One particle of dust is pretty much impossible for us to see, but if we could we would see all different colors of bacteria, fungi, pollen, hair and skin fragments, in addition to some inorganic substances like bits of sand, rock, and salt. Here is a close-up of some house dust.
We can see plant matter in the middle, some flecks of rock, and most of the rest is a mystery because it is too small even for this close-up. But let’s take a look at some of the different parts of dust and how big they are.
Why does dust particle size matter?
Particles that are too small to see do the most damage because they can actually get into our bodies. Our lungs have evolved to resist parasites and pathogens that we might inhale by catching them with hairs called alveoli that are covered in sticky mucus. Once caught, invaders can be pushed back out of the lungs. But anything that dodges the alveoli may get deep into the lungs where gasses like oxygen are absorbed into the bloodstream. Because oxygen is pumped to every part of the body, any toxins or pathogens can also get everywhere.
Your lungs can usually filter things as small as one micron, or 1μm, but the smallest things we can see are around 100 microns or close to the width of a hair. For comparison, the graphite in a mechanical pencil is usually about 0.7 millimeters or 700 microns, and a human hair is about one tenth the size of that at around 70 microns.
Particles above about 10 microns, or about one-seventh the width of human hair, are usually caught by your lungs’ defenses before they get too far. Particles around 1μm may enter the lungs but will stop there. Particles that are 0.1 microns and smaller may enter the bloodstream and have been linked to negative health effects throughout the body such as cancer, dementia, multiple sclerosis, and even diabetes.
Source: Encyclopedia of the Environment
Let’s take a look at some of the possible components of dust and how they are formed to understand a little more about dust particle sizes.
Dust mite size
Dust mites live all over the world and in at least 84% of homes. If allowed to reach the bloodstream, their droppings cause allergic reactions in many people and are one of the main reasons that dust makes us sneeze. The mites themselves are about 250 microns, but are both experts at hiding in fabrics and afraid of light, so you probably haven’t seen one. Their droppings are 10 to 40 microns, but the allergens on the droppings are even smaller.
Dust mites and their droppings are too large and heavy to stay in the air for very long, so most of the exposure will either come from stirred-up dust or dust on pillows and other bedding. The actual allergen is a tiny protein which is on the order of 10 nanometers 0.01 microns, so if the droppings break up into smaller pieces they still can still trigger allergies.
Pollen and mold spores size
Most fungi and many plants reproduce by spreading spores or pollen in the wind, so our whole world is covered with a light dusting of both. If pollen doesn’t find another plant to mate with, it will eventually dry out and biodegrade. Mold spores, on the other hand, are waiting for the right conditions to germinate and become a new individual.
The smallest mold spores are around 3 to 10 microns and also trigger the most allergies and asthma, though they can be as large as 100 microns. The size of pollen grains varies depending on the plant they come from, but in the grand scheme of dusty particles they are fairly large, around 20 to 100 microns.
Most pollen grains don’t find a neighboring plant and break apart into very tiny fragments that can be smaller than 1 micrometer. The same is true for fungal fragments, which can be 200 times more likely to get deep into the lung compared to the whole spore. There is an even greater effect for infants, who may get 4 to 5 times more tiny particles deep in their lungs. Small particles don’t just get past the lungs’ defenses, but also to remain in the air for longer to be breathed in.
Dander and skin
Our skin and the skin of our pets grows from the bottom up, adding one new layer below and shedding one layer on top about every day. Skin typically falls off in hexagon-shaped flakes about 40 µm wide and 0.1 to 0.5 µm thick.
However, just like pollen, skin cells can dry up into fragments smaller than a micrometer, which allows them to bypass the lungs’ defenses and get into the bloodstream. Respiratory allergies to cats, dogs, and other mammals are triggered by these little pieces, not the whole scales.
Bacteria
A significant component of household dust is bacteria, who are part of the ecosystem along with mold, mites, and many other creatures. Bacteria cells are extremely small, usually around 1 micron.
Bacteria come in many shapes and most of them are harmless. But some, such as staphylococcus here, can cause infections if they get into an open wound.
Everything else
In addition to the above, resuspended particles from the soil can account for a portion of the dust in the home and in the air. Typically the soil itself is fairly innocuous, but can be laden with pollen, mold spores, car exhaust, and other contaminants. Soil particles can be virtually any size and will always drift in from outside.
Any object, no matter its size, can also carry harmful VOCs that may have settled out of the air. Tobacco smoke, for example, adds harmful VOCs to settled dust in a form of third-hand smoke that could affect people in the room days after the last cigarette was smoked. Heavy metals such as lead can also be found in house dust. Just like the allergens, when attached to small particles they can penetrate deep into the lungs and enter the bloodstream. Children tend to have a higher exposure to dust containing allergens, VOCs, household chemicals and other pollutants due to their proximity to the ground where dust settles. So no matter the source of your dust, it’s best to sweep, wipe, or vacuum it up.
Keep an eye on our Facebook, Instagram, and Twitter accounts for more science of the invisible, information on Molekule and what we’re doing about air quality.
