What do Home Depot, the Mormon Church, and the U.S. Coast Guard have in common?

Answer: Radioactivity.

According to the Nuclear Regulatory Commission, the construction retailer, the church denomination, and the guardians of our coastline own hundreds of fluorescent exit signs containing the radioactive gas tritium. So, in fact, do various school districts, retail stores, and federal and state agencies. And if the signs are handled and disposed of improperly, tritium could make its way into our drinking water. The NRC was prompted to step in following Wal-Mart’s recent disclosure that 15,000 tritium exit signs have mysteriously disappeared from its stores nationwide.

On January 16, the NRC sent notices to 61 organizations that own 500 or more tritium signs to check the signs against their records and report any lost or missing signs to the agency. The recipients of the “demand for information” letter include the Department of the Navy, the Smithsonian Institution, the U.S. Postal Service, and the West Point Military Academy, as well as several pharmaceutical, defense, and aviation companies nationwide.

In all, more than two million tritium exit signs are estimated to be in use in the United States. The signs are popular because they do not require electricity and provide emergency light and direction during evacuations.

From 2001 to 2007, Wal-Mart bought 70,000 tritium exit signs to install in its stores and warehouses, according to the NRC. In 2007, after discovering that some signs had disappeared, the company started a nationwide audit of its facilities. The result: a staggering 15,000 signs were lost, missing, or otherwise unaccounted for.

An NRC advisory states that the tritium signs pose “little or no threat to the public health and safety and do not constitute a security risk.” Others are not so sure. “Fifteen thousand missing tritium exit signs at 20 trillion picocuries each means that 300 quadrillion picocuries of tritium could be making its way into people's drinking water,” warns David Lochbaum, director of the Nuclear Safety Project at the Union of Concerned Scientists. “Or, nearly four million gallons of water could be contaminated above the EPA's drinking water standards. And what if 15,000 missing tritium exit signs is a low estimate?”

Damage may already have been done. In January 2006, the Pennsylvania Department of Environmental Protection notified the NRC that more than half of the DEP’s water measurements downstream of landfills in the state showed tritium concentrations above the Environmental Protection Agency’s safety drinking water limit.

The source of the contamination, according to state officials: improperly disposed-of exit signs. “There is no other source of tritium in the private sector that could be causing such levels of tritium,” read a DEP letter to the NRC.

Daphne Moore, a Wal-Mart spokeswoman, said the company has replaced all its tritium exit signs with LED and phosphorescent models. As for the 15,000 missing tritium signs, Moore said they might have been sent to storages “that couldn’t be located” or might have been disposed of in an “unknown manner.”

In recent years, tritium leaks have also occurred in nuclear power plants around the country, most notably Braidwood Station in Illinois, which experienced several leaks between 1996 and 2006. Some of the radioactively contaminated water migrated into drinking wells used by neighboring communities. The concentrations of tritium in those wells did not surpass the EPA’s safety limits, but the leaks prompted lawsuits by area residents and the state of Illinois against the plant’s operator, Exelon Corp.