Schools of all sorts are looking for ways to re-open while minimizing coronavirus transmission risks. Harvard University’s School of Public Health recently issued a downloadable document on “Schools for Health”. In it they suggest a number of administrative and engineering approaches for reducing virus transmission in a classroom and school setting. It’s interesting and worth a look.

Since I teach and do research in some aspects of HVAC (Heating Ventilation and Air Conditioning) and indoor air quality, those parts of the report caught my attention. They are suggesting that people consider using portable air cleaners in the classroom, especially in situations where the HVAC is non-existent or poor. They don’t give a lot of numerical detail behind that recommendation, but it’s fairly easy to work it out. So I’ve done some quick calculations to see where air cleaners might be useful from a more quantitative perspective.

The Harvard report suggests using 1 air cleaner per 250 square feet of classroom area (about 1 per 23 square metres). The portable air cleaner should have a HEPA filter (high efficiency particulate air), and it should have a CADR of 100 cubic feet per minute (about 170 cubic metres per hour). CADR is “clean air delivery rate” and is typically cited by the manufacturer of portable air cleaners for their device at specific operating conditions.

As an example of their recommendations, a classroom with dimensions of 10 metres by 8 metres (about 33 by 26 feet) would need about 3 air cleaners to meet the Harvard criteria. (This is a small classroom by university standards, but seems reasonable for elementary and secondary school levels, based on my recollections.)

The question is, how much effect will these air cleaners have on the classroom air quality? People don’t want to go to the expense if the effect is not significant.

To figure this out, we need to consider the “Air Changes per Hour” (ACH) that apply to the classroom. ACH is essentially how many times per hour the entire volume of air in the room is replaced by fresh air. In a room with no HVAC and just windows, the number of ACH is typically around 1 to 6 (depending on how many windows are open, which way the wind is blowing, etc.). In a classroom with HVAC, typical ACHs range from around 6 to 20, according to normal engineering practices. The low end (6) would represent relatively poor HVAC, and the high end (20) would be quite a lot of air movement.

Based on the portable air cleaner specifications recommended by Harvard, and some calculations comparing the air treatment in those cleaners with the ACH, I came up with the chart below that shows how much improvement in air quality might be gained with the air cleaners.

Looking at the chart, we can see that using the HEPA air cleaners potentially improves the air quality by 100% to over 200%, in situations where the ACH is low. I’ve labelled this “Natural” in the chart because this is the typical situation where there is no HVAC and we’re relying on windows (natural ventilation).

For classrooms with HVAC, the beneficial effect reduces quite quickly once the ACH exceeds 10. In other words, if the classroom HVAC is decent and can move fresh air into the room, the air cleaners won’t do much more for you. One would question the cost-benefit ratio under those circumstances.

If you have a classroom and wonder what the ACH is for that room what do you do? Unfortunately it’s not so easy to know without doing air flow measurements, or at least having access to the building HVAC specifications (which may or may not be up to date). Building maintenance people will often have access to air flow measuring devices.

So the bottom line: portable HEPA air cleaners may be a useful idea if the classroom has no or poor HVAC (perhaps some older schools). If the HVAC is properly designed and operating the air cleaners are probably not so beneficial. Focusing on higher levels of HVAC fresh air intake and better filtration of recirculated air are likely better ideas for the latter situation (as discussed in the Schools for Health document).

Caveat: the results in my chart rely on some typical engineering assumptions about air movement within the room, etc. Actual results may vary. Also, as noted in the Schools for Health document, careful placement of the portable air cleaners needs to be considered so that they don’t blow infectious aerosols from one person to the next in the classroom. Air cleaners and HVAC also don’t replace the need for hand hygiene, distancing, and surface sanitization. No air cleaner in the world will help you if the person next to you coughs or sneezes directly on you!