What is particulate matter?

Particulate matter (PM), also known as aerosols, is a mix of fine dust and tiny liquid droplets suspended in the air.  

Even though much particulate matter is so small as to be invisible to the naked eye, it is possible to see others in high concentrations like smoke from a chimney, a candle, or cigarette. Unlike gaseous pollutants (ozone, NO2), the fine particles are not characterized by their chemical composition. In fact, the term particulate matter refers to a wide variety of elements, from the organic (pollen or spores for example) to the mineral (soot and smoke). These particles also come from lots of different sources and in as many shapes and sizes as you can imagine. 

PM is classified by size, not by what it’s made up of. For example, when you read that PM10 is high today in Los Angeles, the number 10 actually refers to the diameter of the dust being measured. In this case the PM10 means that the dust measures less than 10 micrometers. That’s smaller than a human hair. 

Particulate matter is categorized this way because the size of the dust actually determines how bad the PM is for our health. The smaller the size (and number), the more dangerous they are to breathe.

Here’s the list of PM size categories:

  • PM10 (coarse)
  • PM2.5 (fine)
  • PM1 (very fine)
  • PM0.1 (ultrafine)

PM2.5 and PM10 are well-known and form the foundation of regulations around the world. However, the smaller particles are still not commonly regulated. There’s more work to be done to study health effects and develop monitoring technology for these tiny pollutants.


What causes particulate matter? 

Particulate matter can come from natural sources (fine sand, wildfires, volcanic eruptions and sea salt for example) or from human activities, usually involving some kind of incomplete combustion—a fire or explosion.


wildfires forestWildfires cause high emissions of particulate matter


The most common, human generated sources of PM are:

  • Industrial processes
  • Combustion heating (wood burning, coal, oil, etc.)
  • Transportation (diesel vehicles in particular)
  • Intensive agriculture
  • Energy production
  • Garbage disposal (burning)


Particulate matter can also be a big problem indoors. Smoking is definitely a major source of PM indoors, but there are also less obvious things that can create this type of pollution. For example cooking, making toast, or even lighting a candle can create high levels of particulate matter indoors.


What are the health effects of particulate matter?

The impact of particulate matter on our health has been well documented in many studies over the last decade. According to the World Health Organization (WHO), particulate matter is the most dangerous type of pollution when it comes to human health. Even low levels of PM can have serious consequences. Particulate matter has been classified as a carcinogenic by the International Agency for Research on Cancer (IARC), and there are countless studies that link particulate matter exposure to shortened life spans.

Particulate matter can penetrate into our respiratory system, causing simple eye or throat irritation, or more serious issues in our lungs, hearts, and brains. Particulate matter has been linked to diabetes and high blood pressure, and some studies have even linked prenatal PM exposure to serious issues like premature births and developmental issues for the baby.

The size of each particle is directly linked to how dangerous they are. The smaller they are, the deeper they can penetrate into our bodies, causing more damage. PM2.5 can actually pass through the membranes in our lungs into the bloodstream. Whereas PM10 is largely filtered by the nose and remains in our respiratory system. 


Where and when do we see particulate matter pollution peaks?

Meteorological conditions play a big part in particulate matter peaks. For example, in the winter when there is lots of cold air at ground level combined with a high pressure system, particulate matter does not disperse and starts to accumulate in the air. In big urban centers, it’s very common to find high levels of particulate matter along busy roadways and transportation arteries. Extreme levels of particulate matter are also commonly found in subways around the world (usually a result of the braking systems).

It has been possible for many countries in the northern hemisphere to reduce particulate matter pollution in the air over the past decades by adding filters to car exhaust systems and industrial smoke stacks, improving other aspects of industrial processes, and reducing the carbon used in energy production. Even with these changes and improvement, it is still common to find high levels of particulate matter.