Cardiovascular and cancer mortality inversely associated with long-term ambient ultraviolet exposure across US Health Service Areas with UK Biobank corroboration

For more than a century, scientists have observed a puzzling contradiction in how sunlight affects human health. We know that too much sun, particularly the kind that causes a burn, increases the risk of skin cancer. Yet, large studies have long hinted that people living in sunnier parts of the world tend to die less often from heart disease and internal cancers. For a long time, the leading explanation was that sunlight helps the body make vitamin D, and that this vitamin protects the heart and fights cancer. However, when researchers tested this idea by giving people vitamin D pills in large, controlled trials, the pills did not lower death rates from these diseases. This failure suggested that the benefit of sunlight might come from something else entirely—something that happens when the skin is exposed to the full spectrum of light, rather than just the chemical produced inside the body.

A new study has now mapped this relationship across the entire United States, looking at how long-term exposure to ambient ultraviolet radiation correlates with death rates from heart disease and cancer. The researchers did not rely on individual habits or self-reported sunbathing; instead, they looked at the actual sunlight hitting the ground in different regions and compared it to the number of deaths recorded in those same areas. By analyzing data from over 52 million older adults for heart disease and over 115 million for cancer, they found a clear pattern: areas with higher levels of ultraviolet light consistently had lower death rates from these two major causes. The study suggests that for every significant increase in sunlight exposure across a region, thousands of deaths could be prevented annually. This finding holds true even after accounting for poverty, education, and other social factors that usually influence health outcomes.

The researchers focused on Health Service Areas, which are groups of counties that share a local hospital market, allowing them to compare regions with different amounts of sunlight while keeping the population size large enough to get reliable numbers. They used data from 2018 and 2019, combining it with measurements of ultraviolet radiation collected over the previous two decades. To ensure the results were not simply a reflection of wealth or access to care, they adjusted their calculations for the Social Vulnerability Index, a measure of how well a community can cope with stressors like economic hardship or lack of resources. Even with these adjustments, the link between sunlight and lower mortality remained strong.

The results showed that higher ultraviolet exposure was associated with a significant drop in deaths from cardiovascular disease. In the regions studied, an increase in sunlight equivalent to the difference between Boston and Atlanta was linked to roughly 36,000 fewer heart-related deaths per year. Similarly, for cancer, the same increase in sunlight was associated with nearly 50,000 fewer cancer deaths annually. The protective effect was most pronounced for lung cancer, which showed the strongest link to reduced mortality, followed by cancers of the breast, pancreas, and colon. The study also looked at specific types of cancer and found that ten out of eighteen specific cancer sites showed a protective association with sunlight, while only one, melanoma, showed the expected harmful link.

To verify these findings, the researchers compared their US data with a separate, independent study of over 400,000 people in the United Kingdom. In that cohort, individuals who lived in areas with higher residential sunlight exposure also had lower risks of dying from heart disease and cancer. The patterns matched closely: in both countries, more sunlight correlated with fewer deaths from the leading causes of mortality. This consistency across two very different populations and study designs strengthens the idea that the relationship is real and not just a statistical fluke.

However, the story is not the same for every group of people. When the researchers broke down the US data by race and ethnicity, the patterns became more complex. The protective effect of sunlight was clear for non-Hispanic White and non-Hispanic Black populations, but the results for Hispanic populations were different. In this group, higher sunlight exposure did not show the same protective benefit and, for some cancers, appeared to be associated with higher death rates. The researchers noted that this difference did not follow a simple gradient based on skin color, as one might expect if the mechanism were purely about how much vitamin D the skin could produce. Instead, they suggested that other factors, such as occupational exposures, environmental risks, or social determinants not fully captured by their data, might be influencing the results for this specific group.

The study explicitly challenges the idea that vitamin D supplements are the key to these health benefits. Since previous large trials of vitamin D pills failed to reduce death rates, the authors argue that the benefit must come from the sunlight itself acting on the skin in ways that a pill cannot replicate. They propose that sunlight may trigger a series of biological responses, such as the release of nitric oxide to improve blood flow and lower blood pressure, or changes in the immune system that help the body fight off cancer cells. These mechanisms would explain why the benefit is seen with sunlight exposure but not with vitamin D supplements alone.

While the study provides strong evidence of a link, the authors are careful not to claim that more sun is always better. They emphasize that the protective effect is associated with regular, non-burning exposure, whereas intermittent, intense exposure that causes sunburn remains a known risk for skin cancer. The findings suggest that the relationship between sunlight and health is nuanced, depending on the type of exposure and the specific disease. The study concludes that while the data points to a significant survival benefit from moderate sunlight, it does not provide a specific prescription for how much sun an individual should get. Instead, it highlights a need to rethink the balance between the risks of sunburn and the potential systemic benefits of regular, moderate exposure, and to explore whether controlled light therapy could one day be used to improve outcomes for heart and cancer patients.