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A Definition of Problem Particle Sizes & Types in Indoor Air AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Particle size vs. air filter characteristics vs. indoor air quality
• The definition of micron, common indoor air particle sizes in microns, and how they behave indoors
• How to Assess the Effectiveness or Efficiency of Air Filters for Heating and Air Conditioning Systems
• How air filters work
• Air conditioning / heating system filter product sources and recommendations

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

We provide a lot of information about finding, cleaning-up, testing, and preventing problem mold in buildings: see our Mold Inspect/Test website. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

What airborne particle sizes are an IAQ concern?

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 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).

Small airborne particles, say in the range of 5u 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. Certainly some air filters which capture large particles may nonetheless pass the smaller ones in the 1-5u size right on into the "conditioned" air. 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 technolgies than the small particles just named here or the large particles named next.

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 Alernaria 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.

Pollen grains 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.

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 point of reference, a human hair might typically be about 50 to 80u in diameter. A human red blood cell is about 8u. Most bacteria are smaller. E. coli is about 2u (viruses are much smaller). ("u" 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.

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

Technical Reviewers

• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Wikipedia provided background information about the definition of HEPA and airborne particle interception.

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