WASHINGTON — Sunscreen with an SPF of 30 has been found to delay the onset of skin cancer in mice, a pair of scientists says.

The two researchers from Ohio State University say that they’ve tested various SPF30 sunscreens on genetically engineered mice. They have skin similar to humans, and after they’re exposed to a chemical and ultraviolet-B rays they’re prone to develop melanoma whether they’re sunburned or not.

The researchers found, however, that the onset of melanoma was delayed, and tumors reduced, when SPF30 sunscreen was applied in between the chemical and the exposure to the light.

Sunscreens are sold as cosmetic products, so the government only tests them for their safety and their ability to protect from sunburn. Ultraviolet sunlight is a major risk factor for melanoma, and sunscreens help prevent burning from that ultraviolet light. But whether sunscreens directly prevent melanoma hasn’t been known. And some people develop melanoma without ever getting sunburned.

“Over the past 40 years, the melanoma incidence rate has consistently increased in the United States,” said Christin Burd, an assistant professor at Ohio State and the principal investigator in the study.

The mouse model “allows us to test the ability of a sunscreen to not only prevent burns but also to prevent melanoma,” she said.

“This is a remarkable accomplishment. We hope that this model will lead to breakthroughs in melanoma prevention.”

The study isn’t definitive: The mice were exposed to about a week’s worth of UVB light, which is much more than a dose of sunscreen can handle. Researchers also want to test sunscreen against all ultraviolet sunlight, and also to isolate which ingredients of sunscreen work against melanoma.

The researchers said they saw some differences between brands of sunscreen, but they don’t have enough to go on yet to make any declarations about which brands might work best.