What is particulate matter and AQI?

Mar 20, 2018 · 5 min read

What is PM 2.5?

Residents of the world’s most polluted cities — places like Los Angeles, Houston, and Beijing — are often all too familiar with the term “particulate matter”. Put simply, particulate matter is the aspect of air pollution that has the greatest impact on our health. It’s everything we ingest, from ash to flakes of rubber from car tires.

Particulate matter is especially concerning given the associated health risks. Those regularly exposed to harmful particles display a variety of symptoms. On a small scale, it can cause irritation, coughing, and shortness of breath. Long term, it can reduce lung function, cause irregular heartbeats, trigger asthma attacks, and result in premature death. People suffering from lung and heart disease, the elderly, and children are at the greatest risk.

Particulate matter can also have disastrous effects on the environment. The most notable effect is the haze and smog Los Angeles is known for. But to a more drastic extent, it can change soil composition, destroying crops and forests. The particles can even find their way into our water supply, increasing acidity and causing other problems.

However, not all particulate matter is created equal. The classification of “PM 2.5” is reserved for particulate matter that is smaller than 2.5 microns in diameter. While particulate matter is always harmful, smaller particles are the most dangerous. They’re able to inch their way into the smallest crevices of your lungs. Once there, they can linger and wreak havoc. And because the smallest particles are invisible, it makes them even harder to avoid.

Smoke from cars

What is AQI?

AQI is a quick abbreviation for Air Quality Index. In the simplest terms, the AQI informs the public on how polluted the air is at any given time. As the AQI increases, larger and larger percentages of the population are expected to experience health effects.

For instance, the United States ranks air quality on a scale of zero to five-hundred. Anything from zero to fifty is green or “good”. But then each step up is worse, going from moderate to unhealthy for sensitive groups, unhealthy, very unhealthy, and hazardous.

Typically, AQI is calculated based on five categories of pollution: ground-level ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Each is measured and weighed differently, and the final product is a generalization of the current air quality.

What makes AQI particularly complicated, however, is that there is no universal approach to measurement. In fact, each country has its own method. To some extent, this is a result of the varying air quality standards and regulations from country to country. This can make global comparisons difficult. But it is important to note that the vast majority of AQI measurements are similar and based on the same measurements.

These differences, however, are why many opt to measure PM 2.5 AQI in place of traditional AQI. As PM 2.5 simply refers to the size of particulate matter, an objective measurement can be taken. PM 2.5 AQI will indicate the number of these small particles in the air. This allows for a variety of insights regarding air quality. As a result, it is sometimes considered the superior option, especially when combined with direct measurements of other pollutants.

Where does particulate matter come from?

Smoke coming from factories

It might be easier to ask where particulate matter doesn’t come from! In fact, nearly everything in the world produces it. From natural sources to manmade, the air we breathe is full of solid particles. And while some are more dangerous than others, many are simply unavoidable. Luckily, the most dangerous particulate matter tends to be created by us — meaning we can make a commitment to eliminate it.

Natural causes of particulate matter include volcanoes, dust storms, forest fires, tornadoes, and hurricanes. Strong winds can toss around everything from dust to ash. However, natural particulate matter tends to be larger in size. In other words, PM 2.5s (the most harmful particulate matter) tend not to come from all-natural sources.

Instead, the smallest particulate matter is most often a result of human action. Combustion is the leading source of PM 2.5 emissions. Particularly, vehicle use, coal-based energy production, and industrial factories cause the most harm. However, human action such as burning wood and tossing road dust while driving also result in larger particulate matter entering the atmosphere.

How is PM 2.5 AQI Calculated?

PM 2.5 AQI is most often calculated by regional and national governments. Using a beta attenuation monitor, scientists can estimate the number of micrograms in the air. Generally, they estimate the number in mass/weight per cubic meter of air. The monitor is complex, but it’s easy to imagine that it sucks in the air and sorts out larger particles, leaving only PM 2.5.

Each particle classified as PM 2.5 is not equal, however. The first ten micrograms measured count for a total of forty-two AQI points in the United States (remember that only a ranking of zero to fifty is considered “good”). But once the air has one-hundred micrograms, each ten only add five points to the AQI. This is due to the severe effects of PM 2.5 — after a certain point, the maximum damage has been inflicted on your lungs.

Why does PM 2.5 AQI Matter?

PM 2.5 AQI is one of the best measures of air quality. And particulate matter smaller than 2.5 micrograms is some of the most dangerous present in the atmosphere. As a result, asking “why it matters” is the same as asking “why does it matter to have clean air?”. At the end of the day, it’s all about quality of life, health, and environmental friendliness. When air quality is poor, severe health impacts are just around the corner.

But more importantly, being aware of PM 2.5 AQI allows us to address declining air quality. For instance, a study followed adults over a period of nearly forty years. They found that decreasing PM 2.5 concentration by just 2.5 micrograms resulted in a 3.5% reduction in mortality rates. That improvement is unreal, not even considering the other health benefits of clean air!

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