As the pandemic slouches toward its third year, flying is making a comeback of sorts. From the crater of April 2020, when the global industry saw overall demand decline by nearly 95%, passenger air travel has built back up to an estimated 40% of pre-pandemic levels in 2021.

“Airports have done a lot and invested significantly in measures to provide for the health and safety of air travelers and workers,” says Christopher Bidwell, senior VP of security for Airports Council International—North America (ACI-NA).

The actions taken by airports have helped to enhance consumer confidence and raised people’s willingness to fly once more. But some of those gains may prove short-lived. The Omicron variant of COVID-19 has provided yet more evidence that this disease will keep evolving and may, many scientists fear, escape the guards available to vaccinated individuals. Beyond this short- and medium-term issue looms the prospect of the next pandemic.

Airplanes have been a central focus of public scrutiny, but airports themselves—which in 2021 represented a $130 billion global market segment and a $6.6 billion U.S. segment—are an integral part of the picture when it comes to pandemic safety. Airport authorities “remain as the last line of defense between controllable local outbreak and catastrophic global pandemic,” wrote one Singaporean academic in 2018. While airports have made a number of changes in an attempt to restore public confidence since the COVID-19 pandemic began, experts who study the role these institutions play in the spread of infectious disease say far more is needed.

A visit to an airport today looks different from either the zombie-movie emptiness of the first phase of the pandemic or the bustle of pre-pandemic days. Airports have done a reasonable job adapting to the basic public health restrictions of the new normal, says Patricia Schlagenhauf, a professor at the University of Zurich’s Centre for Travel Medicine and codirector of the WHO Collaborating Centre for Travellers’ Health. In many places, norms like wearing masks and social distancing rules, plexiglass barriers, and hand sanitizer stands have all become par for the course in airports just like they have in grocery stores.

After the pandemic started, Bidwell says his organization quickly convened a panel to issue advice to North American airports. That panel, composed of industry members from the U.S. and Canada, doesn’t have a scientist on staff or someone whose dedicated job it is to keep abreast of developments. Instead, says Bidwell, the members of the panel bring studies they’ve encountered in their work. The panel also watches for new guidelines from public health agencies such as the CDC, he says.

His organization referred Fortune to a report it produced in June 2020, more than a year ago. That document, written in the first phase of what has proved to be a much longer public health emergency than originally anticipated, contains 42 high-level recommendations for recovery from the COVID-19 pandemic. The public health interventions it calls for—plexiglass, preparations for recovery from the bottom of the slump, physical distancing, enhanced cleaning—are already underway.

In one of the first moves, says Bidwell, airports across North America began spotting their floor with now-ubiquitous stickers that illustrate how far apart people ought to stand. They also devised signage to remind passengers of the importance of wearing masks.

In airports, as in those other settings, compliance remains an issue. Earlier this year, the Federal Aviation Authority reported that three-quarters of the thousands of reports of unruly passenger behavior it received in the first half of 2021 were the result of airline passengers refusing to comply with the federal mask mandate, which has been in force in transportation networks across the U.S. since early 2020.

The FAA has been active in fining unruly passengers, Forbes reported in October, issuing more than $1 million in fines. But the Transportation Security Administration, which oversees airport security, has issued a total of only $2,350 in fines. Anecdotally, Fortune staff who worked on this story have also observed mask-wearing not being enforced at a number of airports both in the U.S. and elsewhere.

“Given the significant real estate at airports, they don’t have the resources to put people at every location,” says Bidwell.

Schlagenhauf has one suggestion for improving the status quo: photo-epidemiology. This process involves using artificial intelligence to scan the airport, looking for people who aren’t properly wearing a mask. Once those people have been identified by the A.I., it can send real airport employees to remind the non-mask wearers. Even the knowledge that such a system is in place would likely increase compliance, she says.

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Mask-wearing, distancing, hand sanitizer, plexiglass: These kinds of interventions are sufficient in places like grocery stores, where exposure to disease is limited. But airports are different.

“Airports are very important building blocks of a pandemic,” says Christos Nicolaides, a professor at the University of Cyprus who studies how contagions spread. Big spaces where crowds of people coming and going from different parts of the country or world mix, eat, and shop: It’s a recipe for spreading contagious diseases quickly, he says.

The Omicron variant’s entry into the U.S. was linked to a single passenger who returned from South Africa, the first country to detect the variant, through San Francisco International Airport. Authorities believe the passenger wasn’t the only case in the U.S., but this case highlights the important role airports play as centers for disease transmission.

We don’t know what that passenger, or the others who introduced the Omicron variant to the U.S., did at the airport when they arrived in the country. Perhaps they had a meal between flights, drank a beer at the airport bar, or did some shopping—all activities that airports encourage a captive audience of passengers to take part in. Maybe they stopped just long enough to pick up their luggage from the carousel and catch a cab.

We also don’t know yet exactly how contagious Omicron is, though it’s safe to say it is far more transmissible than any other variant yet found. It’s on the rise in the United States, and examples around the world suggest it may become dominant here and globally. Individuals taking some maskless moments to eat a sandwich or not wearing a face covering properly in the reentry line could have infected other passengers and sent the variant spiraling across the country.

Getting airports right matters for far more than just the air industry’s bottom line. As the pandemic stretches into its third year, families and friends around the world are desperate to reunite.

At her clinic in Switzerland, Schlagenhauf says a significant fraction of the people she sees—a load still far reduced from before the pandemic—are “VFR travelers”—people flying in order to visit friends and relatives.

GlobalData, a publicly traded data and analytics consulting company based in London, estimates that VFR travel—historically the second most common reason to take a flight, after leisure travel—will be “vital” to the industry’s recovery. It expects to see VFR’s growth outpace that of leisure travel after declining by more than two-thirds in 2020, from approximately 230 million international departures to just 69.2 million.

“That’s a very strong need,” Schlagenhauf says, “and I think it’s also a human right, to be able to travel.”

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SFO and three other large international airports have now enhanced screening for Omicron, but it likely won’t be enough to contain the variant, which is already circulating within the country. Scientists who study pandemic prep have been warning for decades that airports aren’t prepared for pandemics, says Mohit Arora, a postdoctoral researcher who studies infrastructure at the University of Edinburgh.

The changes that airports have made so far have largely been the simple ones—installing hand sanitizer dispensers being one example—and those that most align with their economic goals, says Arora. Take passengers self-scanning their identification, something that Bidwell says was accomplished by simply rotating the existing scanners 360 degrees. Self-scanning and the self-serve TSA kiosks that are being piloted in several sites throughout the country have reduced contact between travelers and the airport, he says, but it was also a direction that the air travel industry was trying to move in anyway.

“To some extent, these measures are helpful,” Arora says. But, he says, there’s a lot more that needs to be done, and the problem isn’t a new one. During the 2009 H1N1 pandemic and even earlier, during the SARS outbreak of 2002–2003, “there have been significant debates about pandemic policy,” he says.

Those debates are ongoing during the COVID-19 pandemic as well. But they’re not just about the current emergency, the experts interviewed for this story say: They’re about long-term preparation for a world where the probability of pandemic illness exposure for the public is much higher than it has been in previous eras.

Some of the policies that those who study airports have called for would result in much bigger changes to airports than have happened so far during this pandemic, but could result in much lower disease spread. Nicolaides is an advocate for the idea of siloing passengers further, based on where they came from and where they’re going.

“That way you can keep track of what’s going on,” the University of Cyprus professor says. Of course, that measure would also seriously decrease passenger exposure to retail, which accounts for about 40% of revenue across airports globally.

In addition, there’s a fundamental space issue in airports, says Arora, and it’s leading to incredibly long lines as passengers try to find enough space to social distance while queuing. That issue undoes many of the gains of having less contact when someone does eventually make it to the front of the line, he says. Measures like timed entry and more divided queues could change the degree of exposure.

The biggest change that’s needed, says Schlagenhauf, is standardization. “People need really clear information,” she says.

Right now, Bidwell acknowledges, measures like signage aren’t standardized. “Different airports have different brandings,” he says. That means travelers are encountering an unfamiliar environment with new and specific rules, and even the signs may look quite different from other airports, something that has been shown to decrease efficiency and safety in other traffic control contexts.

Standardization needs to include signage but go far beyond it, Schlagenhauf says. The second a passenger buys a plane ticket, that person is inherently part of a network that touches nearly every point on the globe, even if they personally are going only from Rochester, N.Y., to New York City. She says the guidelines for safety should be coordinated with that global exposure in mind. “The risk doesn’t just start at the airport. It’s the whole journey,” she says. “All travelers are associated with travel-related infection exposure.”

That’s a major change in thinking. These kinds of endeavors have not been widely undertaken by airports, says Arora. Presently, he says, the status quo is to do what was done before the pandemic, but with some new precautions. “That’s not really a change in business model,” he says.

Earlier this year, U.S. airports received $20 billion in federal support as part of President Biden’s infrastructure deal. Some of that money has been spent on these pandemic safety measures by individual airports, says Bidwell, but his organization doesn’t know how much or how exactly it was spent.

In fairness, airports have put real work—and real money—into adapting. Many of the top 10 airports in the U.S. contacted for this story referred Fortune to their blogs, press releases, and customer-facing information, all of which demonstrated the degree of change they have made in the past two years.

Some modified their HVAC system, as in the case of Phoenix Sky Harbor International Airport, to turn over 60% to 100% more air. In addition, “we have increased monitoring and condition assessment of more than 34,000 filters in more than 400 air handlers through our building automation system,” spokesperson Heather Shelbrack told Fortune in an email. The federal money that the airport received was spent on “debt service payments, concessions relief, and operation expenses,” she said in the email.

Miami International Airport highlighted the fact it worked to earn ACI World’s annual airport health accreditation in 2020 and 2021, succeeding both years. In the application from this fall, shared with Fortune, Dan Agostino, the airport’s assistant director of operations, and Jessica Marin, operations special projects administrator, identify some of the difficulties airports face.

“…Wherever possible the airport tries to encourage physical distancing, per CDC recommendation,” they note. “However, due to increase in traffic, it is only feasible in limited times and locations.”

But they can only do so much on their own. Arora says the market forces that drive air travel mean that the bigger disease prevention measures he advocates are unlikely to be adopted by industry on its own. “When you enter an airport, most of the time you’re processed like a commodity,” he says. “That mindset has proven to be very dangerous in an environment like a pandemic.”

Accountability for getting airports right stretches far beyond individual airport owners and the associations that represent them, says Schlagenhauf. “It’s all part of the public health response on a national level and an international level,” she says.