WASHINGTON (AP) -- In the space of a week, Chris Vaccaro was furloughed from his government post, called back to work and then furloughed again in head-spinning events that left him feeling like a human yo-yo.
"It's been odd having to switch your mindset off and on," said Vaccaro, communications director at the National Oceanic and Atmospheric Administration.
Vaccaro is among thousands of federal employees whose status changed from nonessential to essential at some point over the course of the partial government shutdown, now in its third week. Agencies from the Federal Aviation Administration to the Centers for Disease Control and Prevention have recalled some of their furloughed workers to deal with safety issues and other emergencies.
For Vaccaro, it happened Oct. 3, as government agencies were preparing for the threat posed by Tropical Storm Karen along the Gulf Coast. But once the storm dissipated two days later, he was back on his couch at home. About 200 furloughed Federal Emergency Management Agency workers were similarly called back to deal with the storm, but at least half were furloughed again when it didn't live up to expectations.
The White House's Office of Management and Budget didn't specify the number of federal workers who have been recalled. When the partial shutdown began, about 800,000 of 2.1 million workers were furloughed.
"As the shutdown drags on, agencies will be forced to adapt to changing circumstances," the OMB said in a statement Tuesday. The agency said unexpected events will require agencies to bring back workers, especially for situations affecting the safety of life or property. But in other cases, agencies will have to furlough additional employees if the shutdown continues and funding runs out.
"Agencies are continually monitoring ongoing activities to ensure they are complying with applicable legal requirements while also doing their job," the OMB said.
In the largest recall of furloughed workers, about 350,000 civilian defense employees were told to report back to work after government attorneys concluded they could eliminate furloughs for "employees whose responsibilities contribute to the morale, well-being, capabilities and readiness of service members."
Of the remaining 450,000 furloughed federal employees, some have trickled back for various reasons.
The CIA initially furloughed a "significant" but undisclosed number of workers when the shutdown began. But a week later, CIA Director John Brennan said reduced staffing levels posed a national security threat and he began bringing back employees deemed necessary for intelligence collection and analysis.
At the CDC, most of the agency's scientists who track food safety outbreaks have been furloughed. But many were brought back Oct. 8 in light of an outbreak of salmonella in raw chicken that has sickened nearly 300 people in at least 17 states.
The FAA similarly recalled about 800 furloughed employees during the second week of the shutdown, including 600 inspectors and other safety employees who check to see whether airlines are properly maintaining their planes.
Those employees who have been deemed essential by their agencies are still working without pay. While the House has voted to reimburse furloughed government workers and the Senate is expected to follow, they will not be paid until the shutdown ends.
The Housing and Urban Development Department could recall up to 698 furloughed employees for temporary work on an as-needed basis, HUD spokesman Jerry Brown said Tuesday. The largest number of employees recalled temporarily for a single day so far in the shutdown was about 400, Brown said.
The recalled employees work between two and eight hours on tasks such as making payments on housing vouchers, processing guarantees for FHA mortgages and managing properties that are deemed vital services. Overall, HUD has just 307 full-time employees it considers essential out of a workforce of 8,709.
At NASA, a few hundred furloughed workers were recalled to prepare for the Nov. 18 launch of a robotic probe to Mars. But the space agency still has 97 percent of its workers furloughed.
Furloughed employees at the Grand Canyon, Mount Rushmore and several other national parks returned to work over the weekend because some states offered to pay the National Park Service to reopen the sites. Arizona agreed to pay the Park Service $651,000 to keep the Grand Canyon open for seven days. Utah's five national parks -- Zion, Bryce, Arches, Canyonlands and Capitol Reef -- reopened Friday afternoon and Saturday morning after the state sent $1.67 million to the U.S. government to pick up the tab for 10 days in hopes of saving its lucrative fall tourist season.