What is Particulate Matter (PM)?

PurpleAir

PM - which stands for particulate matter or particle pollution - is an intricate mixture of liquid droplets made up of acids (like nitrates and sulfates), ammonium, water, black carbon, organic chemicals, metals, soil material, and air-borne particles. 

EPA (Environmental Protection Agency) groups PM into two categories:

  • PM10-2.5  are "coarse" particles, commonly found near roadways and dusty industries. These particles range in diameter from 2.5 to 10 microns. Furthermore, the existing "coarse" particle standard (known as PM10) includes all particles less than 10 microns in size.
  • PM2.5 or "fine" particles, commonly found in smoke and haze have diameters less than 2.5 microns. 

It is hard to imagine a diameter of less than 2.5 microns and have a visual idea of how really small it is. To have a clearer idea, a single strand of hair is about 70 microns in diameter!

PM2.5 Scale

Image 1.1 Size comparisons for PM particles (EPA, 2022)

    How does PM affect my health?

    Due to its small particle size, PM2.5 can travel deep into the respiratory tract, causing adverse health issues. Exposure to this particles can affect lungs and heart, potencially causing some of these effects:

    • Nonfatal heart attacks 
    • Irregular heartbeat 
    • Aggravated asthma 
    • Decreased lung function 
    • Coughing or difficulty breathing

    People with heart or lung diseases, children, and older adults are the most likely to be affected by particle pollution exposure.

    In the city or suburbs, where there are many PM2.5 emission sources such as localized traffic, construction, and even wood fireplaces, the air quality may differ significantly from block to block. PurpleAir Map's hyper-local, community-driven data allows everyone to understand their environment's air quality and make informed, healthy choices.

    How do PurpleAir sensors work?

    PurpleAir sensors use PMSX003 laser counters to measure particulate matter in real time, with each laser counter alternating 5-second readings averaged over 120 seconds.

    Each laser counter uses a fan to draw a sample of air past a laser beam. The beam from these class IIIa/3R lasers will reflect light from any present particles onto a detection plate, like dust shimmering in a sunbeam. The reflection is measured as a pulse by the detection plate, and the length of the pulse determines the size of the particle while the number of pulses determines the particle count.

    These particle counts are used to calculate the mass concentrations of PM1.0, PM2.5 , and PM10  for standard indoor (CF_1) and outdoor particles (ATM).