Sunlight can protect people from lung cancer, reported The Daily Telegraph on December 18 2007. A study has shown that “after smoking, restricted access to ultraviolet light from the sun is one of the most important causes of the disease”, the newspaper said.
The Times said that the “closer you live to the equator the lower your chance of getting lung cancer”. Rates were highest in countries furthest from the equator, where exposure to sunlight is lowest and the reason may be “that exposure to more sunlight increases the production of vitamin D in the skin”, the newspaper explained.
The story is based on a study that looked at the rates of lung cancer across 175 countries. However, the researchers did not measure vitamin D concentrations in individuals. This type of study can suggest theories about why cancer rates differ between countries, but detailed studies using individual data and measuring risk factors are needed to demonstrate exactly how UV exposure to the skin can protect the lung.
Where did the story come from?
Dr Sharif Mohr and colleagues mostly from the Department of Family and Preventive Medicine at the University of California, San Diego, carried out this research. The study was funded by a congressional allocation through the Department of the Navy and it was published in the peer-reviewed medical publication: Journal of Epidemiology and Community Health .
What kind of scientific study was this?
This was an ecological study which made comparisons of lung cancer rates based on geographical data. The researchers plotted the rates of lung cancer against the distance from the equator (latitude) for 175 countries. They then used statistical modelling to look for the associations between latitude, and the level of ultraviolet (UV)-B radiation and the rates of cancer, adjusted for age in 111 of these countries. Other factors such as cloud cover and cigarette smoking were also assessed (independently of each other), using the statistical modelling technique of multiple regression.
The database of the International Agency of Research on Cancer provided age-adjusted rates of lung cancer in 2002. Age adjustment means that the number of new lung cancers occurring in each country (per 100,000 population) was altered to take into account the differences in the age ranges of each of the countries, allowing them to be compared fairly. Information about cigarette smoking was only available for 111 of the 175 countries. The researchers used the smoking data from the years 1980 to 1982, with the assumption that as it is the most important cause of lung cancer, the rates of smoking would be linked strongly with rates of lung cancer 20 years later.
The results were reported as the R² statistic (the coefficient of determination) which represents the "strength" or "magnitude" of the relationship between the factors studied (in this case UVB exposure, cloud cover, atmospheric sulphate or cigarette consumption). A very strong association would have a R² statistic close to 1.0 and a weak association will have a R² statistic close to zero.
What were the results of the study?
The researchers showed that latitude was positively related to the incidence rates of lung cancer in men (R² = 0.55) and women (R² = 0.36). In this analysis, the closer that the R² statistic is to 1 the stronger the relationship that has been demonstrated.
In men, cigarette consumption was positively related to risk, (R² = 0.71) and the level of UVB radiation was inversely associated, that is, the higher the exposure to UVB the lower the risk of lung cancer. There were other positive associations with the environmental features that absorb UVB, in particular cloud cover (R² = 0.49) and aerosol optical depth (a measure of sulphate) in the atmosphere (R² = 0.23). When the researchers used more advanced modelling techniques to take account of all four factors they found that the four factors accounted for much of the relationship (R² = 0.78).
The researchers noted some differences in the model for women; the level of UVB radiation was inversely associated with incidence rates (as it was for men), but cigarette consumption, total cloud cover and aerosol optical depth showed less of an association. As with men, the model showed that these four factors were strongly linked to lung cancer risk (R² = 0.77), with smoking contributing the most to the risk (R² = 0.66).
What interpretations did the researchers draw from these results?
The researchers say that lower levels of UVB radiation were independently associated with higher rates of lung cancer in 111 countries.
What does the NHS Knowledge Service make of this study?
Ecological studies and the graphs produced by plotting countries on graphs of latitude against incidence of cancer are intuitively understandable. However, the researchers alert us to some drawbacks to this type of study.
- Smoking is by far the most important cause of lung cancer and the proportion of cancers that could be prevented by removing this cause alone is quoted as 75% to 85%. The authors have tried to investigate the causes that account for the remaining 15% to 25%. Their models however, still show only a small contribution of UV exposure to the overall association.
- All individuals within the countries listed as having high levels of UVB radiation may not necessarily have experienced high exposure, as the effects of UVB may be tempered by urbanisation or industrialisation. This is likely to have reduced the association demonstrated in this study.
- Other factors, such as diet and physical activity levels or socioeconomic status were not included in the models and it is unclear how far these may account for some of the differences between countries.
This study of the relationships between disease and risk factors at a country level provides an indication that there may be value in investigating the actions of UV light and vitamin D on an individual level, further studies will be needed to achieve this.
Sir Muir Gray adds...
This is no antidote to cigarette smoking, and of course sunlight carries its own cancer risk.