Caught between a rock and a hard place, American dermatologists are coming out to defend the use of sunscreens despite a new study showing their main ingredients are unsafe to both humans and marine life.

A scientific study published Monday in the peer-reviewed journal JAMA found it took only one day of use for several common sunscreen ingredients to enter the bloodstream at levels high enough to trigger a government safety investigation. The study was conducted by the Center for Drug Evaluation and Research, an arm of the U.S. Food and Drug Administration (FDA).

The study also discovered the blood concentration of three of the ingredients continued to rise as daily use continued. These ingredients remained in the body for at least 24 hours after sunscreen use ended.

“The study findings raise many important questions about sunscreen and the process by which the sunscreen industry, clinicians, specialty organizations, and regulatory agencies evaluate the benefits and risks of this topical OTC medication,” a part of the study read,

The researchers looked at four active ingredients normally found in sunscreens or sunblock lotions: avobenzone, oxybenzone, ecamsule and octocrylene.

The study’s 24 participants were asked to apply one of four types of sunscreen spray, lotion or cream four times daily over a week to areas that wouldn’t be covered by a swimsuit. Researchers then measured the concentration of these four different active ingredients in the participants’ blood.

If the blood absorption of any of these ingredients exceeded 0.5 nanograms per millilter (ng/mL), the FDA recommends a person undergo “nonclinical toxicology assessment including systemic carcinogenicity and additional developmental and reproductive studies.”

Researchers found levels of all four chemicals in the participants’ bloodstreams far exceeded 0.5 ng/mL within just day. More disturbingly, three of the ingredients remained in the bloodstream for seven days.

Oxybenzone reached the threshold within two hours of one application. On day seven, the concentration was over 20 ng/mL. Oxybenzone is also toxic to coral reefs, which forced Hawaii to ban it last year.

“Because biological rationale has so often proven to be misleading, drug manufacturers must demonstrate through empirical research in humans that benefits outweigh risks for an intended use of a product in a specified population. Has sunscreen met these current requirements for safety and effectiveness?” asked the study.

Despite the results, the American Academy of Dermatologists (AAD) insists it’s important to continue using sunscreen. It said these four sunscreen ingredients have been used for several decades without any reported internal side effects in humans.

Skin received adequate protection from UV rays when participants applied sunscreen at a thickness of 2mg per cm2. Jean Beaufort/PublicDomainPictures

“Skin cancer is the most common cancer in the United States, and dermatologists see the impact it has on patients’ lives every day,” AAD president George J. Hruza said. “Unprotected exposure to the sun’s ultraviolet rays is a major risk factor for skin cancer.”

AAD spokesman Dr. David Leffell said studies need to be performed to evaluate this finding and determine if there are true medical implications to absorption of certain ingredients. In the meantime, people should "continue to be aggressive about sun protection."

More Americans are diagnosed with skin cancer each year than all other cancers combined, according to the Skin Cancer Foundation. Worldwide, melanoma ranks as the 19th most common cancer in both men and women.