Many researchers have looked at the relationship between DNA damage and repair and cancer. Some cancers result from uncorrected gene mutations, some of which are inherited. Others inherit a susceptibility to damage by certain environmental agents (For example, albinos have inherited an absence of skin and hair pigments; they absorb so much sunlight and ultraviolet-light, which in turn causes DNA damage.).

Some cancer may be the result of inadequate DNA repair.¹⁷ Researchers in Texas have linked a reduction in DNA repair capacity to lung cancer. They found that smokers with lung cancer have a DNA repair capacity five times lower than healthy controls.¹⁸

Certain genetic diseases that feature diminished DNA repair capacity are associated with high rates of cancer. For example, patients with xeroderma pigmentosum, an inherited disorder resulting in reduced DNA repair, have a predisposition to skin cancer.¹⁹

DNA damage and inadequate repair have also been linked to other chronic diseases of aging. One group of Italian researchers has suggested that DNA damage from outside toxins will cause either cancer or chronic disease, depending on what cells it affects.
Looking primarily at cigarette smoke as a toxin, these scientists point out that smoking generates chemicals that damage DNA bases. This results in large disruptive molecules called DNA adducts. The researchers postulate that when these adducts occur in cells that divide and reproduce throughout our lives (such as the cells that line our lungs), these cells can become cancerous.

If, however, the DNA adducts collect in cells that do not divide, such as pneumocytes, the key cells in our lungs, the accumulated damage will induce chronic diseases, for example, emphysema.²⁰ Other researchers would argue that the process that leads to emphysema in humans is quite complex²¹, but the Italian scientists suggest that DNA damage may be the first step in this debilitating condition.