The study followed over 110,000 nurses over 20 years, using questionnaires.

It found that women who had five or more blistering sunburns between the ages of 15 and 20, compared to those that had none, were 80% more likely to develop melanoma (the most aggressive form of skin cancer).

Other risk factors included red hair colour, high sunburn reaction as a child/adolescent and sun bed use – all of which were found to be associated with an increased risk of all three types of skin cancer.

An unexpected result was that increased exposure to ultraviolet rays (radiation produced by the sun, as well as sunbeds and lamps) in adulthood was associated with an increased risk of non-melanoma forms of skin cancer (squamous cell carcinoma and basal cell carcinoma), but not melanoma.

However, this does not mean it is relatively safe to have high levels of UV exposure as an adult, as the UV exposure was not accurately measured in this study. Additional factors, such as how much time the women actually spent outside and if they exposed their skin to the sun, were not taken into account.

Skin cancer is one of the most common cancers in the UK, and this study confirms the necessity for taking precautionary measures to stay safe in the sun.

Where did the story come from?

The study was carried out by researchers from Brigham and Women’s Hospital and Harvard Medical School, and was funded by the Brigham and Women’s Hospital and grants from the National Institutes of Health.

The study was published in the peer-reviewed medical journal Cancer Epidemiology, Biomarkers and Prevention.

In general, the Mail Online covered the story accurately, but one of their headlines was misleading and potentially dangerous. They reported that “UV radiation exposure in later life does not affect melanoma risk”. Although the estimated UV exposure as an adult was not associated with melanoma in this study, there were major limitations in how the estimate was made, which was not addressed in the coverage.

Prolonged exposure to UV radiation, at any age, is not recommended. At best, it can cause premature ageing of the skin. At worst, it can increase the risk of developing skin cancer. Though this study only found an association with other non-melanoma types of skin cancer, UV exposure is also a well-established risk factor for melanoma.

What kind of research was this?

This was a cohort study of US nurses over 20 years. The researchers wanted to investigate the relationship between a number of potential risk factors and the likelihood of developing skin cancer.

An observational cohort study is appropriate for looking at the strength of a relationship between exposure and risk of developing a disease, but it cannot prove that the exposure causes the disease.

However, there is a wide body of evidence that suggests prolonged exposure to sunlight increases the risk of skin cancer.

What did the research involve?

The study followed 116,430 US nurses from 1989 to 2009. The researchers used health-related questionnaires every two years to look for associations between factors such as sun exposure, hair colour, lifestyle and development of skin cancer.

The nurses were aged between 25 and 42 at the beginning of the study. The amount of sun exposure they had over the course of the study was measured by “UV [ultraviolet] flux”.

There are two main types of UV: ultraviolet A (UVA) and ultraviolet B (UVB).

UV flux is an estimate of the amount of UVB and part of UVA waves that hit the earth’s surface, which takes cloud cover into account.

It is calculated for each state in the US using Robertson-Berger meters, which are electronic devices that measure UV radiation. The researchers estimated the amount of UV flux acquired by each woman over the length of the study using their address and accounting for changes of address. At the beginning of the study, the nurses lived in 14 different states; the researchers hoped this would capture different levels of exposure. They were then categorised into low, medium and high exposure.

The questionnaires included other potential risk factors, such as:

• number of moles on the legs
• sunburn reaction as a child/adolescent
• number of blistering sunburns between the ages of 15 and 20
• natural hair colour
• use of sunbeds
• family history of melanoma
• smoking and alcohol intake
• body mass index (BMI) and physical activity levels
• amount of night shifts
• menopausal status

If the women reported that they had squamous cell carcinoma (SCC) or melanoma, their medical records were reviewed to confirm the diagnosis. It was not deemed necessary to validate any report of basal cell carcinoma.

Women were excluded from the statistical analysis if they:

• were not Caucasian
• had any cancer at the beginning of the study
• had missing residence information
• did not report the type of skin cancer

What were the basic results?

Out of the 108,916 women:

• 6,955 developed basal cell carcinoma (BCC)
• 880 developed squamous cell carcinoma (SCC)
• 779 developed melanoma (445 had invasive melanoma, where the cancer has spread below the top outer layer of skin [epidermis]).

Women with a history of five or more blistering sunburns between the ages of 15 and 20, compared to those who had none, had:

• an 80% increased risk of melanoma (Relative Risk [RR] 1.80, 95% Confidence Interval [CI] 1.42 to 2.28)
• a 68% increased risk of SCC (RR 1.68, 95% CI 1.34 to 2.11)
• a 68% increased risk of BCC (RR 1.68, 95% CI 1.55 to 1.82)

Cumulative exposure to UV rays, adjusting for all of the other factors, found:

• no association with exposure to and risk of melanoma
• women in the highest fifth of exposure were more than twice as likely to develop SCC than those in the lowest fifth (RR 2.53, 95% CI 1.11 to 5.77)
• women in the highest fifth of exposure were more than twice as likely to develop BCC than those in the lowest fifth (RR 2.35, 95% CI 1.79 to 3.07)

Other factors that increased the risk of all types of skin cancer were red hair colour and high sunburn reaction as a child/adolescent, as well as sunbed usage. The risk of two or more types of skin cancer was increased by a family history of melanoma, the number of moles on a person’s legs and greater alcohol intake.

How did the researchers interpret the results?

They conclude that the “risks of BCC and SCC were associated with sun exposures in both adulthood and early life, whereas melanoma risk was predominantly associated [with] sun exposure in early life. Host factors, including red hair, sun reaction as a child/adolescent and number of blistering sunburns between ages 15 and 20 years of age, were strong predictors of all 3 types of skin cancer”.

Conclusion

This study has provided further evidence of the link between skin damage from sun exposure and the increased risk of all types of skin cancer.

Strengths of the study include the large size of the cohort and the fact that over 90% of the women were followed up for the whole 20-year duration of the research.

However, there were several limitations. The accuracy of using UV flux to determine exposure is debatable, as it only captures the level of UV rays that a woman living in that state could be exposed to. It does not measure how much skin exposure a woman has actually had. For example, it did not assess basic factors, such as the amount of time spent outdoors, whether the woman wears clothing or a hat to cover the skin while in the sun, the use of sun cream or the type and frequency of holidays in the sun.

The study only included Caucasian women, so it is unclear how applicable the results are to men and people of other ethnicities.

It was also reliant on accurate self-reporting and recall. Some of the women were 42 when the study started, and they may not have remembered how many times they had blistering of the skin from sunburn 27 years earlier.

Overall, the risk factors that are already known were associated with an increased risk of all three types of skin cancer.

The study also highlights the importance of young people being careful to avoid sunburn as, aside from being highly unpleasant, it could increase the risk of developing skin cancer, both in the short term and in later life.

It should be noted that this study does not change the existing advice for reducing the risk of skin cancer.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.

Links To The Headlines

Suffering sunburn five times increase skin cancer risk. The Daily Telegraph, June 2 2014

Getting badly sunburnt just FIVE times as teenager makes you 80% more likely to develop the most deadly type of skin cancer. Mail Online, June 2 2014

Links To Science

Wu S, Han J, Laden F, et al. Long-term Ultraviolet Flux, Other Potential Risk Factors, and Skin Cancer Risk: A Cohort Study. Cancer Epidemiology, Biomarkers & Prevention. Published online May 2014

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