Kellogg Co. said Wednesday that higher-than-normal amounts of certain chemicals in its package liners caused the unusual smell and flavor that prompted a recall of 28 million boxes of its cereal in late June.

The food maker recalled Apple Jacks, Corn Pops, Froot Loops and Honey Smacks after about 20 people complained, including five who reported nausea and vomiting.

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Consumers reported the cereal smelled or tasted waxy and others said the taste or smell was similar to that of metal or soap. Others simply described it as stale.

The company, based in Battle Creek, Mich., said it has identified elevated levels of chemicals called hydrocarbons as the source. Those chemicals include methyl naphthalene.

Kellogg said those chemicals are normally found at lower concentrations in the wax and film used for food packaging. The company said the wax used in its liners is commonly used as a protective coating for foods including cheese, raw fruits and vegetables.

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Little is known about the risks of moderate exposure to methyl naphthalene.

The Food and Drug Administration said it is reviewing Kellogg’s information and conducting its own risk assessment.

Kellogg said the amount of the chemicals wasn’t at a level thought to be harmful and that it is working with its supplier to ensure the situation does not happen again.

The advocacy group Environmental Working Group said Kellogg and the FDA have an obligation to follow up, given the lack of knowledge about the chemical. The group said it also underscores the need for improved food and food packaging oversight.

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