More people are flying every day, as Covid restrictions ease and vaccinations accelerate. But dangerous variants have led to deadly new outbreaks, raising questions about just how safe it is to travel now.

In most single-aisle models, you are constantly breathing a mixture of fresh and recirculated air.

On this plane, air is blown in from ceiling ducts and sucked out through vents near the floors. Half of the air that is sucked out is released from the plane, and the other half is filtered and eventually sent back into the cabin.

With the help of researchers, we simulated more than 2 million air particles to understand how they flow within the cabin, and how potential viral elements may pose a risk.

Air is refreshed roughly every two to three minutes — a higher rate than in grocery stores and other indoor spaces, experts say. It’s one reason, in addition to safety protocols, that there have not been many superspreader events documented on flights.

The high exchange rate on planes forces new and existing cabin air to mix evenly, with the goal of minimizing pockets of air that could become stale or linger for too long.

But that doesn’t mean flights are completely safe. This is what happens when someone wearing a mask sneezes on board.Note: Particles are not to scale.

As air blows from the sides, particles move toward the aisle, where they combine with air from the opposite row.

Not all particles are the same size, and most don’t contain infectious viral matter. But if passengers nearby weren’t wearing masks, even briefly to eat a snack, the sneezed air could increase their chances of inhaling viral particles.

This is what would happen if people sneezed in different parts of the plane. To prevent air from circulating throughout the cabin, the ventilation system keeps it contained to a few rows.

By design, the ventilation system is integral to how a plane operates: The system is powered by the engines that propel the plane, constantly sucking in outside air that is then pressurized and conditioned to control for temperature.

Pressurization plays a key role because air at cruising altitude is thin — good for flying fast, but not great for providing oxygen to breathe.

After air snakes into the plane and is conditioned, it eventually climbs up riser pipes to the ceiling ducts that help distribute the air into the cabin.

Throughout the flight, cabin air is periodically sucked through two HEPA filters into a manifold under the floor, where fresh and recirculated air are mixed. Each filter has 12 panels of densely pleated fiberglass mesh that catch most microscopic particles.

Ventilation systems vary slightly among plane makers, but most have similar filtration and recirculation methods.

Once air has been pulled out of the cabin, the portion that will not be recirculated leaves the rear of the plane through a valve that helps to constantly adjust cabin pressure.

The Risks Beyond Flights

How air flows in planes is not the only part of the safety equation, according to infectious-disease experts: The potential for exposure may be just as high, if not higher, when people are in the terminal, sitting in airport restaurants and bars or going through the security line.

As more people fly — nearly 1.5 million people passed through U.S. airports on Friday — congestion and crowding in parts of the airport can make physical distancing a greater challenge.

Airports vary in size and passenger volume, configurations and on-location businesses, Harvard researchers found. That could increase the chances of exposure depending on where people linger and for how long.

Going to in-terminal restaurants, for example, can be risky because masks are routinely removed and kept off to eat.

The Harvard researchers found that many airports were not designed to mitigate the airborne spread of respiratory pathogens. Although some airports have installed new or additional filtration systems, distancing, vigilance and other safety practices are still crucial.

“The challenge isn’t just on a plane,” said Saskia Popescu, an epidemiologist specializing in infection prevention. “Consider the airport and the whole journey.”

This content was originally published here.

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