Breathing is a fundamental part of being alive. But how much do you really know about the air you breathe? Matthew Ward Agius pulls unseen killer particles out of thin air. This article was originally published in the Cosmos Print Magazine, September 2024.
Close your eyes and take a deep breath. Just a fifth of a lungful of air is life-giving oxygen. The rest? It’s about four-fifths nitrogen, an unreactive gas that goes into our lungs and sits around waiting to be expelled as we breathe out. Plus there’s a 0.9% skerrick of argon, an inert gas used in fluoro lights.
That said, no two huffs of air are truly alike. It’s the remaining 0.1% of air that gives a gulp its distinguishing features. Mixed with those majority gases are tiny traces of neon, carbon dioxide, water vapour and hydrogen.
This gassy mix is mostly the same anywhere on the planet. But open your car window as you drive down a busy road in peak hour? The air is likely to be noticably polluted compared to a breath riding your bike on a quiet country lane.
Air is also alive! It’s choc-full of bacteria, fungal spores and pollen, which – for better or worse – find their way into the airways of far bigger and more complex organisms like us. There are also viruses, which activate the moment they find an unassuming host.
These biological materials confer air with a deadly attribute. Bad microbes can cause disease, pollen and dust can trigger allergies. For some, this is a temporary nuisance, for others, a killer.
These are not the only ways air can kill, so health scientists around the world are sounding an alarm.
What is particulate matter?
Particulate matter is microscopic solid material. Abbreviated to PM alongside the material’s width in micrometres (PM10, PM2.5 or PM0.1), it can be made of just about anything.
PM is a catch-all term because it’s frankly impossible to classify every unique speck of floating solid. Tiny flecks of dust and pollen – both classed as PM – are best known for exacerbating allergies, causing acute lung symptoms and, if exposure is prolonged, leading to chronic cardiovascular or respiratory issues.
There are also many chemical transformations that turn gaseous pollutants into PM. Potentially dangerous pollutants include sulphur dioxide, nitrogen oxides and other products of natural and anthropogenic combustion.
Pollution in the air
at the University of Swansea, UK. Credit: Courtesy of Martin Clift.
Long unseen, the hidden influence of PM, and air pollution in general, is slowly being exposed. The fact remains that 99% of the world’s people live amid air pollution that exceeds the limits outlined by the World Health Organization.
“Every breath you take, you inhale about a million particles, and we know through in silico [computational] modelling that the deposition of at least 50% of those will go into the lowest parts of our lungs,” says Martin Clift, an inhalation toxicologist at the University of Swansea, UK.
While half of these particles might make it to the extremes of lung tissue, the other half will be blocked in the nasal passage. In the nose they’ll be cleared by mucus transported up and away from the lungs, or by a good sneeze.
For particles that remain in the lungs, the body’s immune system will attempt to finish them off. “The human body’s great like that,” says Clift. “But some of them will stick around and they’ll just reside there over time.”
Particulate exposure
In November 2023, a study published in the journal Science revealed the power of particulates. It revealed that 460,000 Americans had deaths attributed to PM2.5 exposure from coal power stations between 1999 and 2020, following an analysis of US Medicare records.
Australian-led research released in March this year evaluated the impact of PM2.5 on a global scale. Led by Yuming Guo, a biostatistician at Monash University, it put about a million deaths per year globally down to short-term PM2.5 exposure.
“Many of these gases that we’re breathing in can also exchange down the concentration gradient to the lungs and enter our bloodstream,” says Jason Kovacic, a cardiologist and chief executive of the Victor Chang Cardiac Research Institute in Sydney.
“They circulate through the body, where they can have a whole range of adverse effects. There’s a whole raft of different things that we know about and there are probably effects we’re not even aware of that are going on.”
As well as the health burden, estimates show substantial financial impacts. The OECD estimates the global cost of premature death, and pain and suffering from pollution-related illness, was US$3.2 trillion in 2015, and could rise to US$18-25 trillion by 2060. The European Commission puts the cost at €1 trillion annually.
“They’re even likely to be conservative estimates,” says Kovacic, “because the true, pervasive nature of the cost of pollution is hard to quantify.”
The lung lab
Clift’s research group is trying to plug a vital gap in the practical understanding of what epidemiologists and statisticians are seeing play out in health data.
We visit a shelf in Clift’s laboratory in Wales. Here you’ll see ‘lungs-in-a-dish’: in vitro human lung cell cultures spun up by scientists at the University of Swansea. They mimic sections of the lungs with LEGO-like modularity.
These aren’t fragments of lungs from a human, but they are about as close as you can get to the real thing. Of course, subjecting a living human to a nitrogen dioxide chamber would fail the first ethical hurdle. And, as the Swansea researchers have argued, rodent models aren’t able to suitably mimic human lung function.
In one experiment, Clift’s team has dished up the lungs’ alveolar region, the place “where the air meets the blood”. Cells mimic cauliflower-like buds at the end of the lungs’ bronchiole branches. Some are designed to imitate susceptibility groups, such as those with asthma.
The researchers expose these dish lungs to different scenarios using an aerosol machine.
“We can mimic occupational exposure in a warehouse, for example, but then we can also mirror just consumer exposure walking through the streets of London,” Clift says.
The Swansea researchers are careful to replicate particle concentrations that a set of lungs might encounter in the real world. When their models are exposed, they can see how these emulated structures respond.
“We know when we’re exposing [the lung cultures] to all of those different pollutants, or particles, or fibres, that the cell system is mimicking what would be happening in the healthy human lung. We can very much control the [particle] concentration that’s deposited, so we can relate that with what humans are actually exposed to.”
It’s hoped that subjecting these dish lungs to relevant aerosol exposures could help guide new air quality standards and policies adopted in cities around the world.
Pollution and policy
With almost everyone on the planet experiencing sub-standard air quality, some scientists see policy as the fast route to improved air.
At the forefront of efforts to communicate the risks of air pollution is physicist Lidia Morawska, a distinguished professor and director of the International Laboratory for Air Quality and Health at Queensland University of Technology.
Morawska was recognised as one of Time magazine’s 100 Most Influential People in 2021, thanks to her determined effort to show that SARS-CoV-2 (the coronavirus that causes COVID-19) was spread by aerosols, raising the importance of air quality as a health issue during the height of the pandemic.
After the World Health Organization took heed of her counsel, ventilation quickly became the buzzword for improving indoor air quality. “Our aim is clean indoor air, and it has to encompass protection against all the risk from inside, from outside, and taking into account other aspects like thermal comfort,” Morawska says.
To emphasise the importance of pollution as an issue, she relates a simple metaphor: “Imagine you are coming to a restaurant, and they give you a glass of water. That water looks murky, so you look at it with disgust and express what you think… they quickly bring you a clean glass of water. But what if they offer you bad air quality?” she asks.
When alfresco dining on a busy, high-traffic street, would you like a lungful of particulate matter with that?
Brandishing her carbon dioxide monitor, Morawska says these simple devices could be a small step towards improving how people think about air.
As well as being a pollutant, carbon dioxide can be treated as a proxy for exposure risk to airborne disease pathogens.
In March, Morawska led the publication of a ‘blueprint’ for indoor air quality regulations in Science. Morawska’s group emphasises that indoors is where air is breathed most of the time.
Their proposal would see authorities legislate new indoor thresholds for PM2.5 (at 15 micrograms/cubic metre/hour), carbon dioxide (800 parts per million) and carbon monoxide (35 milligrams/cubic metre/hour).
The challenge, Morawska says, is fashioning regulations that are actionable.
For most, improving air quality won’t be a cheap solution. Countries willing to take on the short-term cost, they hope, could lead major change in quick time.
Where such measures are cost-prohibitive, other steps could be used to regulate indoor pollution, such as smarter ventilation methods, air filtration, purification and disinfection.
“In general, this is not something which should be left to individual responsibility, or even the responsibility of individual building managers,” she says. “It should be mandated such that it’s very clear what to do, how to do it, and such that individuals don’t have to worry that there’s pollution in the air.”
There’s still a long way to go, but policy to protect human health could also help – or be helped by – the other great challenge of the times: climate change.
Twin predicaments
Not only does rising atmospheric pollution from human activity drive up global temperatures, it’s also the cause of the air taint scientists are worried about.
Fossil fuel-based electricity generation and industrial processes together account for more than half of the world’s greenhouse gas emissions, with the agriculture-forestry-land use sectors (22%) and transportation (15%) the other big emissions sectors contributing to carbon rise.
Nitrogen dioxide and carbon monoxide? Look to your tailpipe or internal stove. Sulphur dioxide? Fossil fuel combustion for power and industry. Ammonia? Fertiliser and agricultural animal dung. PM? It comes from of the above, plus, of course, the natural combustion of biomass in wildfires, which may become more frequent or intense in some parts of the world as temperatures rise.
Writing in the Journal of the American College of Cardiology, Kovacic and his colleagues warn of these twin predicaments. Reviewing hundreds of studies covering millions of patients, the risk of long-term cardiovascular problems such as ischemic heart disease, circulatory mortality and stroke are significantly increased by pollutant exposure, with impacts experienced from within the womb until a person’s dying day.
Some studies suggest around five million lives could be saved each year by turning off fossil fuels and switching to clean energy sources.
“Some of the things we need to do about global warming, they’re the same things we need to do to fix cardiovascular health,” Kovacic says.
Already in Australia, some jurisdictions are intervening to scrub out sources of pollution. Victoria, for instance, has banned gas connections for new buildings. Last year, the ACT government announced a new policy to phase wood heaters out of Canberra by 2045.
Kovacic’s recommendations extend beyond merely achieving the ‘green switch’ that moves away from combustible fuels for energy, though that’s his key aim. He also sees the value of public and clinician education programs to improve air quality literacy.
“That was why we did [the research], to raise awareness and make sure that physicians, doctors, politicians and patients are all aware of this problem.”