Photograph of Aspergillus niger and Chaetomium mold spores Definition of Indoor Air Particle Sizes & Types
What airborne particle sizes are hazardous?

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Guide to airborne & dust particle sizes:

Here we give a definition of Problem Particle Sizes & Types in Indoor Air, The definition of micron, common indoor air particle sizes in microns, and how they behave indoors.

The page top photograph shows what is probably Aspergillus niger black mold spores, 2-4u in diameter, along with those lemon-shaped and nice looking Chaetomium sp. mold spores. More detailsand a close-up photograph of these particles are given just below.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What airborne particle sizes are an IAQ concern?

Photograph of Stachybotrys chartarum mold spores

At left are large black ovate Stachybotrys chartarum mold spores and smaller Pen/Asp mold spores.

Aspergillus niger is often a toxic mold; Chaetomium sp. mold spores are likely to be at least allergenic.

Definitions of PM particle sizes and What Particle Sizes are Considered Air Pollutants or Hazardous

Particle sizes when airborne particulates are considered as respiratory hazards or "pollution" are popularly discussed also using PM (Particulate Matter) particle sizes as follows:

PM-10 Particles

Definition of PM10: particles in the size range of 10 to 2.5 microns in diameter, or PM10 to PM2.5 inhalable particles.

Diameter is used to refer to the particle width, so a PM10 particle that is 10u wide might be of greater dimension in length.

These particles may be in liquid or solid form.

Examples of PM10 particles:

As an example or reference point: the width of a human hair ranges from as thin as 17u to as fat as 181 u. As another reference point, 10u is ten times the size of a common small UFFI particle that may be created when an exisisting section of UFFI insulation is disturbed during remodeling or demolition.

PM10 particles are also referred to in the literature as coarse particles.

PM-2.5 Particles

Definition of PM2.5: particles with diameters that are generally less than 2.5 micrometers in diameter are referred to as PM2.5 sometimes written as <PM2.5

PM2.5 particles are also referred to in the literature as fine particles.

Examples of PM2.5 particles

Smoke, outdoor haze, some mold spores such as some species of Aspergillus sp. or Penicillium sp. We also find talc particles in the PM2.5 range and we may find crushed or damaged fiberglass particles in the PM2.5 size range though normally fiberglass particles are considerably larger.

Source: U.S. EPA, "Particulate Matter (PM) Pollution, Particulate Matter (PM) Basics, What is PM, and how does it get into the air?" [PDF], retrieved 2018.07/06, original source:

What mold, house dust dust, allergen fragment, mite fecal, cat dander, or other airborne particle sizes are a concern for indoor air quality?

In the photograph shown here the large black Stachybotrys chartarum mold spores can be seen against our eyepiece micrometer which, after calibration, shows that these particular spores were about 7u x 15u in size.

The brownish tubular structures in our photograph are fungal hyphae. Another, smaller fungal spore is in the background. What's not addressed by some of the science in the air filtration and IAQ field is just what particle sizes are a worry.

In general, larger particles, say 30u or 50u or long fibers, say 200u, are so big that they tend to be filtered in the nose of a human breathing that air. (1u here means 1 micron in size).

Definition of "Small" Airborne Particles & Their Hazards

Small airborne particles, say in the range of 5um (5 microns) and below are so tiny that they tend to be breathed more deeply into the lungs and might be more of a pulmonary (lung) health or IAQ concern for some building occupants.

For purposes of discussing air pollution and health concerns around airborne particles, those particulates that are 2.5u and smaller are considered small and particularly dangerous.

Certainly some air filters which capture large particles may nonetheless pass the smaller ones right on into the "conditioned" air.

New York Times has reported an increased concern among scientists who study the potential dangers of very small airborne particles. But the concern is not entirely new.

"Fine atmospheric particles - smaller than one thirtieth of the diameter of a human hair - were identified more than 20 years ago as the most lethal of the widely dispersed air pollutants in the United States. Linked to both heart and lung disease they kill an estimated 50,000 Americans each year." [The article continued to focus on secondary aerosols that may be more dangerous than previously thought.] [2]

A 2012 study found that on days when concentrations of traffic pollutants were elevated the risk of stroke among humans increased by 30%[4] and a separate study in 2006 reported a link between short-term exposure to air pollution and cardiovascular disease[5]. The traffic-related air pollutants identified in the first studies included black carbon (BC), carbon monoxide (CO), NO2, ozone (O3), PM [particulate matter] smaller than 2.5um in aerodynamic diameter, and sulfur dioxide (SO2).[6]

In summary, these results support the hypothesis that elevated levels of particulate air pollution, less than the current limits set by the Environmental Protection Agency, are associated with an increase in the rate of hospital admission for the exacerbation of [congestive heart failure] CHF.[5]

In 2012 the NY Times further reported on a study that reported:

"The smallest particles of pollution, those finer than 2.5 microns in diameter ... are particularly effective in infiltrating the body. There is some evidence that they can even penetrate the brain through nasal passages... linking pollution to cognitive decline" [3][7]]

Of interest, as we discuss in our review of How Air Filters Work, is the observation that very very small particles in the less-than-one-micron range are actually captured more easily by some air filter technologies than the 2.5u range small particles just named here or the larger 20-50u sized airborne particles named next.

Photograph of Alternaria sp. mold spores

Larger airborne particles, perhaps in the 20-50u size, which affect a person by carrying allergenic proteins or fungal mycotoxins into the body, might be still a concern (in un-filtered or otherwise contaminated building air) even though they get stuck in the nose or upper respiratory tract.

Some allergists have told us that they can tell by the nature of a patient's complaint what they're probably allergic to and what size and types of particles are in the patient's environment. When inhaled in a breath of air, these comparatively large fungal spores are more likely to be trapped in a person's nose.

If a patient has chronic rhinitus, for example, they may be responding to large mold spores like Alternaria sp. which may be as big as 15 x 50u.

If a patient has lower respiratory complaints (pulmonary or in the lungs) they might be responding to very small mold spores like some of those in the Aspergillus sp. or Penicillium sp. size range, which we often see in our lab can be down in to the 1u range in size. The photograph shows some Alternaria sp. mold spores which are pretty big.

Typical Stachybotrys chartarum "toxic black mold" spores that have received lots of media attention and public worry are a rather large warty, sticky mold spore (intended to be spread by cows walking through moldy straw) which is oval and is usually about 10 x 20u in size. Stachybotrys chartarum's still more irritating family member, Memnoniella echinata is a round black spore of about 10-12u in diameter.

Stachybotrys chartarum and Memnoniella echinata are not normally airborne mold spores, so if we find one or both of these in building air or in settled building dust or in the HVAC system, most likely a surface with that mold growing on it is or was nearby and it has been recently disturbed, say by demolition activity conducted without proper dust management.

Photograph of poppy pollen grains

Pollen grains (shown above) vary considerably in size but generally are bigger than many most many mold spores. Ragweed pollen might be about 20u in diameter.

Shown here are some stunning and still larger poppy pollen seeds collected outside of our forensic lab in Poughkeepsie, NY.

Photograph of a dust mite fecal pellet

A guanine-containing dust mite fecal pellet and other insect fecals vary in size but some are pretty big, maybe 30 x 150u, much bigger than most mold spores, as are most pieces of dog or cat dander.

The photograph provided here shows both common dust mite fecal pellets and, in the same photo you can see much smaller and nice for comparison, some hyaline (colorless) Aspergillus sp. or perhaps Penicillium sp. mold spores which are in the one-micron size range.

Often when we examine an indoor dust sample in the microscope and when we look closely at mite or some other insect fecal pellets we observe that the pellets are comprised of mold spores that the insect has been eating. Usually those same mold spores are present in the dust sample.

But when the mold spores themselves are not seen in the sample, we know that mold was nearby, and our little insect assistants have provided us with the evidence of the presence of of indoor mold. Eating at least some kinds of mold spores apparently does not bother them one bit.

Definition of a micron - how big is a micron?

How big is a one micron particle? How easily do such particles move throughout a building?:

Just as a few points of reference comparing particle sizes,

("u" or "um" here means micron or 1/1,000,000 of a meter or a millionth of a meter or about 1/25,400 ths of an inch if you prefer).

A one-micron Aspergillus sp. mold spore is so small that we find that they move in the air like a gas, right up from a moldy crawl space and through the building, and these particles tend to stay airborne much longer than their larger cousins. In absolutely still air (which never ever occurs inside a normal building), such a particle might remain airborne for more than 40 hours.

Walking outside (where there is plenty of air movement and plenty of mold spores) a person is breathing in a few of these spores with each breath.

Fungal spores may be amplified indoors if there are problems with the heating or air conditioning systems

Breathing in a lot of some kinds of mold spores or other particles can be a problem wherever one is, but indoor allergens, toxic spores, or other irritants may be more of a problem indoors where they are not diluted by outdoor air, where some people spend lots of time, and where these problem particles are being picked up by a heating or air conditioning system, blown through the duct work, amplified in quantity by ductwork or air handler conditions, and then delivered personally to people in the living space.

Still Smaller Particles that May be Found in Indoor Environments

Photograph of Legionella bacterium, provided by the U.S. Department of Labor, OSHA

Bacteria - for an example see Legionella BACTERIA & HVAC Equipment - photo at left.



And see our discussion of ultra-small particles


We provide a lot of information about finding, cleaning-up, testing, and preventing problem mold in buildings: see


Exposure Limits for Small Airborne Particles Sizes PM10 & PM2.5

Current National Ambient Air Quality Standard exposure limits for the small ranges of airborne particles including Coarse Particles of size PM10 and Fine Particles of size smaller than PM2.5 are given here.

U.S. EPA PM Exposure Limits

OSHA Respirable Dust Exposure Limits

Note: in the U.S. OSHA regulates certain particulates (eg. asbestos, silica, zinc oxide or metal dusts) and other dusts that may be considered toxic due to contaminants such as dust contamianted with PCBs, but does not give an exposure limit or PEL for manyu types of dusts. Instead OSHA gives an exposure limit for " soluble / non-toxic dusts as nuisance particulates" or for "particulates not otherwise classified".

These OSHA PELS or exposure limits are given as follows:

References for Airborne Particle Size Exposure Limits by Particle Size Range

Continue reading at INDOOR AIR QUALITY IMPROVEMENT GUIDE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



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