Sun protection advice based on UV index may not be correct
International health agencies recommend that no sun protection is necessary when the UV index is under 3. However, new data from a New Zealand NIWA research group published last month (www.nature.com/scientific reports) raised significant questions about the validity of this advice.
The issue is that the UV index value of 3 was chosen as the level of UV intensity which will lead a fair skin person to burn after one hour of exposure. This assumes no one spends more than one hour at a time outside, which is clearly flawed. Their data suggests that similar skin damage can be done from a long exposure at low UV dose, as is seen in short exposure at a higher UV dose.
This sits somewhat contrary to data about the body's ability to adapt to low dose sun exposure reported in this blog on 4 February 2017 below. The current study quotes recent evidence that there is a measurable increase in DNA damage from small increases in UV exposure even at low level. The February report however looked at a reduction in DNA damage with time, suggesting the body can adapt over a period of weeks if the exposure levels are low. The beauties and controversies of medical science!
They do raise another interesting observation. In winter, UV exposure to the face and neck is higher than the UV index would suggest. This is a result of the sun sitting lower in the sky, and therefore more dose hits these vertical surfaces than it does the horizontal surface that is used to measure the UV index.