WASHINGTON — In the wake of an on-line petition aimed at convincing Subway restaurants to stop using the chemical azodicarbonamide (ADA) in its sandwich bread, the Center for Science in the Public Interest is urging the sandwich chain as well as McDonald’s to eliminate the chemical from their products.

The chemical at issue is an additive used for many years as a flour maturing and dough strengthening agent. The web site FoodBabe.com, which issued the petition, says azodicarbonamide is used in Subway rolls in North America but not in Europe or Australia.

While ADA routinely had been added by millers to flour for many years as a maturing agent, milling sources said the practice largely stopped about five years ago when the Food and Drug Administration concluded ADA’s dough strengthening properties necessitated listing azodicarbonamide on product labels. In the baking industry, certain companies have continued to use ADA for its texture and volume enhancing properties while others have sought alternative additives. The F.D.A. has said ADA “may be safely used in food” in amounts not exceeding 45 parts per million measured by weight of flour.

In its backgrounder, the C.S.P.I. said safety testing of azodicarbonamide has been inadequate.

“Two suspicious chemicals form when bread with azodicarbonamide is baked,” Lisa Lefferts wrote in a Feb. 4 letter. “One of the breakdown products is semicarbazide, which caused cancers of the lung and blood vessels in mice, but poses a negligible risk to humans. A second breakdown product, urethane, is a recognized carcinogen. When azodicarbonamide is used at its maximum allowable level, it leads to slightly increased levels of urethane in bread that pose a small risk to humans.”

Because bread may be produced without azodicarbonamide and its use “slightly increases exposure to a carcinogen, this is hardly a chemical that we need in our food supply,” she wrote. The C.S.P.I. urged the F.D.A. to consider whether banning of the product is required.

“At the very least, it should reduce the amount allowed to be used,” Ms. Lefferts said. “Chains like Subway and McDonald’s needn’t wait and should get rid of it on their own. ‘Food Babe’ blogger Vani Hari deserves credit for drawing the public’s attention to this substance.”