How Particulate Matter Moves and What It Does When We Ingest It

KBMF Environmental Reporter David Hutchins explains what particulate matter is and why it is harmful to human health.

Today I wanted to talk about particulate matter, and why some people in Butte are concerned about it even though we are meeting all of the EPA standards.

So, what is particulate matter? It’s a term for the mixture of solid particles and liquid droplets found in the air, it’s also known as particulate pollution. Some particles, such as dust or smoke, are large or dark enough to be seen with the naked eye, others are microscopic. Sources can be natural, like volcanoes, dust storms and wild fires, or they can be anthropogenic like the emissions from fossil fuel combustion.

And there is good reason to be concerned about particulate matter. The effects of inhalation include asthma, lung cancer, respiratory disease, birth defects, and premature death. Ambient air pollution was responsible for 4.2 million deaths in 2016, according to the World Health Organization.

But researchers have found the size of the particle plays a large role in how dangerous they are. Our respiratory tracts are designed to remove particulate, and the larger particles are filtered in the nose and throat via cilia and mucus. But the smaller particles, less than around 10 microns, can settle in the bronchi and lungs.

It can be hard to visualize how small these particles are. 10 microns is small, about 1/7th of the diameter of a human hair. Particles in this size range include mineral dust, pollen, and mold. But an even smaller fraction, those less than 2.5 micron diameter are even more dangerous. We call this fraction PM2.5, they are less than 1/25th the diameter of a hair, and they are comprised of mostly combustion products.

PM2.5 easily make their way to the lungs and penetrate the gas exchange regions or alveolus, causing inflammation. It is here that these tiniest particles cause most of the adverse health effects attributed to air pollution.

And this is why the US and most countries regulate the small size fractions of particulate. Here, the EPA’s National Ambient Air Quality standards limit 7 different criteria pollutants, including nitrogen dioxide, sulfur dioxide, carbon monoxide, ozone, lead, and, of course particulate.

The standard considers PM 10 and PM 2.5 and includes primary and secondary standards. They are measured by concentration, the mass of the particulate per volume of air, in these cases micrograms per cubic meter. For instance, PM 10 is not to exceed a daily average of 150 ug/m^3 more than once per year.

Particulate matter regulation has largely been successful. From 2000 to 2017 there was a 41% decrease in the national average for PM2.5 concentrations. And this has a significant positive effect on human health outcomes like asthma. Levels have been improving in Silver Bow county, too. We were listed as in non-attainment for PM10 for years but recent improvements have initiated a delisting process. Pm2.5 has been dropping as well, in part due to a woodstove changeout program. A study on PM2.5 in the Greeley area found woodsmoke was a significant contributor in the winter so the county has been removing old woodstoves and installing high efficiency stoves.

So, why are some folks in Butte so concerned about particulate matter? The health department and EPA tell us we are within standards, but everyone has experienced the dust that collects when the wind blows right.

Some people in the Greeley neighborhood feel they are the most heavily impacted, and have been asking for years for assurances that their health is not at risk. Adding to the concern, a recent study conducted by a researcher at Montana Tech suggested there may be metals exposure linked to the dust, but presented more questions than answers. So what’s going on?

Well, as is often the case Butte is wholly unique. There are few, if any, examples of a city our size in close proximity to both massive historic and current mining operations, and the national air quality standards were not written with us in mind.

They were written for the most vulnerable, the kids with asthma in polluted urban areas, and as I explained, this is a very legitimate concern. Those combustion products are a serious health threat. But it’s possible people in Butte have a legitimate concern that has not been nearly as well studied or addressed.

We live in a highly mineralized area, it’s no coincidence, the town is built on top of some metal-rich ground for a reason. The dust that is kicked up into the air often contains many of these metals, and is often in those larger size fractions. When our respiratory tracts act as they are designed and remove this particulate, it is often ingested. This exposure pathway has not been thoroughly studied, and neither has chronic exposure to a low-level broad mixture of metals.

So, people are trying to find answers. Montana Resource, in partnership with the BSB health department, recently launched a voluntary air monitoring study. They will be monitoring and analyzing those larger fractions of particulate matter, both for concentration and for metal content. The Greeley School air monitoring station, now bristling with new instruments, will serve as the study site. MR’s operation is already subject to an extensive air quality permit through the Montana Department of Environmental quality, but they want to be a good neighbor and know if they might be contributing to the problem. Research also continues at Tech examining exposure to a variety of metals and how particulate matter might have a role. Data like these will go a long way in our understanding of the situation and it’s encouraging to see citizen’s concerns taken seriously and investigated. Stay tuned as we’ll be reporting on the findings as they are released.