Health Canada insists a chemical found in hamburger buns and doughnuts — as well as yoga mats and shoe soles — is safe to eat.

But as some restaurant chains voluntarily move to pull the controversial food additive, scientists at home and abroad are raising serious questions about the research that underlies that safety assurance.

Azodicarbonamide, commonly called ADA, is banned in Europe. Canada is one of only five countries listed in the International Food Additive Database that permits its use in flour as a dough-conditioning agent to make the finished product light, soft and resilient.

ADA is also found in the undercoating of jarred food lids — everything from baby food to mayonnaise.

European Food Safety Authority’s overarching 2005 review of semicarbazide in food and food contact materials that contain azodicarbonamide (PDF)

Health Canada’s 2004 assessment of semicarbazide in foodPDF)

Health Canada’s most recent assessment of the additive and its chemical by-products relies on what its own scientists have called “incomplete” and “poor quality” research. A key study to gauge Canadians’ risk of exposure was based on data collected in 1972, and the federal government admitted it has not established a safe, acceptable daily intake — which raises alarm among some food safety experts.

“You need to get data,” said Maricel Maffini, senior scientist co-chairing the “chemicals in food” research project for the Natural Resources Defense Council in Washington, D.C. “… They have to use the newest and best science to figure out what is the safe dose, the acceptable daily intake. If what people are eating exceeds your acceptable daily intake, then you have a bigger problem because that is the definition of unsafe.”

The additive is ubiquitous in fast-food restaurants. In Canada, it is found in the buns and yeast-based doughnut sold by Tim Hortons, the pizza dough and garlic bread at Pizza Hut, the English muffins and most buns that top McDonald’s burgers, and the four-inch seeded bun that KFC uses for its BLT Deluxe and Zinger BLT sandwiches, to name a few.

Earlier this month, the Subway restaurant chain pledged to stop using ADA in all its outlets after a grassroots social media campaign led by an American food blogger lobbied for its removal.

Subway said that even though the federal Canadian and U.S. governments had both approved it as a safe additive, the company has been seeking a substitute for the past year. After calls from the Star, Starbucks and Yum! Canada, which operates KFC and Pizza Hut, also promised the ingredient will be removed from their foods.

“We have made a decision as a global company to take steps to remove this ingredient from our products on a worldwide basis,” said Starbucks’ spokeswoman Carly Suppa.

The chain will stop using ADA in its sunflower turkey cheddar sandwich bread this month and in its apple fritter this summer.

Yum said it is working with suppliers to switch to new recipes “as soon as possible” but would not provide a timeline.

Spokeswomen for McDonald’s and Tim Hortons both emphasized the popularity of the government-approved ingredient and referred questions of safety to Health Canada. Neither chain indicated plans to alter its recipes.

Joyce Reynolds, an executive with Restaurants Canada (formerly the Canadian Restaurant and Foodservices Association), whose 30,000 members include chains and independents nationwide, said her group called Health Canada after Subway announced it plans to pull the additive.

“We immediately called the Bureau of Chemical Safety to get their assurances that it was a safe product,” Reynolds said. “We have to rely on the expertise of scientists at Health Canada and the FDA … and the fact that it has been in bread products that people eat every day for half a century.”

The Environmental Working Group, a Washington, D.C.-based research firm that called on government and industry this week to ban ADA, found the ingredient in nearly 500 products sold in U.S. grocery stores, including croissants, cinnamon rolls, cheese danishes, ravioli, French rolls and 12-grain bread.

Such a survey is possible in the United States because the Food and Drug Administration requires manufacturers to label all additives used in standardized bakery products.

Not so in Canada.

“If azodicarbonamide has been added to flour and this flour is used as an ingredient in bread, or any other bakery product … azodicarbonamide would not have to be labelled on the final prepackaged food,” Health Canada spokesman Gary Holub confirmed. “A consumer would therefore need to inquire with individual bakeries to determine if their product uses flour that contains azodicarbonamide.”

While Europe pulled the additive after researchers found small levels of a carcinogenic chemical was created during the heating process in baby food manufacture, Health Canada’s 2004 toxicological evaluation of ADA found double the amount in bread products.

In a February letter addressed to industry “stakeholders,” such as restaurants and bakeries, Health Canada said the additive underwent a “thorough safety assessment prior to approval” in 1964.

Thomas Neltner, a chemical engineer and lawyer who published a massive project last year with Pew Charitable Trusts on the safety of food additives, finds little comfort in that statement.

“All the studies (on ADA) in the FDA’s toxicology database were unpublished studies and they were all dated 1959. They weren’t looking for the kinds of problems we look for today,” said Neltner, who now heads up the Natural Resources Defense Council’s food chemicals project with Maffini.

“A decision made in the 1960s based on science from the 1950s needs to be reviewed. The fact that a chemical has been used for 50 years doesn’t mean it’s safe — because we don’t have a way of checking these things.”

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When ADA was approved for use in Canada, researchers had not yet detected ADA’s potentially cancer-causing by-products, semicarbazide and urethane, which are created during the baking process. That finding didn’t occur until 10 years ago.

Health Canada says it is aware of these by-products and assured industry they would “not be considered to pose a health risk.”

“What we’re looking for is an agent directly capable of damaging DNA,” said Mark Feeley, associate director for Health Canada’s Chemical Safety Bureau. “A DNA-damaging agent in experimental animals has a very high probability of also damaging DNA in humans. If it had been shown to be a strongly genotoxic agent, our position would be distinctly different.”

Health Canada referred the Star to several studies that it said helped allay its concerns about the chemical’s potential to do harm. Perhaps the most significant, carried out by the National Food Administration in Uppsala, Sweden and published in Toxicology Letters 10 years ago, concluded that semicarbazide had no “DNA-damaging effect” when exposed to two different mouse strains.

Still, the researchers offered a cautionary note, warning “it is difficult to exclude genotoxic risk in humans” based on the results.

The European Food Safety Authority’s 2005 review classified semicarbazide as a “weak, non-genotoxic carcinogen” and found the risk minimal to humans in Europe, a conclusion partially based on the new data but also based on the risk of exposure. Since Europe ordered the substance banned from use in or proximity to food products, the health risk was judged to be very small.

“Even if it’s not an enormous cancer risk, it’s an unnecessary cancer risk,” said Lisa Lefferts, senior scientist at the Center for Science in the Public Interest in Washington, D.C., which also is calling for a ban on ADA. “People shouldn’t have to be worrying about it. You just shouldn’t use azodicarbonamide in bread.”

Other food additive experts caution that the science is not conclusive enough to rule out human health risks.

Because the law prohibits the use of chemicals suspected of causing cancer, government and industry are often preoccupied with whether an additive is capable of doing just that.

“But there are other things besides cancer,” said Maffini of the Natural Resources Defense Council.

A 2003 review by the European Food Safety Authority found embryo-fetal death and cleft palates in surviving fetuses of pregnant rats given high doses of semicarbazide, skeletal deformations and lung and vascular cancer in female (but not male) mice dosed through food or water, and malformations of the brain and kidneys and bleeding in other rats injected with high doses.

More extensive testing for toxic effects was recommended.

“To me, something that causes limb defects is pretty important,” Maffini said. “Having kids that don’t have their legs or arms well-formed is pretty serious to me. It’s not cancer but it’s still a problem. Putting all the eggs in that cancer basket is not the best way to do a safety assessment. You have to look at the overall evidence.”

For now, even though Health Canada’s exposure assessment for ADA and its by-products of semicarbazide and urethane are alarmingly dated — based on consumption data collected by Statistics Canada more than 40 years ago — Ottawa says it has no plans to reopen the file.

Barbara Lee, director of Health Canada’s Bureau of Chemical Safety, said the government is “not considering any changes to the approved food additive uses of azodicarbonamide.”

In the meantime, if another agency finds a problem, Health Canada will respond.

Said Lee in Health Canada’s letter to industry: “Should new evidence become available indicating that the low level of azodicarbonamide that is permitted in bread, flour and whole wheat flour represents a possible health risk to Canadian consumers, Health Canada would take immediate and appropriate action to address that risk.”