NEWS • 23.10.2020

Harvard’s misunderstanding calculator

In August this year, the Foundation “Schools for Health” published a guide that told us how to know if the ventilation rate in the classrooms was adequate for the correct start of the school year given the current situation of the COVID-19 pandemic. The study indicates a method to determine the air renewal capacity of the ventilation systems in schools and whether the flow moved by them was sufficient or not.


Keeping schools closed comes with massive, long-term individual and societal costs. Many children cannot effectively learn, grow, engage, socialize, be active, eat healthy food, or get support until schools reopen. Parents and caregivers cannot go back to work until children go back to school. Knowing that schools will reopen at some point, we set out to answer this question: what strategies should schools consider to reduce risk of COVID-19 transmission?

Dimensions of the classroom


The impeccable technical treatment of the guide expected the possibility of not being able to ventilate properly and opened the door to fill this gap with Air Purification Systems based on HEPA filter. The flow not provided by incoming outdoor air was compensated by the same flow rate of purified air. To do this, it linked us to a selection calculator of units based on their technical characteristics.


This link takes us to an Excel sheet where just entering the dimensions of the classroom gave us the characteristic of the necessary Purifier Unit. This characteristic was based on the CADR (Clean Air Delivery Rate) that the equipment had to comply with.

The fact of only considering the volume of the room leaving aside the occupation is not in accordance with the Spanish regulations, but that it was based on ASHRAE 62.1 standards led me to investigate this, at first glance, a contradiction. Indeed, the calculations were exact, their application correct, but not applicable to the reality we face in Spanish schools. 

ASHRAE method

If we analyse how the calculations are applied, we must go to the “README” sheet, where it explains in detail the data taken into account for this. What we find in it is that the ASHRAE method for ventilation is applied as:

  • 5 l/s person
  • 6 l/s m²


This approach would be correct under ASHRAE standards, but it would be outside of the standards in Spain, due to:

  • RITE ventilates people, not surfaces
  • The RITE flows are two, schools and classrooms in general 45 m³ / h person and nurseries 72 m³ / h person

The truth is that this aspect could be ignored if the final result were practical, but it is not. The fundamental parameter of the calculation is based on the capacity, rather, the capacity density, which the calculator took as the basis of calculation. This value was 25 people in a 1,000 ft² (92 m²) room, unthinkable in Spain where in 40 m² we comfortably accommodate those 25 people.

Misuse of calculator

The misuse of this calculator leads to under sizing errors of 70% to 100% for schools and from 112% to 150% for kindergartens, not insignificant, but above all to the error of feeling safe for having done well under high guaranteed standards.


The fact is that for a 50m² classroom it indicates us as good, with “typical natural ventilation” of 1.5 renewals / hour and with an occupancy of 13 people, a purified contribution of 3.1 renewals / hour.

The rates of our ventilation indicate between 7-8  air renewals/hour for an occupancy of 25 people in  a room of 50m², well said, 1,125 m³/h filtered air through pre-filter F6 and filter F8, from which if we subtract that “natural ventilation” of 1.5 renewal/hour, more than doubtful, indicates a purified air rate of 6.5 renewals/h. Data very distant from that obtained by the calculator.


As indicated above, this error multiplies in day care centers. As a final summary we must indicate that:

  • Although the calculator is extremely correct under the ASHARAE Standard, its direct application in Spain is unviable and dangerous
  • Calculating air renewal flow rates without knowing the capacity is very risky


Our AirSanit (air handling and purification unit) has been developed thanks to studies carried out by our R&D. AirSanit has already obtained the Anti-COVID Certification from the National Institute of Aerospace Techniques of Spain.


This is just an example of how we continue work to put all our experience and knowledge at the service for the global fight against COVID-19.

“AirSanit is one of the methods schools can use for their fight against viruses, like COVID-19.”

José Arbolebas (Special Projects and Training)

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