How Little we Know about Particulate Matter (and Why It Matters)

Whenever we hear about bad air quality, a few pictures come to mind- industrial Beijing at its worst, or the busy streets of New Delhi, sometimes children wearing masks through the fog of Mexico City. But we rarely think of major US cities as having exceptionally bad air. After all, the United States has on average a dozen times less particulate matter than the most polluted countries, with an annual pollution level 20 percent below the safe benchmark established by the World Health Organization.

Screenshot--18--1 But for those of us who live in Salt Lake City, it’s obvious that on some days, our air quality seems to be everything but safe. This is why non-profit organizations such as the Utah Clean Cities Coalition, Utah Physicians for a Healthy Environment, Breathe Utah and the Utah Clean Air Partnership dedicate resources to increase awareness of how unhealthy levels of air contaminants affect Utah, and educate us on the lesser-known explanations of where these pollutants originate.

Most of the particulate matter pollution suspended in Salt Lake City air comes from on-the-road and idling vehicles, industrial facilities such as coal power plants and residential burning stoves and fireplaces. These sources produce PM2.5 which is a label given to all inhalable microscopic particles 2.5 micrometers in diameter (1/20th of the thickness of a human hair) or smaller. This is most organizations’ larger concern, since once inhaled it can become trapped in the lungs.

Additionally, although most respiratory health threats associated with poor air quality are believed to be caused by PM2.5 exposure, there are other categories of pollutants (such as PM10, ozone, carbon monoxide, carbon dioxide and other hazardous industrial chemicals) in the air we breathe. These categories are also likely to jeopardize public health, and simply lack the attention that researchers and health practitioners have placed on PM2.5.

Credit: Patty Jiang IMG_20171110_150354

Following annual temperature drops, most urban areas surrounded by mountains will experience notorious winter inversions. An inversion is a phenomenon where the air closest to the surface of the air gets colder (or the normal temperature gradient is inverted), the difference in density between it and the layers of air above is too large for them to mix and pollutants caught in the cold, dense air are sealed by the less dense, warmer lid of air above, like a bowl. Salt Lake City, being the largest city in the Salt Lake Valley, experiences temperature inversions most winters, which suspend PM2.5 in the air for days at a time.

During a Salt Lake City inversion, air pollution sensors can measure concentrations as high as 50 micrograms per cubic meter.This is twice the concentration classified as safe by WHO guidelines.

According to the Center for Disease Control, unsafe exposure to fine particulate matter has been linked to eye and respiratory tract irritation, lung cancer and problems at birth like prematurity and low weight. Older adults, young children, those with pre-existing respiratory conditions and heart disease are at a higher risk when it comes to inhaling PM2.5. Many will follow advice from official sources and change their behavior by limiting time outdoors or wearing masks, and some Utah schools have even started holding recess inside during winter months.

The Utah Recess Guidelines, use a color scheme based on the EPA’s Air Quality Index to classify PM2.5 levels as good (green), moderate (yellow), unhealthy (orange), very unhealthy (red) or hazardous (purple). Schools have a flag reflecting the Department of Health’s forecast for the day as one of the five categories. Green and yellow flags let staff and students know that recess and P.E. can be done outside, as usual. An orange flag is a warning that children with asthma and other respiratory conditions should stay inside, while red or purple flags mean everyone should stay inside.These guidelines could easily be used as a model for the general population to know when we’re having a bad air day in the city.

Modifying our lifestyles to fit in safer, more restricted routines is not only inconvenient or harmful to a child’s quality of life, but represents an increased risk of suffering from ischemic heart disease (the leading air pollution-caused killer in the US), and negatively impact the economy. In order to minimize particulate matter exposure efficiently, we need to become better educated about the state of air quality in our cities. In an attempt to disseminate crucial health-related information, much about the nature of these health threats is left unanswered. To start, what is in PM2.5?

To understand what is in this 2.5 micron particulate matter, we first need to know how its concentration is measured. Devices called light-scattering sensors collect air with PM2.5-sized particles in a chamber and shine a laser beam onto it. Then, using a light detector, the number of particles can be calculated as a function of how much the beam was scattered. This works extremely well at telling us how concentrated these particles are, but it tells us nothing about the particles’ nature.

According to the EPA, it’s mainly soot particles that make up the “2.5-micron or smaller” readings, but dust, pet dander and aerosol particles from household products can also fall within this range. They can, therefore, make it into the PM2.5 count of a conventional air pollution detector, which is used to measure both ambient and indoor air quality.

Surprisingly, studies carried out inside homes to measure indoor air pollution show activities like cooking or opening and closing windows also having any of the above-mentioned common particles drew potentially misleading peaks in PM2.5-measuring devices. And although we cannot ignore that these do aggravate respiratory issues in sensitive groups (and that PM2.5 readings anywhere outside a house coming from combustion exhaust could similarly be underestimated), the average healthy adult should not plan on limiting time spent in the living room or wearing a mask in their own home to stay safe.

We can and should, however, become concerned with the availability of more accurate and detailed information about air pollution in Salt Lake by learning more about the sources of particulate matter in the air they breathe, supporting ongoing research on the nature and distribution of these and relevant contaminants and join initiatives that promote awareness of our city’s alarming air quality.


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