Despite an active threat, the White House has slashed funding for radiological protection.

Denying terrorists access to radiological materials that can be used in a dirty bomb attack—one that could bring our economy to a standstill and render areas uninhabitable for decades—is a major security challenge.

Should a dirty bomb or multiple dirty bombs be detonated in any American city, port complex, airport or train station, the result could be loss of life and widespread panic. Billions of dollars would be spent in clean-up and medical costs, not to mention lost wages and serious interruptions in the flow of commerce and everyday life.

We know that terror groups remain highly interested in such devices. Al Qaeda's senior leadership has publicly expressed a desire to acquire weapons of mass destruction, including radiological weapons, with which to attack the United States. Since September 11, U.S. officials have recovered many documents detailing al Qaeda's pursuit of these weapons.

In 2006, Abu Hamza al-Muhajir, then the leader of al Qaeda in Iraq, called for nuclear scientists and explosive experts to help his organization in making biological and radioactive weapons.

That same year, British citizen Dhiren Barot pleaded guilty to conspiring to detonate a radioactive dirty bomb. He planned to target underground parking garages in the U.K. and U.S. institutions such as the International Monetary Fund, the World Bank, the New York Stock Exchange, and offices belonging to Citigroup and Prudential Financial.

Just two weeks ago, Adnan el-Shukrijumah, an American citizen on the FBI's most-wanted list, was named as an accomplice in the New York subway bomb plot with Najibullah Zazi. Currently at-large, Shukrijumah—a trained nuclear technician allegedly tasked by al Qaeda with carrying off an "American Hiroshima"—once sought radioactive material from a university in Ontario, Canada. News reports allege that this was an attempt to construct a dirty bomb.

President Obama has rightly focused on the threat of nuclear terrorism by pledging to locate and safeguard loose nuclear material in the world within four years. This critical goal served as the theme of an April Nuclear Security Summit convened by the president in Washington with over 40 countries in attendance.

But an exclusive focus on "loose nukes" could mean missing the threat we face in our own backyard: the dirty bomb, which is far easier to assemble from ingredients that are widely available in this country. Highly dispersible radiological materials like cesium-137 or cobalt-40 are used every day in medical procedures at hospitals and in universities. These components of modern medicine are underprotected.

Many hospitals are eager to upgrade security for these radiological elements—provided they receive the resources to do so. But the clock is ticking. Entry to most hospitals is easy. Radiology departments usually control access, but a basic tool kit can be used to break into a blood irradiator and seize a substantial amount of radiological material. Once combined with a simple explosive device, you've got a crude radiological weapon. While there have been a handful of successful pilot projects to secure these ingredients, they simply don't go far enough.

Despite those chilling facts, the White House has slashed the domestic radiological protection budget over the past few years. The current budget proposal would cut these domestic resources by half. At the same time, it would more than double the funding for "loose nuke" programs.

This policy is pennywise and pound foolish. We cannot allow a dirty bomb to become America's Achilles' heel because we've lost sight of that threat.

Ms. Harman is a Democratic congresswoman from California and chair of the Homeland Security Subcommittee on Intelligence & Terrorism Risk Assessment. Ms. Collins is a Republican senator from Maine and ranking member on the Senate Committee on Homeland Security and Governmental Affairs.

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