The Invisible World Around Us: How Can We Capture And Clean The Air We Breathe?

What are aerosols?

By definition, aerosols are a “suspension of solid or liquid particles present in a gaseous medium”. That means smoke from a wildfire is an aerosol, and so is the annoying pollen that floats in the summer breeze, causing hay fever. Aerosol size can range from a few nanometers to 100 micrometers (µm) in diameter. To put things into perspective, red blood cells are roughly 7.5 to 8.7 µm in diameter, and a strand of coarse hair is around 100 µm. So, aerosols can be just visible to the naked eye to basically invisible. Don’t underestimate them for their size, though; aerosols can influence climate, weather, and even transport dangerous diseases. 

Aerosols, but the biological version

Aerosols can be subcategorized into bioaerosols, which are basically airborne particles consisting of biological components. These include bacteria, viruses, and even flakes of human skin. And they’re pretty much everywhere. Every time someone sneezes, coughs, or even talks loudly – congratulations, bioaerosols are created. 

Depending on the species and conditions, some can remain viable for several hours, and some are found to thrive in the upper troposphere. Bioaerosols can be harmless, non-pathogenic airborne microbes that exist as part of the microbial ecosystem, with some contributing to cloud formation and rain. However, pathogenic ones can be troublemakers and being airborne is a way to transmit and spread to other hosts. 

Consider the following:

• Viruses: One of the most well-known bioaerosols is the SARS-CoV-2 virus that caused the COVID pandemic. These viruses were transmitted as aerosolized droplets; heavier droplets were deposited on surfaces, whilst smaller droplets could remain in the air for hours. 
• Bacteria: Many bacterial species use aerosol transmission to cause respiratory infections. These bacteria include Mycobacterium tuberculosis (the culprit behind tuberculosis) and Bordetella pertussis (the agent that causes whooping cough).
• Agricultural fungal spores: Farms can be bioaerosol hotspots. Excluding the pollen released from plants that can cause irritation and hay fever, fungal spores that are airborne transmitted can attack a range of crops. An estimated 10-23 percent of pre-harvest crops are lost to fungal diseases.  

Scientists have made huge strides in methods to test patients and developing vaccines to fight infections, but when it comes to detecting airborne diseases, we’re still falling behind – even though catching these invisible threats in the air might be just as crucial. 

How do we detect tiny things in the air?

This is where air sampling comes in. Scientists and engineers have developed different techniques to target different aerosols. These techniques include the use of pumps, filters, electrostatic precipitators, and impactors to extract the aerosols from the air for analysis. For example, electrostatic precipitators trap airborne particles onto a collector plate using an electric field – no magic, just high voltage mixed with a sprinkle of physics. These small devices are handy to place in all sorts of locations, including sampling air at high altitudes.

“We’ve developed electrostatic collection techniques that we can use to collect particles in a really small format device, and they’ve already demonstrated that they could attach that to a drone, fly up thousands of metres into the air and collect particles out of the air,” Professor Ian Johnston, director of the Biodetection Technologies Hub at the University of Hertfordshire, UK, told IFLScience. 

Once captured, the particles can be washed off for analysis, such as imaging under the microscope, growing the microbes in the lab, and sequencing their DNA. These air sampling techniques are clever and they work to some extent, but they are nowhere near perfect. 

“One of the big challenges with monitoring airborne transmission is [in] your devices that you might use to try and collect bacteria, viruses, […] pathogens from the air; [it’s] in understanding how reliable they are,” explained Professor Johnston. “If they’re not very efficient, then the challenge you have is trying to find something that might cause an infection in a person in huge spaces of air. So, you’ve got to effectively sample large volumes of air and do that in such a way that you retain the ability to identify those virus particles that you’ve collected afterwards. One of the problems with that is that there has not been much investment in those technologies.”

Having to sample large volumes of air means the technique would be quite time-consuming, which leads to another challenge. Bioaerosols have a limited viability period, and to make matters worse, certain techniques used to detect them can kill them before they reach the analysis point. 

So, investments in biodetection for improved technology are very much needed.

Air filtration: making the air we breathe a little cleaner

Once you know there are countless microbes, pollutants, and particles swirling around in the air, the next question is obvious: how do we stop breathing them in?

Filters are the best collection system to take stuff out of the air.

Professor Ian Johnston

One of the answers is filtration. But do they actually work?

“They definitely work,” reassured Professor Johnston. “[W]e’ve worked on many different collection systems and filters are the best collection system to take stuff out of the air.” 

The most common household filter, the mighty HEPA filter (short for High Efficiency Particulate Air), can trap up to 99.97 percent of particles, including tiny ones with 0.3-µm diameters. That means these filters are effective for capturing dust, pollen, fungal spores, bacteria, and many viruses from the air. 

Luckily, air filters are becoming more popular, especially in public spaces. These systems quietly trap airborne nasties before they have a chance to spread, making the indoor air safer and healthier for everyone breathing it in – as long as the filters are changed when required!

“Whether people replace the filters often enough, and whether […] you’ve bought one that really can empty the air in the room quick enough, is a different kettle of fish,” Professor Johnston said.

Adding these filters in all buildings is easier said than done. Air filtration requirements are different for building types; for example, hospitals may require super clean air to avoid patients getting infected, whilst the pressure change in the room from filters with the highest ratings might be uncomfortable for hotel guests. And keep in mind, these filters come with serious price tags, not just to install but for ongoing maintenance too. 

If you’re just looking at disease transmission in workplaces, can we reduce by 50 percent the amount of time off that people have in the winter just through illness?

Professor Ian Johnston

Moreover, air filters usually do not reduce carbon dioxide in indoor air. Studies have shown that the carbon dioxide levels are linked to respiratory pathogen transmission risks. 

“There’s been really valuable information that’s been gained by understanding how much that stale air means you’re likely to have much more exhaled breath, potential viruses, and other pathogens in the air. So, it was very valuable. I don’t think it’s necessarily particularly valuable as an ongoing metric, but it’s very valuable for scientists to understand how important that is,” added Professor Johnston.

So, let’s not forget Mother Nature’s fresh air. “[B]ringing in fresh air from outside is more effective and cheaper. If you could just do air exchanges, then you don’t have to worry quite so much, and it’s easier.”

Lessons learnt from COVID 

COVID-19 was devastating because of the pandemic it caused, but it gave us an insight into airborne transmission in indoor spaces and spread awareness on the dangers of bioaerosols. “I think it’s interesting to understand what transmission processes are occurring, to understand whether they’re a threat, whether there’s anything we need to do to mitigate a threat, or whether it’s just the general scheme of how everything works,” Professor Johnston told IFLScience.

New knowledge and ongoing research on bioaerosols can help prepare for and prevent future outbreaks and could help cut down the spread of seasonal colds and the flu. “I mean, we know that when there’s an unusual event like a COVID outbreak that absolutely if there had been better mitigation in place, fewer people would have died. There would have been less catastrophic damage to the global economies. There were things that could have been done to kind of alleviate those problems.”

You might not think it matters, but you’re breathing it in all day long.

Professor Ian Johnston

“What’s unclear is that at a smaller, more local level, whether there are things that can be done that have similar effects. If you’re just looking at disease transmission in workplaces, can we reduce by 50 percent the amount of time off that people have in the winter just through illness?” Professor Johnston continued.

“You know we’ve all caught flu. We’ve all had bad coughs and colds. If you’re off for a week and you didn’t need to be, then the productivity of the country could be significantly increased through what are effectively […] minor changes like changing ventilation systems – not so minor, it’s actually quite expensive. But in the grand scheme of things, as you build new buildings, if you learn how to solve these things, then they can become much more important for the future economies.”

COVID-19 taught us key strategies to lower the risk of infection, which included improving ventilation, wearing face masks, and reducing crowding indoors, with studies showing that airborne spread indoors is preventable. 

Futures of aerosol detection technology

In the battle against new emerging diseases, researchers are working together to improve and discover new technology for aerosol detection. Current research, Professor Johnston told us, includes developing “[a]pproaches that might enable us to collect particles from the air by different mechanisms. Electrostatic mechanisms, dielectrophoretic mechanisms, things that are effectively low power, low noise, and could be run for long periods of time without disrupting everybody.”

New research isn’t just about making detection devices. “We build instrumentation, but there are other people, for example, that are colleagues down in the University of Bristol who are working on techniques to assess the kind of aging and the effects of change of pH […] on what happens to virus viability within micro droplets. So, there’s really good work going on there to understand what happens [and] are there environmental conditions that we can impose that mean that it’s likely that the virus will become less infectious.” 

Aerosols may be tiny and sometimes invisible to the eye, but they can cause massive disruptions to life; luckily, new research is being conducted to help understand and reduce their risks. 

“[F]rom my perspective, just that they’re absolutely fascinating and they’re everywhere. They’re in every breath you take,” explained Professor Johnston. “You might not think it matters, but you’re breathing it in all day long. Understanding what you’re breathing in seems really important.”