A trial is under way to investigate the health benefits and risks of spending time in the sun.
Cancer Research UK says it hopes its study will help solve years of controversy surrounding vitamin D, known as the sunshine vitamin, and how much sun is too much.
The amount of sun exposure needed to make enough vitamin D varies from person to person, largely dependent on skin colour.
But at the moment there are no clear guidelines about what level of sun exposure is needed to produce enough vitamin D without causing excessive DNA damage to the skin, which can cause skin cancer.
The trial will use simulated sunlight to determine how much sun is needed for different skin types.
Study leader Professor Lesley Rhodes, from the University of Manchester, said: "There's no doubt that too much sun can seriously increase skin cancer risk, but we know there are also important benefits to going out in the sun, such as making enough vitamin D.
"This study is about finding that balance, so we can offer people of all skin colours the best possible advice about how much sun is safe for them."
Seventy-five healthy volunteers aged 18-45, and with a range of skin colours, are being recruited to take part in the study in the Photobiology Unit, Dermatology Centre at Salford Royal in Greater Manchester.
Participants will be exposed to simulated sunlight - equivalent to that of a summer's day in Manchester - for short periods of time.
Blood and urine samples will be taken after exposure and examined for chemicals that indicate vitamin D levels and DNA damage.
Small skin samples will also be taken from some volunteers and analysed for any signs of DNA damage.
Kalani Hargrove, who is studying sociology at the university, is one of the first volunteers to take part in the trial.
"Before I signed up for the study I was given an information sheet that explained everything.
"Taking part has been really simple and straightforward so far and it's been fascinating to see first hand how medical research is carried out.
"I'm delighted to be taking part in research that will hopefully lead to a better understanding of how people of all skin colours can enjoy the sun safely."