Disaster-response teams are used to being the ones running toward the thing that everyone else is running away from.

But with the risk of radiation exposure looming in Japan, where explosions have rocked nuclear power plants in recent days, even some of the most hardened professionals and volunteers are hesitating.

“The wild card here is what’s going to happen next,” said Christoph Gorder, senior vice president for global programs at AmeriCares, “and can the Japanese government get the situation under control, or will it get worse? So it is really a very worrisome X factor out there, and it’s something that needs to be taken extremely, extremely seriously.”

Friday’s 9.0-magnitude earthquake buckled roads, toppled buildings, crushed residents and unleashed tsunami waves. Thousands are dead, and Japanese officials says thousands are missing.

But for some aid workers, it is the unknown — how the continuing chaos will affect the country’s nuclear reactors — that is most frightening.

“It’s something we’re acutely aware of,” Gorder said. AmeriCares has Japanese volunteers helping on the ground; a couple of those volunteers have fled south, to safer areas of the country.

Experts are working with their medical director to gauge risk, Gorder said. “It’s a complicated issue. Radiation comes in a lot of different forms. Some of it is fairly benign, with a short half-life. Some is extremely toxic. We’re not nuclear physicists — we’re looking at it more from a health point of view.”

While aid workers frequently find themselves in dangerous situations, radiation is different, said World Vision spokesman Laura Blank. “It’s so scary because you can’t see it. Some health concerns you can see the symptoms; some, like cholera, there are things you can do to prevent it. Nuclear radiation is simply being in the wrong place at the wrong time.”

A team from World Vision went to the evacuation area in Fukushima to tell local officials that they are available to help with supplies for families staying there. “There are special masks and suits the team would wear if they get close to the area,” Blank said. And they wouldn’t go into a situation that was too dangerous for staff, she added. But it’s a balance: trying to help the people most in need and making sure staff members are safe.

“Humanitarian aid inherently has risk,” she said. “I think the staff accepts that.”

International Medical Corps has a team in Japan and is continually assessing the situation to decide whether to send more workers. “We worked in places like Afghanistan, Somalia, Iraq, the Democratic Republic of Congo – there are dangers to our work, but the safety of our staff is paramount,” said Margaret Aguirre, director of global communications. “It’s a tough situation.”

Virginia Task Force 1, a team from Fairfax County Fire and Rescue Department, landed in Japan on Sunday.

When asked if team members had concerns about the potential for radiation exposure as fires continued to burn Tuesday at reactors north of Tokyo, Fairfax County Fire and Rescue spokesman Dan Schmidt said repeatedly, “There’s no increase in radiation levels at their location.” The team is working in Ofunato, on Japan’s northeastern coast.

“There’s a lot of unknowns right now,” Schmidt said.

At Mercy Corps, which is working with a Japanese-based partner group, a spokesman said it would not send staff into areas that have been evacuated or deemed unsafe because of concerns about radiation.

The basics of the situation stay the same: There’s this incredible, incredible human need. This is a new kind of risk,” communications director Joy Portella said. “But the desire to address that incredible human need is still there.”

For Mercy Corps, as for other international aid agencies, she said, “We’ll do what we can.”

The American Red Cross is sending money not volunteers. It had raised more than $34 million by Tuesday afternoon. The Japanese Red Cross has 2 million volunteers, including a team in Nagasaki specially trained to treat patients with radiation contamination.

At a gathering Tuesday of about a dozen local groups in downtown Washington, the Red Cross was the recommended charity.

“The important thing in Japan right now is money,” said John R. Malott, president and chief executive of the Japan-America Society of Washington, D.C. “They don’t need people showing up who mean well but are not professional relief workers.”

Money “allows people to buy exactly what they need,” he said at a crowded meeting in the society’s headquarters on L Street NW. “The last thing they need is to have well-meaning people sending boxes of food with expired dates.”

Staff writer Michael E. Ruane contributed to this report.