Lack of sunlight may increase the risk of lung cancer, suggests a study of rates of the disease in over 100 countries, published in the Journal of Epidemiology and Community Health.
Lung cancer kills over a million people every year around the globe.
The researchers looked at the association between latitude, exposure to ultraviolet B (UVB) light, and rates of lung cancer according to age in 111 countries across several continents.
They took account of the amount of cloud cover and aerosol use, both of which absorb UVB light, and cigarette smoking, the primary cause of lung cancer
International databases, including those of the World Health Organization, and national health statistics were used.
Smoking was most strongly associated with lung cancer rates, accounting for between 75% and 85% of the cases.
But exposure to sunlight, especially UVB light, the principal source of vitamin D for the body, also seemed to have an impact, the findings showed.
The amount of UVB light increases with proximity to the equator. And the analyses showed that lung cancer rates were highest in those countries furthest away from the equator and lowest in those nearest.
Higher cloud cover and airborne aerosol levels were also associated with higher rates of the disease.
In men, the prevalence of smoking was associated with higher lung cancer rates, while greater exposure to UVB light was associated with lower rates.
Among women, cigarette smoking, total cloud cover, and airborne aerosols were associated with higher rates of lung cancer, while greater exposure to UVB light was associated with lower rates.
The associations for a protective role for UVB light persisted after adjusting for smoking.
The link between cancer and sunlight is chemically plausible, say the authors, because laboratory research has shown that vitamin D can halt tumour growth by promoting the factors responsible for cell death in the body.
"Although cigarette smoking is the main cause of lung cancer, greater UVB exposure may reduce the incidence of the disease," they conclude.