It began in late 2007 as a routine audit. Retail giant Wal-Mart noticed that some exit signs at the company's stores and warehouses had gone missing.

As the audit spread across Wal-Mart's U.S. operations, the mystery thickened. Stores from Arkansas to Washington began reporting missing signs. They numbered in the hundreds at first, then the thousands. Last month Wal-Mart disclosed that about 15,800 of its exit signs – a stunning 20 per cent of its total inventory – are lost, missing, or otherwise unaccounted for at 4,500 facilities in the United States and Puerto Rico.

Poor housekeeping, certainly, but what's the big deal?

In a word: radiation.

The signs contain tritium gas, a radioactive form of hydrogen. Tritium glows when it interacts with phosphor particles, a phenomenon that has led to the creation of glow-in-the-dark emergency exit signs.

It's estimated there are more than 2 million tritium-based exit signs in use across North America.

It turns out that Ontario-based companies SRB Technologies (Canada) Inc. of Pembroke and Shield Source Inc. of Peterborough have sold the lion's share of these signs, which use tritium produced as a by-product from the operation of Canadian-made Candu nuclear reactors.

The health effects of tritium exposure continue to be a hot topic of debate. It's not strong enough to penetrate the skin, and in low quantities regulators and industry groups say tritium is safe. But when inhaled or ingested it can cause permanent changes to cells and has been linked to genetic abnormalities, developmental and reproductive problems and other health issues such as cancer.

"The problem is that because it's hydrogen it can actually become part of your body," says Shawn-Patrick Stensil of Greenpeace Canada. "The radiation doesn't emit far, but when it actually becomes part of your cell it's right next to your DNA. So for a pregnant woman, for example, it can be really dangerous."

General exposure from one broken sign might be the equivalent of getting up to three chest X-rays, even though today we no longer give pregnant women X-rays. If tritium is ingested, for example, by a child who breaks a sign with a hockey stick, it's much more potent. If only 5 per cent of the tritium in a large exit sign is ingested, it would be equivalent to 208 years of natural background radiation, according to a report from the Product Stewardship Institute at the University of Massachusetts.

And what about exposure from thousands of signs dumped near a source of drinking water, or packed with explosives in the back of a truck that has been driven into a crowded building?

"I'm sure thousands of them would create a credible dirty bomb," says Norm Rubin, director of nuclear research at Energy Probe in Toronto. "Most experts think the main purpose of a dirty bomb is to cause panic, disruption and expensive cleanup rather than lots of dead bodies. A bunch of tritium, especially if oxidized in an explosion, would probably do that job fine."

Tritium is also a component in nuclear warheads. In 2005, SRB Technologies got permission from the Canadian Nuclear Safety Commission to export 70,000 of its tritium exit signs to Iran. Foreign Affairs Canada blasted the regulator for allowing shipment to a country that's attempting to develop weapons of mass destruction. The shipment went through.

South of the border, the U.S. Nuclear Regulatory Commission appears more concerned with tritium contamination of landfills and the threat of leaching into drinking water. The agency regulates the use of tritium devices, requiring the reporting of lost, stolen or broken property and proper cleanup and disposal.

"Throughout the whole process we stayed in very close contact with the NRC and received their guidance," said Wal-Mart spokesperson Daphne Davis Moore. "We no longer use these signs in our stores."

Wal-Mart's poor recordkeeping was a wake-up call for the nuclear agency, which in January sternly reminded users of the signs of their regulatory obligations. At the same time, it assured the public there's nothing to worry about.

Still, the agency was concerned enough to demand that any organization possessing 500 or more tritium exit signs conduct audits and report their findings within 60 days. The list included Home Depot, AMC Theatres and a number of universities and schools.

Wal-Mart Canada says it has a few tritium exit signs in most of its stores. "We've gone back over our records and have not found any reason for concern," said spokesperson Kevin Groh. "We are doing an audit to get an accurate inventory." The difference, in Canada, is they don't have to do it. Users of the signs are not licensed in Canada as long as the product is properly marked as radioactive, according to the Canadian Nuclear Safety Commission. This makes it difficult to determine exactly how many tritium signs exist in Canada and where they end up.

Stensil of Greenpeace said it's a strange way for a government to treat a radioactive device, but he's not surprised. He said the federal government has always had lax rules when it comes to tritium, partly because Canada, through its Candu nuclear plants, is one of the biggest producers of the substance in the world.

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Dorothy Goldin Rosenberg, who teaches environmental health at the University of Toronto, said there's a double standard in Canada when it comes to regulating tritium. Permissible levels in drinking water here are 100 times greater than in Europe and more than 400 times greater than in California.

She was shocked when told about the 15,800 missing tritium signs at Wal-Mart, but even more surprised to learn that use of such signs isn't tracked or monitored in Canada.

"Most people haven't even heard of tritium," she lamented.