BATTLE CREEK, MICH. — The substance found in packaging liners of more than 28 million boxes of Kellogg Co.’s cereal in June has been identified as methylnaphthalene, a petroleum-based compound.

“Working with external experts in medicine, toxicology, public health, chemistry and food safety, we identified elevated levels of hydrocarbons, including methylnaphthalene, normally found in the paraffin wax and film in the liners,” said J. Adaire Putnam, director of corporate communications at Kellogg. “This specific wax is commonly used as a protective coating for foods, including cheese, raw fruits and vegetables, and is approved by the F.D.A.”

Ms. Putnam said methylnaphthalene is regulated in the Code of Federal Regulations and approved by the F.D.A.

According to the Environmental Working Group, which is a Washington-based non-profit organization that includes scientists, engineers and policy experts pushing for national policy change, methylnaphthalene has two forms: as a component of crude oil and coal tar and also as a pyrolytic byproduct from the combustion of tobacco, wood, petroleum-based fuels and coal. E.W.G. research showed the compound is produced in enormous quantities in the United States, and health agencies know very little about its safety.

“This episode casts further doubt upon the ability of the F.D.A. to evaluate and assure the safety of food packaging,” the E.W.G. said. “Methylnaphthalene gives off a strong taste and odor at quite low levels. People can discern its presence at concentrations as low as 7.5 to 10 parts per billion in air and water. In the case of Kellogg’s cereals, it affected the smell and taste of the products. Thousands of other food additives, however, would not be so obvious to consumers.”

In light of its findings, the E.W.G. has called on Kellogg and the F.D.A. to make public what they know about methylnaphthalene.

“In light of the lack of information on the safety of this chemical, E.W.G. calls on Kellogg’s to disclose the exact chemical composition and concentration of methylnaphthalene and any other compounds that leach from the cereal packaging, and to make public its safety data and its assessment of human health risks to those who were exposed,” the E.W.G. said. “We call on F.D.A. to do the same.

“The F.D.A. should independently assess and make public its evaluation of the potential impacts of this chemical and ensure that food producers take all necessary steps to ensure that packaging materials do not continue to sicken consumers.”

According to the Department of Health and Human Services Agency for Toxic Substances and Disease Registry, consumers are not likely to be exposed to methylnaphthalene by eating foods or drinking beverages.

“These materials are unlikely to come in contact with naphthalene or methylnaphthalenes during production or processing,” the A.T.S.D.R. said. “Naphthalene and the methylnaphthalenes are also unlikely to be present in tap water.”