More than one kind of bomb can spread radioactive contamination, and Maine Sen. Susan Collins and Rep. Jane Harman, D-Calif., are rightly worried about that.

In a column this week in The Wall Street Journal, the two noted that terrorists don’t need to get their hands on a military-style nuclear weapon to wreak death and destruction over a widespread area.

Although its effects would be more limited than even the smallest nuclear device, a so-called “dirty bomb” could still mimic the effects of such a bomb’s radioactivity, if not its destructive blast radius.

Dirty bombs rely on conventional explosives, which can do considerable damage by themselves, as the fertilizer bomb exploded in Oklahoma City in 1995 proved by killing 168 people and injuring 680 others.

Imagine, however, that Timothy McVeigh had been able to procure a fairly sizable amount of radioactive material to pack into the rental truck that he had already filled with ammonium nitrate. The explosion not only would have wrecked a building, it would have contaminated the site and a far larger area as the radioactive material blew downwind.

Collins and Harman, in an essay headlined, “Al-Qaida Still Wants a Dirty Bomb,” say “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.”

That is, they can be stolen far more easily than a nuclear bomb. And, the authors say, the administration has “slashed the domestic radiological budget over the past few years.”

They call for a full restoration of funding to secure these materials, and they’re right to do so.

One dirty bomb going off in a city center would be one too many.