How risky is flying without a face mask?

After two years of having to wear face masks on flights to and within New Zealand, the requirement has now been lifted.

The government now only requires masks to be worn in healthcare settings and aged care facilities. Airlines could have continued to enforce their own face mask policies - but both Air New Zealand and Jetstar have decided to make them optional.

Air New Zealand said its chief medical officer Dr Ben Johnston had undertaken a risk assessment and advised it was now appropriate to remove the face covering requirement.

"This also aligns with recent changes to government public health advice and settings," a spokesperson said.

Air New Zealand chief customer and sales officer Leanne Geraghty said in an earlier statement they "really encourage" both customers and staff to continue to do what makes them comfortable, and the airline would continue to make masks available.

But how risky is flying without a mask? And how effective is wearing one when others on a plane aren't?

Clearing the air

Air travel was one of the hardest hit industries of Covid-19. Naturally, the industry has been eager to quell fears about flying in a pandemic.

The International Air Transport Association, which represents 290 airlines around the world, says the risk of contracting Covid-19 onboard a plane is "very low".

It explains on most modern jet aircraft, cabin air is made up of around 50 percent fresh air brought in from outside, and 50 percent air that has gone through High-Efficiency Particulate Air (HEPA) filters, which are said to be more than 99.9 percent effective at removing viruses, bacteria and fungi. Thanks to these systems, the cabin air is completely changed every two to three minutes.

As University of Auckland aerosol chemist Dr Joel Rindelaub previously told Stuff, it was true planes have an advantage over some other forms of public transport or indoor environments due to their superior ventilation systems.

But "low risk" is not "no risk", and there have been documented cases of Covid-19 transmission on flights, including a domestic flight in New Zealand.

In September 2020, someone who had just completed 14 days of MIQ caught Covid-19 while on a government-chartered flight from Christchurch to Auckland. The person had been sitting in the row in front of two unknowingly infected people who had also just been released from MIQ.

The case was written about in the US Centres for Disease Control and Prevention's Emerging Infectious Diseases journal.

Layering up

As good as the ventilation might be when you're actually in the air, that's only one aspect of air travel - there's also the time spent going through security, waiting around the boarding lounge, queuing up to board the plane, and waiting to get off the plane.

Researchers at Harvard University looked at ways to reduce the public health risks of flying during the Covid-19 pandemic.

In their first report, released in October 2020, they found the risk of transmission on an aircraft could be reduced to very low levels through a "layered approach", which took into account an aircraft's ventilation system, disinfection and cleaning procedures, and - crucially - passengers wearing face masks.

The authors recommended that passengers and airline employees should be required to wear face masks throughout their entire journey, including time spent in the airport, boarding, in-flight, in the toilet, and getting off the plane.

"Face masks... are a critical element in preventing the spread of respiratory infectious diseases while travelling," the report said.

"In fact, face mask requirements are perhaps the most essential layer of a comprehensive set of measures to reduce transmission of Covid-19 throughout air travel."

It's worth noting that this report predates vaccination and widespread infection. The key driver of Covid numbers is now immunity.

As Professor Michael Plank explains: "Blanket measures such as mask mandates in places like retail, schools and workplaces are likely to have a marginal effect on the number of infections in the long-term. The reason is that, at any given point in time, the large majority of the population will be immune to the virus and so the majority of masks will be having little or no effect."

Scrapping masks

Many countries have this year scrapped mask mandates for travel. Most US airlines have dropped masking requirements since a federal judge struck down the travel mask mandate in April, while the European Union abolished its mandatory face mask requirement on flights and at airports in May.

So has there been an increase in Covid cases on flights as a result? It's hard to say, said Linsey Marr, an engineering professor at Virginia Tech who is an expert in the airborne transmission of infectious disease.

"It's unclear whether there has been increased Covid transmission during air travel because I don't think anyone has tried to look at this question," she said in an email to Stuff.

"In the US, we have certainly experienced increased Covid transmission everywhere because of the BA.5 variant. Anecdotally, I have heard of a lot of people getting infected while travelling, although it's unclear whether transmission occurred during a flight, in a hotel or restaurant, or elsewhere."

Because there are so many variables, it's difficult to quantify just how risky it is to fly without a face mask, she said.

"However, it is definitely riskier to fly without one than to wear one."

The right mask

But are you still protected by your mask on a plane if no one else around you is wearing one? The experts say yes - depending on the mask.

"A good mask can reduce your exposure to virus particles in the air by 95 percent or more, whether other people around you are wearing a mask or not," Marr said.

"This will translate into a reduction in your risk of getting infected during the flight."

Based on studies of masks in other settings, wearing a respirator - like an N95 or a KN95 - was associated with a much lower chance of testing positive for Covid-19.

That means reaching for the surgical masks provided by the airlines may not be of much benefit.

Ben Mullins, an aerosol scientist and professor at the School of Population Health at Curtin University in Perth, said surgical masks were mainly designed to stop the wearer from emitting virus-laden particles, and were generally around 60 percent effective at preventing transmission.

For the best protection, people should wear an N95-type mask, which were 90 to 100 percent effective.

Rindelaub from the University of Auckland agreed an N95-type mask was the best option.

"That is the best way to make sure you are protected if you decide to wear a mask on a plane."

To mask, or not to mask?

Mullins said the decision to wear a mask would depend on your personal risk profile.

"If you've got pre-existing health problems - especially any vascular or cardiovascular issues - then you should wear the best quality mask you can, regardless of what the rules are," he said.

"Those people who are healthy and fully vaccinated might be willing to have a less risk-averse option."

Marr said if you mask during the flight, you should also mask at other points of your trip, such as in crowded parts of the airport and on buses, trains or taxis you take to get to or from the airport, as these also contributed to your exposure.

Rindelaub also said wearing a mask throughout the whole journey was necessary, taking into consideration the lack of ventilation at certain points.

"Especially if you're in the queue at the terminal trying to get on the plane, there's not going to be a lot of ventilation in that hallway," he said.

"The flight itself - in the aeroplane - does have some ventilation to help you out, but if there are people near you that are infected, that's where the risk increases dramatically."

Wearing a mask, he said, was an easy thing to do to help ensure you stay safe - and he would personally be continuing to do so on all public transit, including flights.

"Just because it's not mandated doesn't mean you shouldn't do it. It's still a great idea to mask up when you're flying."

- This story originally appeared on Stuff