HEPA filters capture virtually all viruses and bacteria from air, but don’t ‘eat any form of virus’ on planes

South Africa’s tourism industry is back in business after lockdown regulations were eased on 18 August 2020. People are now allowed to travel across provincial borders for tourism, including on airlines. International air travel is still restricted, however. 

Restricted national air travel has been allowed since late June 2020, when the country’s transport minister Fikile Mbalula announced that planes would be allowed to fly domestically at “100% capacity”. 

Explaining the decision, he said: “All our airlines are fitted with HEPA: high efficiency particulate air filters.” This filter, he said, “is able to eat any form of virus including coronavirus inside the aircraft”. 

But was Mbalula correct when he said HEPA filters are able to “eat any form of virus” in planes? We checked. 

What are HEPA filters?

Most modern commercial airplanes are fitted with high efficiency particulate filters, also known as HEPA filters, which filter the air circulating in a cabin.

A briefing paper from the International Air Transport Association (IATA), which sets technical standards for airlines, says the filters are able to remove “virtually all viruses and bacteria” from the air. 

“Even the most difficult particles in the range of 0.1 to 0.3 micron are filtered out with an efficiency level of 99.995%.” 

For size comparison, a grain of fine sand can range from 125 to 250 microns in diameter. Covid-19 particles are about 0.1 microns in diameter. 

After air passes through a HEPA filter it is mixed with air from outside the plane before being recirculated in the cabin.

The air-handling system ensures that the air inside the cabin consists of 50% fresh air and 50% filtered air. According to IATA, this means that the air in the cabin is fully replaced every two to three minutes. (Note: The organisation’s latest guidelines on the use of the filters can be read here.)

Virus needs to stay in air to be filtered

Prof Shaheen Mehtar is an infection prevention and control specialist based at Stellenbosch University. She told Africa Check that HEPA filters do a good job filtering air. But they can’t capture “any form of virus” in a plane, as Mbalula claimed. 

“When we talk about how human beings generate viruses, we have to think that there’s a huge range of particle sizes,” said Prof Ana María Rule, an assistant professor at the Johns Hopkins Particulate Matter Research Centre in Baltimore in the US. These particles are generated when people cough, breathe or speak.

She said that air-handling systems in planes are able to capture smaller particles that are carried in the air. But bigger particles, such as those produced by spittle, are typically too heavy to stay in the air. They will not be filtered but will rather fall from the air and land on surfaces or passengers.

Diseases transmitted in airplanes despite filters

“I think the evidence is enough to say that, yes, air-handling systems help,” said Rule. But infections are still spread on planes, she added. 

Virus particles need to travel some distance from the person emitting them to be captured by the ventilation system. This means that those in close proximity to an infected person are still at risk of infection. 

There are two ways diseases transmitted by droplets can spread. 

Direct transmission occurs when infected droplets land in a susceptible traveler’s eye, nose or mouth. Indirect transmission occurs when these droplets land on surfaces, like tray tables, seat belts or toilet door handles. A person can become infected if they touch these surfaces and then touch their face. 

Rule pointed to a 2016 study which looked at the transmission of Severe Acute Respiratory Syndrome (SARS), measles and influenza on planes. These diseases are transmitted by droplets when an infectious traveler sneezes, coughs, talks or breathes.

The study estimated that passengers seated within two rows of an infected person had a risk of infection of about 6%. The risk decreased to about 2% when passengers were seated more than two rows from an infected person.    

Experts say wearing masks reduces risk

Domestic air travel is allowed under South Africa’s current lockdown regulations. But passengers are required to wear face masks in the airport and on the plane. 

Bheki Mhlanga, flight operations manager for the South African Civil Aviation Authority, told Africa Check that airports and airlines have implemented measures in line with the regulations. 

This includes disinfecting planes before take off, observing social distancing measures during check-in and boarding, and ensuring that all passengers wear masks. Passengers are also not allowed to eat food while onboard. 

But even with these measures, Rule says that flying at 100% capacity is not without risk, especially as people may remove their masks to drink water, which is still allowed on planes. 

She said leaving room between passengers would reduce the spread of the virus. “I think 75% capacity would make it a little safer.” 

Both Rule and Mehtar told Africa Check that wearing masks throughout the flight would help reduce the chance of passengers becoming infected. 

Conclusion: Virus removed from cabin air by HEPA filters but larger particles not filtered

South African minister of transport Fikile Mbalula claimed that HEPA filters on planes are capable of “eating any form of virus” including the novel coronavirus.

While HEPA filters don’t “eat” viruses, they are able to filter about 99% of bacteria and viruses out of cabin air. But they don’t capture “any form of virus”, as Mbalula claimed. 

The filters can’t capture larger infected droplets, such as spittle, that land on surfaces. They also can’t prevent direct transmission between passengers. We therefore rate this claim as misleading.