6.7. Mechanisms of Particle Deposition

So many of you have heard of it, the PM10 and PM2.5. So, what is this PM10 and 2.5 okay? So 10 stands for 10 microns and 2.5 stands for 2.5 microns of particles. So, what it stands this PM10 is anything that is less than 10 microns, all particles less than 10 microns, this is less than 2.5 microns.

PM10 and PM2.5 refer to specific sizes of particulate matter in the air. PM10 consists of particles that are 10 microns or smaller, while PM2.5 includes particles that are 2.5 microns or smaller. To visualize these sizes, a human hair is approximately 70 microns wide. Therefore, PM10 particles are quite small and can easily suspend in the air we breathe.

Think of PM10 and PM2.5 as different types of dust in your home. PM10 could be like larger dust particles that settle on surfaces, while PM2.5 represents the finer dust that you can’t always see but is there in the air and can easily be inhaled.

So, this 10, this is known as the aerodynamic diameter. The definition of the aerodynamic diameter is the diameter of an equivalent spherical particle of density 1 gram per centimeter cube that has the same Stokes’ settling velocity.

The aerodynamic diameter helps us understand how a particle will behave when moving through a fluid, like air. It is not just about the size but how the particle settles in the air. Particles of varying shapes and densities can behave differently despite having the same

• Chunk Title: Importance of Particle Size in Health Impacts
• Chunk Text: So why is the settling chosen as a reference point to characterize the size of a particle? If you look at the size of particles that exists in the atmosphere, you have a very wide range of particles. These particles if you look at the diameter in microns, you can go all the way you are looking at 100 microns, you are looking at 10 microns, you are looking at 1 micron unit, looking at 0.1, you have 0.01 and so on. You have a very wide distribution.
• Detailed Explanation: The significance of particle size is crucial when considering their impact on human health. Larger particles, such as PM10, tend to settle in the upper part of the respiratory system, like the nose and throat, where they can cause irritation but may not reach the lungs. In contrast, smaller particles, such as PM2.5, can penetrate deeper into the lungs, posing greater health risks, including respiratory diseases.
  
  This means that for effective public health policies, focusing on smaller particles is essential since they are associated with more severe health problems.

Think of a basketball and a grain of sand. The basketball may bump into someone walking on the street, but it is unlikely to get inside a building. The grain of sand, however, can be carried by the wind and easily enter a building through the smallest crack. Similarly, larger particles don’t penetrate the lungs as easily as smaller ones.

These particles in the lower range are considered to be coming from the gas phase from combustion activities and it is their formation due to condensation of gas phase products. So, combustion of fuel is coming in gas phase and as soon as it comes out, it cools very rapidly and it forms a particle. If you take a large particle and start breaking it, you will get the other size, from here this is you get the larger size by mechanical processes like breakage or erosion.

Particles in the air can originate from two main processes: gas phase processes such as combustion, where gases cool and condense into tiny particulate matter, and mechanical processes, where larger particles break down into smaller pieces through erosion or physical impact. It is important to understand that these smaller particles can have significant health implications since they may evade the body's natural defenses and reach deeper into the lungs.

Imagine making a smoothie. When you blend fruits, the larger pieces break down into smaller chunks. Similarly, when a car burns fuel, the larger gases produced can condense into smaller particles that may become airborne and affect air quality. Just like the smoothie can be hard to clean if it gets everywhere, so can these tiny particles when they enter our lungs.

The inertia of a larger particle means it continues moving in a straight line until it encounters an obstacle, like our respiratory system. In certain conditions, particles may collide with surfaces in the respiratory pathway leading to deposition.

Inertial impaction refers to the tendency of larger particles to keep moving straight when air flows around them. When these particles encounter branches or changes in the airflow within the respiratory system, they may collide with the walls and deposit in the nose, throat, or lungs. This can lead to various health issues depending on where the particles accumulate.

Think of a bowling ball rolling down a slightly curved path. If it encounters a wall, it will hit it and deposit there. Similarly, when larger particles travel through the twists and turns of our respiratory system, they can collide with the walls and get stuck, leading to potential health problems.

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