1/ Clean Air Delivery Rate (CADR). The CADR of a portable filtration system is:

CADR = f x Q.

f is the fractional removal efficiency of particles that pass through a device (value = 0 to 1). Q is the volumetric flow rate of air through the device (e.g., m3/hr, ft3/min). ...

2/ A portable air cleaner may be highly efficient (high value of f) but also highly ineffective at removing particles from air if Q is very small. This is the case for a wide range of ion generators. Don't be fooled by claims of high efficiency. It's f x Q that matters.

3/ On the other hand, a portable filtration system might have a lower efficiency (say, 0.5) but a very high air flow rate that leads to significant reduction of particle concentrations in air. It is f x Q that matters.

4/ By its very name, CADR implies an equivalent amount of high quality (clean) outdoor air added to an indoor space (additional ventilation with clean air). Approximation of that ventilation equivalent is quite simple. Remember, it is f x Q that matters!

5/ The CADR for a device is usually given as a single number without units. In most cases the units should be scfm (standard cubic feet per minute). There are some companies that use cubic meters per hour, which is a bit deceiving as it makes the CADR look better than if in scfm.

6/ Make sure that when you see CADR = 300 or CADR = 500, etc., you know what units are being used. To convert from m3/hr to ft3/min simply multiply the former by 0.59. Or conversely, multiply ft3/min by 1.7 to get m3/hr.

7/ Here is an example conversion to a ventilation equivalent (EqACH) starting with CADR in ft3/min.

EqACH = CADR x 60 / (A x h)

Here, the "60" is to convert from minutes to hours. A is the floor area in ft2 and h is ceiling height in ft. EqACH has units of /hr, e.g., 3/hr.

8/ So, if a classroom has a floor area of 600 ft2 and ceiling height of 9 ft, a portable air cleaner with CADR = 300 ft3/min yields EqACH = 300 x 60 / (600 x 8) = 3.3/hr.

9/ If the actual ventilation rate of the classroom is 2/hr, this portable air cleaner leads to an effective ventilation rate (EVR) of 2/hr + 3.3/hr = 5.3/hr. That is a substantial increase and will help reduce virus-laden aerosol particles in classroom air.

10/ On the other hand, many ion generators have CADR closer to 50 ft3/min, leading to EqACH of 0.56/hr and EVR increase from 2 to 2.56/hr in the example above. Many ion generators also emit ozone that, in poorly ventilated classrooms, can be problematic (more in a future tweet).

11/ I used "approximate" above because not all classrooms approach well-mixed conditions and volume (A x h) is not really the empty volume. The volume of students, furniture, etc.., should be subtracted, but correction is small (< 10%). Approach FAR better than rule of thumb.

12/ It is important not to locate a portable filtration system near a wall or corner where air is recirculated from exhaust to inlet of the device. This can reduce effectiveness.

13/ Note that CADR is dependent on particle size, and that is why it is often given for smoke, pollen, and dust separately. For portable HEPA air cleaners the CADR values differ only slightly for these three types of particles (it is good across a spectrum of sizes).

14/ The Association of Home Appliance Manufacturers (AHAM) certifies CADR across the categories described above.

15/ For devices that use non-HEPA filters, make sure that you understand the filter rating. The difference in CADR for the particles listed above will vary to a much greater extent.

16/ The multi-sided MERV-12 or 13 DIY portable air cleaner with a box fan allows for high flows and can lead to high CADR across particle sizes of relevance to transmission of COVID-19. Follow @JimRosenthal4 for more information on these.

17/ Note that some portable air cleaners are being marketed to inactivate viruses in aerosol particles. The concepts described above are relevant for these devices as well. In these cases "f" is fractional inactivation and Q remains the air flow rate though the device.

18/ Before buying into these technologies, ask questions. What is "f"? What is Q for the device? What is their product? Does the device generate chemical by-products that may be irritating or worse for building occupants? Do these products persist in the indoor environment?

19/ Has the system been tested independently? Are the data from those tests available? This is not to say that these devices do not work well. But for now I am sticking with proven technologies and approaches. Much more to come ......