Weak sunlight can still damage your DNA, scientists warn

The results challenge the long standing assumption that spending time outside early or later in the day is largely harmless.

Total UV Dose Matters

Study co-lead Professor Rachel Neale said the research was the first in Australia to examine what happens when human skin is exposed to levels of UV radiation that naturally occur at the Earth's surface.

"You can get the same dose of UV radiation in a short time in the middle of the day or a longer time earlier or later in the day. Our research has shown it is the total dose that is important - it doesn't matter how long it takes to get it," Professor Neale said.

"People potentially get lulled into a sense of security when the intensity of sunlight is weak and they spend too long outdoors without adequate protection. We have shown that is a problem for skin damage."

The Low-Dose UV Study included 58 people with light to olive skin. Researchers exposed an area of each participant's back to ultraviolet radiation at lower (e.g., mid-morning) and higher (e.g., lunchtime) UV intensities across several days.

They then collected biopsies to look for DNA damage and measured molecular and immune responses in the skin.

DNA Damage Without Visible Redness

The researchers were surprised to find signs of DNA damage regardless of whether the UV dose was delivered over a shorter or longer period.

The amount of UV exposure was not high enough to make participants' skin turn pink. Even so, the researchers detected measurable damage to DNA.

Sunlight plays an important role in vitamin D production and overall health, but it is also classified as a complete carcinogen, meaning it can both start and promote cancer. Current guidance recommends sun smart measures (slip, slop, slap, seek, and slide) when the UV index reaches 3 or higher.

Early Warning Signs in Skin Cells

The Low-Dose UV Study focused on molecular changes that can signal skin damage well before cancer develops.

Among the markers researchers examined was p53, a protein that rises when skin cells are stressed or when their DNA has been damaged. They also measured UV-induced DNA lesions.

Study co-lead Professor David Whiteman AM said the presence of skin damage does not mean cancer has already formed.

"Our lab results show these small, incremental doses of UV have caused some damage to the DNA in the skin cells, enough for the cells to then respond to the damage. Over months and years, these episodes of incremental, incidental UV exposure will have a cumulative impact on the skin and can lead to mutations that will initiate skin cancer," he said.

Rethinking What Counts as Safe Sun Exposure

The researchers emphasize that the findings are not a call for people to avoid sunlight altogether. Instead, they say sun safety recommendations should be updated to reflect biological evidence rather than long held assumptions.

"Our aim is to give Australians accurate, trustworthy guidance," they say. "That starts with understanding whether the sunlight we consider 'safe' is truly risk-free at the cellular level."

The findings also support making sunscreen part of a daily routine, especially to protect skin during short and unexpected periods of sun exposure that happen as part of everyday life.

The scientific paper is published in Photochemistry and Photobiology.