere with normal lens development during critical stages of fetal growth.
Other Contributing Factors
Smoking accelerates protein damage in the lens through direct oxidative stress, roughly doubling or tripling cataract risk depending on how heavily and how long someone smokes. Heavy alcohol use has a similar though somewhat weaker association. Obesity increases risk partly through its connection to diabetes and chronic low-grade inflammation. Radiation therapy to the head or upper body can damage the lens, with cataracts sometimes developing months to years after treatment.
Severe nearsightedness (myopia) is independently associated with earlier cataract formation, likely because the structural differences in highly myopic eyes alter the lens’s metabolic environment. Nutritional deficiencies, particularly in antioxidants like vitamins C and E, may remove a layer of protection against the oxidative damage that drives protein clumping, though the evidence for supplements preventing cataracts remains mixed.