Since there are no official standards for indoor air quality, outdoor air quality measurements set by EPA are often used as benchmarks. EPA uses the weight of dust per cubic meter of air (aka, airborne dust mass, mass concentration or density) to determine air quality rather than particle counts. This is expressed as micrograms (ug) per cubic meter of air (m3) and includes dust particle sizes of 10, 5.0, and 2.5 microns (aka, Particulate Matter or PM 10, 5.0, and 2.5 respectively). To put these sizes in perspective, a human hair is about 70 microns in diameter.
The EPA website (www.epa.gov) provides several articles on Particulate Matter (PM) 10, 5.0, and 2.5. A good starting point is: http://www.epa.gov/air/particlepollution/standards.html. Notably, EPA's 24-hour fine particle standard is 35 µg/m3. The 24-hour PM10 standard is 150 µg/m3.
Interestingly, the Carpet and Rug Institute (CRI) uses airborne dust mass measurement to determine the amount of dust that CRI Seal of Approval vacuum cleaners are permitted to release into the air. According to CRI, the best vacuums strictly control dust output resulting in only 35-100 micrograms of airborne dust per cubic meter of air.
The CRI Seal of Approval (SOA) Program awards vacuum cleaners Bronze, Silver or Gold recognition based on these dust containment levels:
Soil Removal Dust Containment
Bronze 40 - 49% ≤ 100 µg/m3 of dust particles
Silver 50 - 54% ≤ 100 µg/m3 of dust particles
Gold ≥ 55% ≤ 35 µg/m3 of dust particles
According to Jim Akey of Lighthouse Worldwide Solutions: "Both particle counts and mass concentration readings are important in determining the overall quality of air in indoor settings." Some devices enable calculating both. For example, some particle counters can translate particle readings to approximate density in g/m3.
As a rule-of-thumb, fine particle counts indoors should be half of what they are outdoors. "In our experience, the 0.1, 0.3 and 0.5 micrometer size ranges should be cut in half indoors," according to Dave Blake of the Northwest Clean Air Agency.
"Customers frequently ask me what are 'typical' readings (particle counts) for the different particle sizes," Akey says. "I have not found any published ranges, and for good reason. There are literally hundreds of variables that will affect the particulate counts. Some of these include weather (there are less particles present on a rainy day, many more on a windy day); geographical location (in a city, out in the country); activities occurring in the sampling location (people moving around, HVAC system running or not, windows open or not, etc.) I have found that after using a particle counter in many similar settings, the user gathers a large amount of data (a baseline) that will reflect a 'typical' particle count in their region."
He continues: "One can also compare particle counts to some of the clean room standards being used by the electronics, pharmaceutical, and medical industries. For instance, sitting at my desk, I generally fall within a Class 100,000 clean room (Federal Standard 209E Class 100,000 is the least clean of these classifications). A Class 100,000 clean room classification states that there can be no more than 100,000 particles at 0.5 microns and greater per cubic foot of air."