(Reuters) - The U.S. Food and Drug Administration issued guidelines on Tuesday detailing the data makers of sunscreen sold over the counter need to produce to prove the ingredients in the products are safe and effective.

The guidance is the latest step aimed at settling longstanding questions about the safety of potential new ingredients in sunscreen.

Americans rely extensively on over-the-counter sunscreens, which are used liberally over large portions of the body to prevent sunburn, early aging and to reduce the risks of skin cancer.

The FDA has been reviewing the issue for more than a decade, and some ingredients, which are still under review in the United States, have been in use in Europe for years.

On Tuesday, the FDA laid out the chemical and human studies manufacturers will need to conduct to win approval to include a new ingredient in an over-the-counter sunscreen product.

Among them, companies will need to provide data from a so-called "maximal usage trial" to determine whether the ingredient is absorbed into the blood and, if so, to what degree - the same standard used by the FDA for all topically applied drugs. (http://bit.ly/2fZ0YK3)

In February 2015, the FDA said it could not approve two new sunscreen ingredients - ecamsule and enzacamene - citing a lack of data to support their safety and effectiveness.

The decision followed a similar conclusion in January that year regarding six other ingredients that had been pending approval.

French cosmetics maker L'Oreal SA applied for a review of ecamsule in 2007 and Buchanan Ingersoll sought a review of enzacamene in 2002 on behalf of Merck & Co Inc. Merck's consumer business is now owned by Bayer AG.

The FDA's review process for sunscreen additives was streamlined with the passage of the Sunscreen Innovation Act in November 2014.

(Reporting by Natalie Grover in Bengaluru and Toni Clarke in Washington; Editing by Sai Sachin Ravikumar)