Remembering just one number could help you prevent cancer and may even save your life, say researchers.

New fun-in-the-sun guidelines reveal the tremendous importance of the number “3” when it refers to UV Index levels. Always check the number in weather forecasts before heading outdoors, researchers advise — and take precautions if it’s “3’ or higher.

The UV Index — short for “ultraviolet index” — is a global method of measuring the intensity of the sun’s rays and their ability to cause sunburn. UV Index readings vary widely from day to day, season to season, and place to place.

They are increasingly included in TV, radio, and newspaper weather forecasts, and UV Index readings for any city can also be found online. What’s more, apps are easily downloaded to disclose current UV readings.

Summer triggers a worrying health dilemma for many Americans. Sun exposure causes skin cancer — with melanoma the deadliest of the disease’s various forms — but the sun is also the main source of vitamin D, essential for calcium absorption and bone growth.

Researchers report that people who cover up the most against sunlight — often for religious or cultural reasons — or who spend most of their time indoors have the greatest vitamin D deficiencies.

The latest "3" UV recommendations come from a collaborative research effort by Cancer Council Australia and some of that country’s leading medical organizations.

Australia has the highest incidence of skin cancer in the developed world with one in two men and one in three women diagnosed with the disease by age 70. The country also has some of the highest UV Index levels. As a result, skin cancer is a major research priority down under and high-profile public education campaigns are commonplace.

But skin cancer rates have been rising everywhere around the world, including the U.S., which has seen a threefold increase in cases since the 1970s.

According to Cancer Council Australia, the new guidelines aim to tell people “how they can balance the need for sun protection to reduce skin cancer risk while maintaining healthy Vitamin D levels for optimal health.”

The council reveals a worrying research finding: 15 percent of adults adjusted their sun protection levels in recent summers, aiming to get more vitamin D, even though they aren’t vitamin D deficient.

Researcher Dr. Rebecca Mason of Sydney University, a former president of the Australian and New Zealand Bone and Mineral Society (which collaborated with the Cancer Council), maintains “sun protection isn’t required when the UV Index is below 3.”

Cancer Council Australia spokesman Craig Sinclair adds that most people maintain adequate vitamin D levels “just from doing typical day-to-day activities.”

Dr. Peter Foley, of the Australasian College of Dermatologists (which helped formulate the guidelines), adds: “Prolonged sun exposure does not cause vitamin D levels to continue to increase but it certainly does increase the risk of skin cancer.”

The U.S. Environmental Protection Agency confirms that “a UV Index reading of 0 to 2 (‘low’) means “low danger.” According to the EPA, a UV Index of 3 to 5 is “moderate,” 6 to 7 is “high,” 8 to 10 is “very high,” and 11 or more is “extreme.”

“Skin cancer protection should remain a priority even for those with vitamin D deficiency,” Cancer Council Australia notes. “Deliberate and extended unprotected sun exposure when the UV Level is 3 or above isn’t recommended.

“Sun protection — including hats, sunscreen, clothing, shade, and sunglasses — should be used when the UV Level is 3 or above when heading outdoors for more than a few minutes.”

The council adds that to get a sufficient amount of vitamin D requires only “a few minutes of mid-morning or mid-afternoon sun exposure to arms and hands on most days of the week.”

In addition, those at risk of vitamin D deficiency should talk to a medical practitioner to determine whether supplementation rather than sun exposure is appropriate.