Here we provide a reference table of airborne & dust particle sizes for different particle types.

We include a second table describing typical airborne particle settlement rate or time as a function of particle size.

The page top photograph shows large mite fecal pellets surrounded by Pen/Asp mold spores that are themselves probably in the 1u size range. Those spore chains tell us that there is or has been active mold growth nearby.

Table of Common Indoor Airborne Particle Sizes

Notes to the table above

• 1u or 1 micron = 1 micrometer = 1 millionth of a meter
• Sizes are given for longest dimension and are only approximate. For example, we find some fungal spores down in the 1u range or slightly smaller such as some species of Aspergillus, while animal hair ranges from around 10u for vicuña wool hairs to over 400u thick hairs found on elephants & giraffes.
• The unaided human eye for people with excellent vision can detect particles down to about 10u or about half the thickness of a typical thin human hair.
• Clauß, Marcus. PARTICLE SIZE DISTRIBUTION OF AIRBORNE MICROORGANISMS IN THE ENVIRONMENT-A REVIEW [PDF] Landbauforsch Appl Agric Forestry Res 65, no. 2 (2015): 77-100.
  Abstract Excerpt:
  The size distribution of airborne particles carrying micro-organisms is a well-investigated subject in the range of aerodynamic diameters (AD) of 0.65 µm to 12 µm for many micro-organism groups and environments. It depends primarily on the sampling location and the type of source as well as the method of aerosolisation.
  
  Highest median shares of large bacteria-laden particles were found in livestock husbandry and in waste management.
  
  Sampling height above ground, air humidity, temperature and solar radiation may also influence particle size.
  
  For moulds, the median size distributions in air largely represent the size ranges of their spores.
  
  There is little knowledge about particles > 12 µm AD and the actual number of micro-organisms in different particle size classes. Few studies suggest that most micro-organisms are in particle size fractions > 10 µm AD.
• Findlay, S. R., E. Stotsky, K. Leitermann, Z. Hemady, and J. L. Ohman Jr. "Allergens detected in association with airborne particles capable of penetrating into the peripheral lung." American Review of Respiratory Disease 128, no. 6 (1983): 1008-1012.
• Niu, Jianjun, Pat E. Rasmussen, Nouri M. Hassan, and Renaud Vincent. CONCENTRATION DISTRIBUTION AND BIOACCESSIBILITY OF TRACE ELEMENTS IN NANO AND FINE URBAN AIRBORNE PARTICULATE MATTER: INFLUENCE OF PARTICLE SIZE [PDF] Water, Air, & Soil Pollution 213 (2010): 211-225.
  Abstract:
  Trace elements, especially those associated with fine particles in airborne particulate matter (PM), may play an important role in PM adverse health effect. The aim of this paper is to characterize elements in a wide particle size range from nano (57–100 nm) to fine (100–1,000 nm) and to coarse (1,000–10,000 nm) fractions of two urban PM samples collected in Ottawa.
  
  Size-selective particle sampling was performed using a micro-orifice uniform deposit impactor, and element concentrations were determined in each different size fraction by inductively coupled plasma-mass spectroscopy.
  
  A general trend of increasing element concentration with decreasing aerodynamic diameter was observed for elements V, Mn, Ni, Cu, Zn, Se, and Cd, indicating they were predominately concentrated in the nanoparticle size range.
  
  Other elements including Fe, Sr, Mo, Sn, Sb, Ba, and Pb were predominately concentrated in the fine-size range. Increased concentration of elements in the nano and fine particle size range is significant due to their ability to penetrate into the deepest alveolar area of the lungs.
  
  This was confirmed by the calculation of median concentration diameters, which were less than 800 nm for most of the investigated elements.
  
  Particle size distribution and element correlation analysis suggest that the elements concentrated in the nano- and fine-size fractions originated mainly from vehicular combustion and emission. Long-range airborne transport and soil or road dust resuspension may also contribute.
  
  Particle size had an important effect on element bioaccessibility for the studied urban PM samples showing a general trend of increasing element bioaccessibility with decreasing particle size. These results emphasize the importance of acquiring information on nano and/or fine PM-bound elements and their bioaccessibilities for accurate element and PM exposure assessment.
• Noble, W. C., O. M. Lidwell, and D. Kingston. THE SIZE DISTRIBUTION OF AIRBORNE PARTICLES CARRYING MICRO-ORGANISMS [PDF] Epidemiology & Infection 61, no. 4 (1963): 385-391.
  Excerpts:
  

  Values are given for the median equivalent diameters and for the inter-quartile range, of airborne particles carrying a variety of micro-organisms.

  Organisms associated with human disease or carriage were usually found on particles in the range 4–20 μ equivalent diameter.

  Many fungi appeared to be present in the air as single spores.
  
  ... The ability of a particle to remain airborne, its ability to pass through filters, the site at which it may be deposited in the respiratory tract and the rate at which it will be removed from the air by sedimentation are all dependent on the size and density of the particle
  
  . In the course of a variety of investigations we have determined the size distribution of particles carrying various species of bacteria and fungi, using the size-grading slit-sampler described by Lidwell (1959). A few of the results obtained have already been quoted in part, but the majority have not been published previously.

Airborne Particle Settling Rates

Complementing data on airborne particle size, we should consider the variations in typical particle settling time when collecting indoor airborne particle samples.

Notes to the table above

1. Average settling time assumes an 8 foot ceiling height and no significant air movement from external causes
2. Watch out: Notice that this research only reflects particle size, not particle mass or weight.
3. Watch out: Any active or passive indoor particle sampling method will see results that are affected by surrounding conditions such as indoor air movements and possibly more subtle conditions such activity of building occupants and even temperature and humidity that may affect indoor convection currents. Of these, turning fans on and off can dominate air movement and can make enormous differences in the collection of airborne particles.
   
   I [DF] have found significant differences in the number as well as mix of airborne particles collected when using an air sampling machine depending on whether or not the room where sampling is being done is occupied or not and especially depending on whether or not building HVAC equipment or fans are turned on and off.
   
   Naturally, the total particle concentration in the test area will also be an important factor (Whyte 2016).
   
   Those details are at AIRBORNE PARTICLE COUNT VARIATION CAUSES
   
   Supporting research and research also in particle settling velocity are included here.
4. Condie, Scott A., and Myriam Bormans. "The influence of density stratification on particle settling, dispersion and population growth." Journal of Theoretical Biology 187, no. 1 (1997): 65-75.
5. Drube, L., K. Leer, W. Goetz, H. P. Gunnlaugsson, M. P. Haspang, N. Lauritsen, M. B. Madsen et al. "Magnetic and optical properties of airborne dust and settling rates of dust at the Phoenix landing site." Journal of Geophysical Research: Planets 115, no. E5 (2010).
6. Beller, Lorin D. HEPA FILTRATION and its EFFECTS on INDOOR AIR QUALITY and PRODUCTIVITY in the WORKPLACE (1992). MS Thesis,
7. Montoya, Lupita D., and Lynn M. Hildemann. "Size distributions and height variations of airborne particulate matter and cat allergen indoors immediately following dust-disturbing activities." Journal of aerosol science 36, no. 5-6 (2005): 735-749.
8. Murakami, S., S. Kato, S. Nagano, and Y. J. A. T. Tanaka. Diffusion characteristics of airborne particles with gravitational settling in a convection-dominant indoor flow field [PDF] (1992).
   
   Abstract excerpt: Particle diffusion with gravitational sedimentation has been investigated. The property of particle diffusion becomes complicated with the increase of particle size because of the effect of gravitational sedimentation.
9. Noble, W. C., O. M. Lidwell, and D. Kingston. "The size distribution of airborne particles carrying micro-organisms." Epidemiology & Infection 61, no. 4 (1963): 385-391.
10. Rutala, William A., Suzanne M. Jones, John M. Worthington, Parker C. Reist, and David J. Weber. "Efficacy of portable filtration units in reducing aerosolized particles in the size range of Mycobacterium tuberculosis." Infection Control & Hospital Epidemiology 16, no. 7 (1995): 391-398.
11. Tan, Huiyi, Keng Yinn Wong, Mohd Hafiz Dzarfan Othman, Hong Yee Kek, Bemgba Bevan Nyakuma, Wai Shin Ho, Haslenda Hashim et al. "Why do ventilation strategies matter in controlling infectious airborne particles? A comprehensive numerical analysis in isolation ward." Building and Environment 231 (2023): 110048.
12. Warburton, C. J., R. M. Niven, C. A. Pickering, A. M. Fletcher, J. Hepworth, and H. C. Francis. "Domiciliary air filtration units, symptoms and lung function in atopic asthmatics." Respiratory medicine 88, no. 10 (1994): 771-776.
13. Whyte, W., and M. Derks. "Airborne particle deposition in cleanrooms: Relationship between deposition rate and airborne concentration." Clean Air and Containment Review 25 (2016): 4-10.
14. Whyte, W., and M. Hejab. "Particle and microbial airborne dispersion from people." European Journal of Parenteral and Pharmaceutical Sciences 12, no. 2 (2007): 39-46.
15. Source: National Safety Associates, Inc., AIRBORNE PARTICLE SETTLEMENT STUDY [PDF] Memphis TN, #855100, September 1989, where the test method employed was a Dual Laser Particle Counter Filter Test System, Model No. NSA 7000 HF, S/N 1 & 2, at 85 cfm with an average pressure drop of 0.295 in. H₂O. The study counted "upstream" or source particles and "downstream" particles and calculated an average penetration and collection efficiency of the filter. - cited by Warburton (1994) and Rutala (1995) and Beller (1992)

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Continue reading at AIRBORNE PARTICLE SIZE DEFINITIONS & TYPES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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