Updated: Jan 8
This is the first part of our four-part series on Indoor Air Quality (IAQ) Advocacy Basics.
Effective IAQ advocacy entails understanding what recommendations exist and what they mean. One source of confusion for indoor air quality (IAQ) advocates has been the diversity of units in which ventilation and filtration specifications can be expressed. Moreover, it can derail the conversation when recommendations and scientific literature have been conveyed in units that school facilities staff are not accustomed to using, namely air changes per hour (ACH). Better understanding airflow basics can allow advocates to insist that ventilation be quantified, and understand the answers when they are.
Recommendations have largely been expressed in three kinds of units, which can be mathematically inter-related for a given room size and occupant count:
Air change rate (Air Changes per Hour, ACH)
Airflow per person (cfm/person or L/s/person)
CO2 concentration in parts per million (ppm of CO2)
Before we can get to these, we need to define the most basic unit of most ventilation dimensions:
Airflow (cfm, L/s, or LPS)
Airflow is the rate of the flow of air per unit time, and is often expressed in units of cubic feet per minute (cfm) or liters per second (L/s or LPS). Engineers, scientists, and HVAC professionals use the variable Q to signify airflow in equations. In our parent advocacy materials, we tend toward being more explicit about the units we anticipate our US IAQ advocates will use, labeling this variable cfm.
Types of Airflow
When speaking about ventilation airflow, it is important to call out when we are talking about Outdoor Air (as in ventilation), which is associated with specific risk reductions in scientific literature (for example, the Hume Foundation study). If IAQ advocates fail to make ourselves clear about this, there is a greater chance that facilities managers may erroneously provide us with answers that are really in terms of supply air.
Supply air does not generally have the same properties as Outdoor Air in terms of reducing airborne illness. Because HVAC filters provide varying levels of efficacy at capturing droplets for various efficiency ratings, facilities managers must perform equivalent outdoor air calculations if they wish to quantify the proportion of supply that can be expected to provide a risk reduction. Based on observations from many parents at many school systems, not all facilities managers possess an understanding of this crucial and basic distinction; we, the advocates, must make it clear.
In contrast to HVAC supply air, airflow from a portable HEPA filter has an equivalency to outdoor air that is very close to 1:1 for particulates and respiratory aerosols. This means that the Clean Air Delivery Rate (CADR) for a portable HEPA filter is generally very close to the overall airflow rate that it can process, measured in cfm. Basically, any air that goes through that filter is going to come out with less than 0.03% of the particulates that were present before. This is known because HEPA filters must remove 99.97% of 0.3 micron particles, and for any other particle sizes, HEPA filters are more efficient.
Ventilation and filtration are often specified in terms of units that build on top of airflow to express an amount of ventilation relative to room size or occupant count. One example of this is...
Air Change Rate (Air Changes per Hour, ACH)
Air change rate, expressed in air changes per hour (ACH), represents an airflow rate relative to the volume of air present in the occupied space. Air change rate (in ACH) is commonly used to express both ventilation and portable HEPA filtration rates.
When speaking about ventilation, air change rate indicates how many times per hour the same volume of air contained in the room will be either cleaned by a portable HEPA filter, or pumped in from outdoors.
Imagine freezing the air in a room into an ice cube and then swiping left. Now, imagine replacing that stale air with an entire room's worth of new air.
That's one air change. The number of times this happens per hour is the air change rate, in Air Changes per Hour (ACH).
Notably, because of the way that air mixes, one air change does not mean that the full volume of air in a given room has been perfectly replaced by Outdoor Air. That also means that one air change cannot rid the room of all contaminants. Furthermore, if the contaminant source (e.g. paint fumes, sick kids, or mold) is emitting contaminants inside the room, then continual air changes are needed. This is the entire purpose of ventilation. Because of the way air mixes, it is necessary to pursue increased air change rates to produce decreased contaminant concentrations, and accordingly, decreased exposure and infections.
Because portable HEPA filters capture particulate contaminants with 99.97% efficiency, ACH can be used directly to specify the volume of airflow through one or more portable HEPA filters, relative to room size. Portable HEPA filters do not introduce actual Outdoor Air, so air changes from portable HEPA filters are sometimes differentiated as effective, or equivalent, air changes per hour (eACH). Lesser filters remove a smaller proportion of contaminants, which can contribute an increase in the equivalent air change rate as calculated by an HVAC professional, albeit to a lesser extent than HEPA filters do. Finally, since portable HEPA filters cannot capture gases like CO2, a personal CO2 monitor will not indicate any change in CO2 levels when portable HEPA filters are used.
ACH is the most common unit we have observed in both the scientific literature regarding observations on risk reduction, and in the authoritative recommendations from experts. Even so, some building operators or facilities managers regard ACH as an imprecise measure because it fails to account for (and scale with) room occupancy. It is important for advocates to understand that if they are being told that ACH is an inadequate metric of ventilation, they should insist that the ventilation then be quantified in some other unit (for instance, cfm). It is a matter of algebra to convert other units to ACH for a given classroom, and then to compare that ventilation rate against the most widely-recommended minimum of 6-12 ACH (which we will cover along with many specific authoritative recommendations in the third blog in this series).
Facilities administrators commonly design ventilation against a given occupancy, so it's good to know the units for expressing ventilation on an occupancy basis, i.e.:
Airflow per Person (L/s/person or cfm/person)
Airflow per person expresses ventilation relative to the number of occupants. Airflow per person is most often expressed in either cfm per person (cfm/person) or liters per second per person (written as either L/s/person or LPS/person). Airflow per person is used to specify a minimum amount of airflow that should be added to the room (usually from ventilation) for each additional occupant in that space.
We haven't seen airflow per person used to specify a quantity of HEPA filtration, although we suppose it could be, by summing the total clean air delivery rate from an ensemble of portable HEPA filters and dividing that by the number of occupants.
Airflow per person by itself does not take into account the age or activity rate of the occupants, which can factor into their breathing rate, lung volume, and contribution to aerosol generation. Air change rate, likewise, does not necessarily account for this factor. One kind of ventilation specification that does take this into account, by using CO2 as a tracer mechanism, is...
CO2 Concentration (parts per million, ppm)
In contrast from air change rate and airflow per person, CO2 concentration does not directly express an airflow rate, but it can be used in combination with occupant counts and ages to estimate one (for details, keep reading).
The ambient concentration of CO2 in outdoor air is usually between 400-450 parts per million (ppm), depending upon where you live. Human respiration is usually the main source of CO2 indoors. Roughly 4-5% of what people exhale is CO2, so without adequate ventilation, the indoor concentration of CO2 and other exhaled gases can build up. In contrast, if ventilation is used to maintain a certain degree of Outdoor Air, then the CO2 concentration should level off at some steady-state value. Because CO2 concentration is easily measured, it can serve as a proxy to measure the degree to which ventilation is being employed (or not employed) to provide a healthy environment and mitigate the spread of airborne illness.
Notably, HEPA filters do not capture gases, meaning that their contribution to air cleanliness cannot be benchmarked by measuring CO2 concentrations.
Also notably, for a given quantity of airflow per person, we calculate that the room will settle at roughly the same CO2 concentration irrespective of the size of the room and number of occupants. We've seen literature from experts that is consistent with that. Stay tuned to this series for details on that.
Computing and Converting
But how do we compute ACH? And how does CO2 concentration relate to airflow so that we can benchmark a given CO2 concentration against other kinds of recommendations? When you need to perform such computations, you can refer to our next blog in this series, which covers Clearing Up Conversion Confusion in IAQ.