Particulate Matter (PM) are tiny specks of solid matter or liquid micro-droplets suspended in the air. Suspended matter in air is referred to as an aerosol, from the Greek αη⍴ [Aeir], meaning air and the Latin Sol, meaning solution.
This broad definition captures everything from the exhausts of cars and chimneys, to dust from soil and the desert, to the clouds in the sky and the mist rolling off a lake or river. Aerosols, and thus particles, are everywhere and they can have a profound effect on our health and the Climate.
“I don’t trust air I can’t see” - Gene Hackman
1. Visible or Invisible?
While there are a couple of ways to quantify PMs, they are most often defined simply by their size range. The most common categories are all particles up to 2.5 μm in diameter, referred to as PM2.5, and particles up to 10 μm in diameter, known as PM10. For reference, 10 μm is less than 5x smaller than the width of a human hair! As PM10 contains all particles up to 10 μm, this means a measurement of PM10 also contains within it PM2.5.
While these particles are too small individually to be seen with the human eye, we are often surrounded by them without noticing, local concentrations can be raised high enough to be seen. Combustion smoke, e.g. from candles, fireplaces or cigarettes, is an example of this as is household dust one might see after vacuuming. The effects of particles can extend much wider than this, however - the reddish brown color of many major cities seen during Sunsets is a result of particulate matter interacting with sunlight. The infamous Asian Brown Cloud is a result of dust and soot and can be seen from Space!
2. Where do they come from?
Sources of particles are numerous, both with natural sources and anthropogenic ones. Burning fossil fuels is a major source of particulate matter - but also in ways we might not expect - cooking is a major source of exposure for many, particularly in the developing world where solid biofuels (wood) are used for cooking fuels. Metal dust is given off by cars and trains from their wheels and brakes, while agricultural activities also release certain kinds of particles related to fertilizers, as well as soil-dust from tilled fields.
Not all particles come from human activities though; and PM is also kicked up by winds passing over deserts or oceans. Volcanoes also release a considerable amount of ash, which has a dark color and blocks light from the sun, resulting in a cooling effect. The results can be quite dramatic: the eruption of Mt. Tambora in 1816 released so much PM, the entire Earth was cooled to such an extent that 1816 was also known as the year without a summer.
3. PM and IAQ: How is it linked?
Particle pollution is one of the most prevalent forms of air pollution in the world today. It is an issue for both outdoor and indoor air quality. PMs often infiltrate indoor air spaces through openings in the building, particularly through open doors and windows. Oftentimes particularly small particles are able to pass straight through filters in the building’s Heating, Ventilation and Air Conditioning (HVAC) system, especially when the filters are improperly fitted, old or have a low-efficiency, known in the industry as the MERV rating. Outside is not the only source of indoor particle pollution, however, as everyday objects and activities, such as cooking, dusting , will also generate them.
Particles are strongly linked to negative health outcomes, in particular respiratory and cardiopulmonary diseases. PMs are also known to aggravate breathing conditions such as asthma. Pollens, which are also particles, are well known for the allergic responses that they can cause. Greenpeace estimated that in the top 5 cities around the world, exposure to PM2.5 may have resulted in as many as 160000 deaths. The WHO estimated that in 2016, some 91% of people lived in an area where PM levels were above safe levels.
4. What can be done about PM?
Fortunately, most particles are some of the most controllable forms of air pollution we know of. Monitoring the air, both outside - to give an early warning as to when there may be unsafe levels of particles about to enter your building - and inside, to know what’s in the air that you are breathing, are essential. To prevent particles indoors, banning activities like smoking will definitely help. Other activities we don’t want to necessarily prevent, like cooking and vacuuming should be done, where possible, with open windows and with correctly chosen and regularly-changed filters in place.
Some particle emissions are unavoidable though, and so we should make sure we have a robust HVAC system in place to ensure that air containing PM is passed regularly through a good filter, so that PM concentrations don’t reach concentrations which are dangerous to our health.
- IMDb - Gene Hackman
- Springer Link - Atmospheric Aerosols
- Environmental Protection Agency - Particulate Matter (PM) Basics
- Nature - The contribution of outdoor air pollution sources to premature mortality on a global scale
- Britannica - Asian brown cloud
- UCAR, Center for Science Education - Mount Tambora and the Year Without a Summer
- Greenpeace - PM2.5 air pollution behind an estimated 160,000 deaths in world’s 5 biggest cities in 2020
- Health and Environmental Effects of Particulate Matter (PM)
- World Health Organization - Ambient (outdoor) air pollution