Particulate matter like dust, pollen, mold, smoke, and even metals are in the air we breathe. Too much particulate matter in the air reduces the lungs ability to utilize oxygen. Over time this leads to difficulty breathing, lung disease and even death.
Air Particle Background
For centuries scientists have known about the adverse health effects of breathing contaminated air.
- Georgius Agricola, a German mineralogist, first described lung disease in coal miners in the 16th century. He found that the disease was caused by coal dust. “Black lung disease” is now widely recognized worldwide as an occupational illness.
- The widespread use of Asbestos beginning in World War II led to many asbestos-related lung diseases 20 years later.
- Smog, a term derived from combining smoke and fog is primarily found in big cities and other industrial areas. It is composed of nitrogen and sulphur oxides, ozone, smoke and other particulates. Smog is considered so dangerous that countries have “smog alerts” where city residents are instructed to stay indoors or to only travel wearing a mask.
The understanding of the dangers of particles in the air led scientists to develop laser air particle counters to quickly and accurately monitor the number of particles in the air.
Air Particle Size
When discussing air particles, it’s important to note how small airborne particles are. For example, a human hair is approximately 50 to 70 um (micrometers or microns) in diameter. Dust, pollen and mold are 10 um or less in diameter, while soot, airborne chemical compounds and metals are in the range of 2.5 um in diameter.
While you can view dust particles with proper lighting, the smaller particles are too small to be seen and can only be monitored using an air particle sensor.
Note that air particles are more than dust, pollen, mold or viruses like Covid-19.
In fact, everything you smell is the result of micron-sized particles entering your nose. While our nasal hair and mucus is perfectly designed to trap large particles, smaller ones can evade this filter and be drawn into the lungs. These particles can lodge in the alveoli in the lungs and cause long-term damage.
Air Particle Counter Explained
While there are many different ways to count particles, the most common is to use a light scattering air particle counter. Known as optical particle counters (OPCs), they use a laser directed through a flow of air.
OPCs work the same way our eyes work when we see dust in the air. Dust particles deflect the light. You’ve seen this if you’ve ever looked across the still air in a dusty room on a sunny day. While the sun allows us to see larger particles, a tuned laser particle counter is able to precisely measure the size and quantity of all the particles in the air.
So how does it work?
Inside the optical particle counter is a fan, a laser and a light detector. As the fan creates a flow of air across the laser beam, the particles in the air deflect the laser's light.
However, instead of measuring the particles, the photon detector measures the deflections of light. Like shadows, the diameter of the deflections are measured by photon detectors that convert the size and frequency of the deflections into a signal that tells not only how many particles are in the stream of air, but their size.
Air Particle Sensor Examples
The need for “clean air” both indoors and outdoors encouraged scientists and engineers to develop new, lower cost optical particle monitoring devices using lasers. An early example of low-cost lasers are the common “laser pointers” used in classrooms and “laser scopes” used on hunting rifles. With further modification, lasers have decreased in size and cost to the point that they are now available to anyone.
An example is the Cubic Particulate Matter Sensor. This small indoor or outdoor sensor contains a laser, fan and photon detector capable of measuring particles in the air between 0.3μ-10μm and real-time output of PM1.0, PM2.5, PM10 in μg/m³.
Overall, the sensor sends 11 different measurements via UART or I2C that can be used to monitor the particle count in real-time including:
- PM1.0, PM2.5 and PM10 GRIMM mass concentration
- PM1.0, PM2.5 and PM10 TSI mass concentration
- Particles count at 0.3um, 0.5um, 1.0um, 2.5um, 5.0um and 10um
GRIMM in Germany and TSI in the US are companies that make laboratory-quality, high quality particle counters. Because of their precision, they are considered as standards for optical particle matter instruments worldwide.