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How to Understand Particle Size and Distribution for Cleaner Air

Particle size is an important factor in air pollution and allergies. We might simply think of airborne particles as a bunch of tiny molecules in the atmosphere. And while this wouldn't be inaccurate, it wouldn't tell the entire story.

Indoor air pollution comes in many shapes and sizes. When you get down to the microscopic level, airborne particles can be called tiny and others that, by comparison, can be gargantuan. The same goes for allergens like fine dust particles.

But why does this matter to you, the homeowner?

Why does particle size make a difference to businesses and municipal buildings?

Because particle size has a direct impact on the specific type of air purifier and filter you need.

particles entering filter

Particle Size: Why Does it Matter?

Our homes are full of different forms of particles and air pollution.

There are biological sources like pet dander from dogs, mold spores and mildew, fumes from paint, chemicals from air fresheners, and more. All of these pollutants are different, but each one is, at a microscopic level, made up of individual particles.

These particles, however, come in different shapes and sizes. While there is slight variation among the similar particles (for example, mold particles vary in size), the differences between different particles (mold vs pet dander, for example) can vary widely.

Understanding particle size, however, is important when choosing an air purifier. This is because each air filter and air purifier can clean particles of different sizes. Some are designed to handle very large particles, while others are made to trap the tiniest of microscopic airborne particles.

How is Particle Size Measured?

There are numerous methods for measuring particle sizes. While some use light to measure the specific size, others use technologies that incorporate ultrasound, electric fields, and even gravity.

In all cases, however, the measurement is indirect. It is not a measurement of the particle itself, which can have funky, obscure shapes. But rather a measurement of the particle reworked into a uniform model.

Think of it this way: particles are not all perfectly round, or square, or any other uniform, comparable shape. Instead, particles will take unique and obscure shapes.

Measuring the particle sizes, then, is like comparing the size of a football and a basketball. They have different shapes and therefore require different methods for measurement.

So, they need to be converted into a comparable format.

Quite frankly, the technology and process for accurately measuring particle size is dense and complex. It is also diverse, so we won't rack your brain trying to explain each one in detail.

However, we will summarize the most common, which is using light scattering technology. Essentially, the airborne particles are stored in a sealed container and high intensity light or lasers are shot inside. A computer connected lens then records the amount of light that is blocked by the particles. With the use of advanced software, the particle counter computes the particle size information.

In most cases, the particle size is converted into a spherical model, and the diameter is measured in a microscopic unit call a micron.

Microns: The Particle Size Measuring Unit

We measure liquids in gallons and pints, distances in feet, yards, and miles, and weight in pounds, ounces, and tons.

Typically, particles are displayed in a unit called a micron, also called a micrometer.

A micron is a measurement of the diameter of the spherical model, and, as you might have guessed, it's extremely small. A micron is one millionth of a meter. In case you need a refresher from high school geometry, diameter is the straight line measuring the width of the circle.

To put a micron into a perspective you can wrap your brain around, there are roughly 100 microns in the width of a human hair. And the airborne particles we are referring to are many times smaller than this.

Particle Size Distribution

Okay, now we understand the basics of particle size, but what about particle size distribution?

As we discussed above, particle size will vary among the same particles. Pet dander particles, for example, can be large and small, so saying all pet dander is X microns, would be inaccurate. To be more precise, you need to know the range of sizes among the pet dander.

Particle Size Analysis

In many cases, the particle size is given as an average. For example, you might say that particle size is one micron. (You'll find more on microns below.) However, this doesn't tell you the range of sizes. A single number cannot describe the largest and smallest particles in a group.

It's better to report the media size along with largest and smallest points, as well as the percentage of particles that fall into this specific range. This is called particle size analysis or particle sizing.

In most cases, particle distribution will be displayed in the classic bell curve, which demonstrates the smallest, average, and largest particles.

When searching for particle size distribution numbers, you'll often come across the term D50. This is a simple term to indicate the median diameter of the particles. (Not necessarily the average, or mean, but the median).

The D50 number mean that 50% of the particles that were measures are larger than this number, and 50% were smaller. So, if a particle when measured you may have a D50 of, say, 4 microns. (Usually written as D50= 4 microns.) This means half the particles were over 4 and half were under.

Pretty simply, right?

Typical Sizes for Common Pollutants

particle size chart
Source: Wikimedia Commons By Jisaac9

Using modeled particle size measurements and the micron as our standard measuring unit, we can now compare different airborne pollutants in an apples-to-apples comparison. With an understanding of the particles size, you can then select the right air purifier.

But first, let's look at the particle sizes of the most common airborne pollutants...

Mold Spores: 3 to 40 Microns

Mold is neither a plant nor an animal. It falls in the fungus category, which is often defined by spore-based reproduction. Mold produces spores which are released into the air for reproduction. If there is sufficient food and a habitable environment, the spores become new mold. Mold spores are typically 3 to 40 microns in size.

Pet Dander: Less than 2.5 Microns to 10 Microns

A pet's skin cells can be released in the air, becoming what we call pet dander. Pet dander is particularly small, measuring about 5 to 10 microns. However, about a quarter of pet dander can measure smaller than 2.5 microns. This small size allows them to become and stay airborne easier. And this is what you are most likely to breathe in and cause allergic reactions for you or your pet.

Household Dust: 0.5 to 100 Microns

Dust is made up of many different materials, including human and pet skin cells, pollen, hair particles, mold spores, and more. Essentially, what we think of as dust can be made up of dozens, even hundreds of different airborne particles. This means household dust can have a massive particle size distribution. In most homes, household dust will range from as little as half a micron to 100 microns.

Bacteria: 0.3 to 60 Microns

There are many different types of bacteria, so the particle size of this pollutant will vary as well. Bacteria can be as small as .3 microns, but it can also be as large as 60 microns, making it visible to the human eye under the right conditions.

Viruses: .005 to .3 Microns

Virus particles are extremely small, usually no more that .3 microns. They can also reach nearly invisible levels of .005 microns. For the record, that's five one-thousandths of a micron! Because viruses reach such small levels, hospitals usually implement air purifiers that capture extremely small particles to prevent the spread of disease.

Tobacco Smoke Particles: .01 to 4 Microns

The particles that make up tobacco smoke are also extremely small. Usually ranging from .01 to 4 microns, tobacco smoke is one of the smallest common airborne particles for indoor air. In addition to particulates, tobacco smoke is also made up of hundreds of gases. These gases are much smaller than any particle and you can be effected by them without being the smoker. For example, second-hand or thirdhand smoke.

Pollen: 10 to 1000 microns

In many cases, you can see pollen floating in the air. This tells you that the individual particles are quite large. Pollen generally ranges from 10 to 1000 microns, although the sizes are influenced by the type of plant and other factors. Because pollen particles are so large, they can often be removed by filters that capture the biggest particles.

Are Smaller or Larger Particles More Harmful to Health?

Size is a very important factor when considering the potential harm caused by particles in the air. When you inhale, you take in air and any particles that are in the air. The breath, along with the particles, travel through your respiratory system to your lungs.

Along the way, particles can become caught in your nasal or sinus system, causing discomfort or potential health problems. As a defense, the body produces mucus to trap particles. There are also tiny hairs that grab unwanted pollutants.

However, the smallest of airborne particles are often able to beat these defenses and make it all the way through to the alveolar, which is the deepest part of the lungs.

This is the place where oxygen is added to the blood stream. Think of it like the onramp to the transportation highway that is your circulatory system. Anything that makes it to the onramp, whether it's oxygen, smoke, or viruses, is then distributed through the body.

Therefore, because smaller particles are more likely to make it to the alveolar, they are considered most dangerous to your health.

HEPA Filters and Their Relation to Particle Size

So, what do particle sizes, microns, and distribution bell curves have to do with HEPA filters?

It's quite important, because the size of particles you need to target will directly affect the type of filter and air purification system you choose.

If you are trying to eliminate an air pollutant that is generally large, such as dust or pollen, you can choose a filter that captures these larger particles but may not capture smaller ones.

However, if you need to capture and eliminate the smallest of particles, such as viruses, pet dander, mold spores, or bacteria, you will likely need a HEPA filter.

HEPA filters have been classified by the US government to meet specific standards of performance. To hold the true HEPA label, a filter must capture at least 99.97% of all particles that measure .3 microns and larger.

If you purchase a room air purifier that has been labeled as HEPA, you can reasonably expect the filter to remove most particulates from your air, including pollen, mold spores, tobacco smoke, and more.

Particle Size of Gases

While particulates can be tiny in size, gases are even smaller.

A HEPA filter is the best solution for airborne particles however gases will simply pass through the HEPA filter fibers.

To solve this problem, a different approach is needed and that is to adsorb the gases. That is, to hold them in like a sponge.

The most common method is to use activated carbon to adsorb gases. This is a good solution for many gases including cigarette smoke smell however you will want a few pounds or more to properly clean the air.

Activated carbon cannot adsorb certain gases well like methane or formaldehyde. For this, additional filter media is needed. A common filter media is potassium permanganate or alumina. This, along with activated carbon is what we use in our EJ Air Purifier and Erik650A filters.

What About Viruses?

HEPA filters remove most particles measuring .3 microns or larger, but what about viruses, which can be as small as .005 microns? In this case, it may be ideal to use an air purifier that uses UV light to kill germs, bacteria, and viruses. In UV-based air purifiers, the air is pulled through the UV-C light, which destroys many of the harmful airborne molecules, creating cleaner, healthier air.

Oransi Has the Perfect Air Purifier to Meet Your Needs

Want more information about particle size and choosing the right filter for your home, business, or facility?

The helpful team at Oransi can help you choose the appropriate air purifying technology for your home.

You can also see our guide on How to choose an air purifier. This covers what you need to consider and look for in an air cleaner.

Contact us today and we'll make sure you have the right information to make an informed, confident decision on your air purifier.
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