A new monitor could revolutionize the way air pollution is regulated

PITTSBURGH — After working as a physicist for the U.S. government for 45 years, Dave Litton was looking forward to retirement in 2016 — but it wasn’t meant to be.

Shortly after retiring from the National Institute for Occupational Safety and Health, Litton got a call from researchers at Carnegie Mellon University who were developing an innovative new air monitor. They asked him to serve as a temporary advisor, but before long, he found himself running the lab.

“My wife says I’m a workaholic,” Litton, who is now 75 years old, told EHN. “I guess retirement is ok for some people, but I like working. It keeps my mind active and keeps me off the streets, which is probably a good thing.”

Jokes aside, Litton agreed to stay on as Senior Scientist for the CMU-incubated startup Airviz, Inc. because the work is thrilling: The air monitor they’re developing has the potential to revolutionize the way air pollution is thought about, measured, and tracked across the globe.

The monitor, dubbed the Duo PM300, is capable of giving a more accurate measure of
PM2.5 than any retail, residential air monitor on the market. It’s also capable of measuring particles much, much smaller than PM2.5 — which many scientists believe are key to preventing health harms from air pollution exposure.

“Ultrafine particles are one of the most relevant topics we can speak about at this time,”
Juan Castillo, the air quality and health regional advisor for the Pan American Health Organization, the North American division of the World Health Organization, told EHN.

There’s some disagreement within the scientific community about what qualifies as an ultrafine particle, but the term is most often used to describe particles smaller than .1 micrometers or 100 nanometers in diameter. By this definition, ultrafine particles are at least 25 times smaller than PM2.5 particles, which are smaller than 2.5 micrometers in diameter, about 36 times smaller than a grain of sand.

Because of their miniscule size, these particles — which can come from industrial sources, fuel and traffic emissions, and wood burning and dust — are more capable of penetrating respiratory and lung tissue, entering our bloodstreams, and
causing health impacts. These impacts can include everything from asthma and respiratory disease to heart problems, high blood pressure, cancer, central nervous system disorders like epilepsy and autism, and mental illness.

Today, accurate ultrafine particle monitors typically cost between $10,000 and $100,000. As a result, they’re few and far between. There are far fewer ultrafine particle monitoring stations than PM2.5 monitoring stations worldwide, and while some states, like California,
have begun building ultrafine particle monitor networks, only three are officially reporting data on ultrafine particles to the U.S. Environmental Protection Agency (New York, Rhode Island, and Florida).

In contrast, Airviz hopes to sell the Duo PM300 for around $300.

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The monitor uses angular light scattering technology. It’s complex, but essentially entails shining tiny lights on air pollution particles, using a sensor to measure the angles at which the light scatters, then using an algorithm to calculate the particles’ surface area and mass based on those measurements. Angular light scattering technology has been around for a while, but it’s only recently that the light sources and light sensors needed to make these measurements have become affordable.

“Our goal is to create a dense network of highly accurate, extremely localized, real-time air quality data, which doesn’t exist today,” Ian Magazine, president and CEO of Airviz, told EHN.

Magazine said that when he’s at his house in Greensburg, Pennsylvania, the air quality data he sees when he checks the federal government’s
AirNow website comes from a monitor in Clairton, Pennsylvania, about 25 miles away — which is how air quality data works for most people.

“Air pollution can vary widely even from block to block in the same neighborhood,” he said. “It’s not very helpful for me to know about the air quality somewhere 25 miles away, and there’s a lot of information I’m missing by not having access to data on these very tiny air pollution particles.”

“Think about how we used to deal with traffic,” Magazine added. “We used to have to listen to the radio to hear about some traffic issues in our general area, but now thanks to things like Waze we all have direct access to real-time traffic data specific to our route that we can use to make better decisions. That’s what we’re hoping to do with air monitoring.”

Have we been monitoring air pollution all wrong?

Airviz’s new monitor is revolutionary in another way: In addition to measuring the mass of air pollution particles, like PM2.5 monitors do, they also calculate their surface area.

Scientists have long suspected that surface area is a better indicator of the toxicity of very small air pollution particles than mass. The exact mechanism by which air pollution causes health harms is still largely a mystery, but the prevailing theory is that it’s related to inflammation. And many scientists suspect that the level of inflammation caused by air pollution particles is determined by how much of their surface area comes into contact with our bodily tissues.

The idea originated in the 1950s, when coal miners started developing black lung disease, explained Andrew Maynard, a researcher, author, and associate dean and professor at Arizona State University’s College of Global Futures who holds a Ph.D. in aerosol physics. But scientists discovered that while it was difficult to measure the surface area of air pollution particles at the time, it was relatively easy to measure their mass.

“So we just measured mass and based all of our air pollution regulations on mass, but those ideas about surface area being more important didn’t go away,” said Maynard, who worked with Litton briefly at NIOSH in the early 2000s, but is not involved in his current research.

Then in the 1990s, said Maynard, scientists noticed that some particles with relatively small mass — the metric used to monitor and regulate air pollution the world over — could be much more toxic than particles with larger mass.

As researchers investigated this further, two theories emerged: One posited that toxicity depends on the number of air pollution particles, while the other says it’s determined by their total surface area. The debate is ongoing.

“I have personally never been fully convinced of the data suggesting that the number of soluble particles is the most important thing driving toxic responses,” said Maynard, who has investigated the issue. “In our research, we found that there was a very difficult correlation between toxicity and the number of particles, but a relatively level correlation as soon as we looked at toxicity versus surface area.”

Several recent studies have found that exposure to air pollution at levels well below legal thresholds is still harmful to human health. Litton believes this is because current regulations rely on the mass rather than surface area of air pollution particles — and it’s possible for the mass of these tiny particles to be relatively low while their surface area is dangerously high.

Think of the difference between a ton of feathers and a ton of bricks — they’d both have the same mass, but the feathers would have a much larger total surface area.

Litton described a scenario where someone with asthma might see on the AirNow website that the air quality index in their region was in the “good range,” but still experience a flare-up after exercising outside. With a monitor like the Duo PM300, they might have been able to see that the surface area of very small particles in the air in their neighborhood was high, despite the low mass of PM2.5 particles, then use that information to take extra precautions, like using an inhaler or running an air filter and exercising indoors.

“Why else are we still seeing health problems when PM2.5 levels are below recommended health thresholds?” Litton said. “Should we just keep lowering the mass threshold, or is there a better measurement we can use?”

In an ideal world, Maynard said, a monitor like Litton’s would be capable of measuring both the surface area and the number of particles. Litton said adding that capability would likely double the cost of the devices, so they’re placing their bets on the surface area theory prevailing — both to keep their monitor affordable, and because he believes the science is heading in that direction. It’s worth noting that as with mass, it’s also possible for the number of very small particles to be relatively low while their surface area is very high (or vice-versa), so these measurements aren’t interchangeable.

Air pollution regulations and standards

The World Health Organization published new air quality recommendations in 2021, and concluded that there wasn’t yet enough scientific consensus about how to best measure and monitor ultrafine particles to recommend regulatory thresholds.

“This type of research is very aligned with the World Health Organization’s recent recommendations on expanding our monitoring and knowledge about ultrafine particles,” Castillo said, referring to Airviz’s recent paper on their work. “Not everyone is lucky enough to have a big air monitoring reference station near their home, so additional sensors can help fill those gaps. We’d just want to be very sure that kind of data is validated and accurate.”

The EPA, which collects and reports air monitoring data from a large, national network of air monitors that measure not only PM2.5, but also carbon monoxide, lead, nitrogen dioxide, ozone, sulfur dioxide, and PM10 (particles smaller than 10 micrometers in diameter), is currently considering revising its 2020 “final rules” on national air pollution standards. The rules originally concluded that there’s not yet enough scientific consensus about how to measure ultrafine particles or assess their toxicity to warrant regulatory changes, but the agency will review emerging research on ultrafine particles as it considers revising its National Ambient Air Quality Standards.

“There’s often a chasm between the best science and effective regulations,” said Maynard. “There’s compelling data showing that the surface area of these particles is a more accurate predictor of health impacts than anything else we’re currently measuring, but that’s difficult and expensive to do. So anybody who’s making that easier and cheaper is moving the needle toward more effective and more scientifically-based regulations.”

Next steps for measuring “wee tiny particles”

A handful of other researchers have investigated ways to make ultrafine air pollution monitors cheaper and more widely available, but none have gotten as close to putting one on the market as Airviz is now, according to Litton.

Using a grant from the National Institutes of Health, they developed several prototypes, which they’re now testing. Next, they’ll seek additional funding to deploy a small network of sensors throughout Allegheny County and western Pennsylvania — which has particularly problematic air quality — for beta testing and data validation alongside more sophisticated (and more expensive) monitors.

If all goes according to plan, they have a tentative agreement with a telecommunications company to use their existing infrastructure to deploy a wider, denser network of sensors nationwide for additional testing and data validation.

“I would say a year and a half to two years is probably a reasonable timeframe for getting this on the market,” Litton said.

“I worked in government long enough to know that this won’t happen overnight, but one of our hopes is that a gadget like ours will provide enough information for regulators to want to change the metric in order to get a better handle on health impacts related to these very small particles,” he added.

The Duo PM300 got its name because, in addition to PM2.5, it measures particles smaller than 300 nanometers in diameter.

“A lot of people want to be purists, saying ‘ultrafine’ only refers to particles smaller than 100 nanometers in diameter,” Litton said. “But we’re saying that if you only include those particles, you’re missing a lot of the important action.”

Litton and his team decided to include particles up to 300 nanometers in diameter after analyzing 14 years of data from a best-in-class small particle monitor in the U.K. that took measurements every 15 minutes, 7 days a week and 365 days a year. They found that if they only looked at particles smaller than 100 nanometers in diameter, they’d be missing around 40% of the total mass of all air pollution particles, and more than half of the total surface area.

There isn’t an official name for particles this size, but Litton once heard a British researcher refer to them as “wee tiny particles,” which he liked.

“The literature suggests these wee tiny particles also get deposited deep inside the lungs like ultrafine particles do,” Litton explained. “So by including everything up to 300 nanometers in size, we believe we’re capturing just about everything that has the potential to cause health problems.”

Magazine said the goal is to empower people to have more control over the quality of the air they breathe.

“When it comes to what we put into our bodies, we carefully read labels to monitor what we eat and what we drink,” he said. “Right now we can’t do that with air, but what we breathe is really the key to life. All 37 trillion of our cells need a constant supply of oxygen to survive, and we constantly take in millions of tiny particles, toxic vapors and gasses without having any idea that it’s happening.”

“Once people start having access to that data,” he added, “I think a new level of awareness about the importance of clean air and better ways to ensure it will prevail.”

Banner photo: Lab manager Jill Anderson holds a 3D printed piece of a Duo PM300 air monitor prototype in the Airviz laboratory at Carnegie Mellon University (Credit: Kristina Marusic for EHN)