(CNN) — The partial US government shutdown raises all sorts of questions for travelers on topics from aviation safety and security to passports and national parks.

Here's where things stand on travel

Many national parks open but with limited facilities

Although National Park Service sites across the country have been closed during previous government shutdowns, many have remained open -- but severely understaffed -- under the Trump administration.

Some park sites have been operating for weeks without park service-provided visitor services such as restrooms, trash collection, facilities or road maintenance.

More than 40 parks have been able to continue to offer key services for visitors, thanks to millions of dollars in donations and services from a number of states, private concession companies and park nonprofit groups, according to Jeremy Barnum, a National Park Service spokesman.

A car drives into Joshua Tree National Park in California past a shuttered entrance station. Loic Pialat/AFP/Getty Images

But that money will only go so far.

"As the lapse in appropriations continues, it has become clear that highly visited parks with limited staff have urgent needs that cannot be addressed solely through the generosity of our partners," Barnum said.

To address those needs, the National Park Service has modified its contingency plan to direct visitor fee revenue toward services, including trash removal, visitor information, snow plowing, custodial services and campground services.

The park service estimates that more than 80 national parks -- including Liberty and Ellis Islands, the National Mall, Yellowstone and the Grand Canyon -- are already using those fee-generated funds.

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Nearly 331 million people visited park service sites around the United States in 2017.

People are getting increasingly worried about conditions at the national parks, said Will Shafroth, president and CEO of the National Park Foundation , a charitable partner of the National Park Service.

The foundation has set up a Parks Restoration Fund to help with repairs and cleanup efforts once the shutdown ends.

Visitors should go to www.nps.gov and select "Find a Park" for information on access to specific parks and sites.

Smithsonian museums and National Zoo closed

A closed sign is posted on the gate of Smithsonian's National Zoo. Carolyn Kaster/AP

The 19 Smithsonian museums and the National Zoo in Washington closed their doors January 2.

"Due to the federal government shutdown, all Smithsonian museums and the National Zoo are closed," the Smithsonian Institution said in a statement. Programming and events were also canceled.

In addition to the National Zoo's grounds being closed to the public, its live-animal cameras -- including the popular panda cam -- are not operating during the shutdown.

The zoo has continued to feed and care for the animals during the stalemate. "A shutdown will not affect the Zoo's commitment to the safety of staff and the standard of excellence in animal care," a statement said.

The National Gallery of Art and its sculpture garden and the National Archives are also closed.

Despite the shuttering of some popular attractions, the government shutdown doesn't mean Washington is closed to visitors.

Destination DC, the tourism office for the nation's capital, has created a list of spots open to help visitors navigate the partial shutdown.

Airspace operational; FAA recalls inspectors and engineers

Air traffic controllers have been working through the shutdown and will receive back pay -- along with other government workers -- when it is over, according to a bill signed Wednesday by President Donald Trump.

The Federal Aviation Administration recalled furloughed inspectors and engineers Tuesday "to perform duties to ensure continuous operational safety of the entire national airspace. We proactively conduct risk assessment, and we have determined that after three weeks it is appropriate to recall inspectors and engineers," an FAA representative said.

"We have seen no unusual increased absenteeism and there are no operational disruptions due to staffing."

A union representing aviation safety workers has blasted the partial shutdown, saying Wednesday that the nation's airspace is "less safe today than before the shutdown began."

The National Air Traffic Controllers Association, a union representing FAA air traffic controllers and other aviation safety workers, said in a press release that the lengthy shutdown is "eroding the layers of redundancy and support necessary to maintain the safety of the National Airspace System."

Airport screening continues, but some screeners have called in sick

Unscheduled absences have increased steadily among Transportation Security Administration employees since the shutdown started.

At times during the last week, the absence rate was more than twice what it was at the same time last year, according to data from the TSA.

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The TSA reported a national rate of unscheduled absences of 6.4% on Thursday, compared with a 3.8% rate a year ago on the same day.

"Many employees are reporting that they are not able to report to work due to financial limitations," a TSA statement said.

"While national average wait times are within normal TSA times of 30 minutes for standard lanes and 10 minutes for TSA Pre✓®, some airports experienced longer than usual wait times," the TSA said.

The agency expects an uptick in travelers for the Martin Luther King Jr. holiday weekend.

TSA encourages travelers to stay up to date on airport and airline information in allotting enough time to get through security.

Some airports have intermittently experienced much longer wait times in security lines, and they have had to cut back their operations temporarily

Passports, visas still being processed; State Department returning to work

The State Department's Bureau of Consular Affairs has maintained scheduled passport and visa services in the United States and at US embassies and consulates during the shutdown, and on Thursday, the State Department announced it is calling staff back to work next week.

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