Indoor air quality (IAQ) has been on a lot of our minds while dealing with an airborne virus. Teams of scientists in the fields of epidemiology, public health, aerosols, fluid mechanics, and those involved in the design and construction of environmental comfort (heating and air conditioning) have helped give us road maps for making indoor spaces safer. In January 2020 I started cracking windows open in the studio as the first reports of an airborne virus were being discussed. I haven’t given a session or hosted a practicum class with the windows closed since – rain or shine, hot or cold.

Fresh Air

Researchers have found that fresh air in indoor spaces impacts multiple areas of our health. In a 2011 review of the available literature, Santa Cruz researchers found that poorly ventilated buildings contributed to sick building syndrome (SBS), inflammation, respiratory infections, asthma symptoms, childhood allergies, and an increased amount of short‐term sick leave days by employees (Sundell et al).

The guidance on how much fresh air should be brought into a building varies depending on who you’re talking to. Ventilation guidelines are measured in how many times a room’s stale air is completely exchanged with outside air and is measured as ACH: Air Changes per Hour. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends a minimum of 0.35-1 air changes an hour for homes and increases their rates based on how many people are gathering in the space, what their activities are, and whether there is an airborne contaminant like virus particles. In healthcare settings ASHRAE and the CDC specify a minimum of 2-3 fresh air changes per hour but acknowledge the need for fresh air changes beyond 50 times an hour to remove contaminants from the room in a timely fashion (more on this later).

CO2 Monitoring

One of the tools I’ve been very interested to explore during the pandemic is the use of carbon dioxide (CO2) meters to monitor how effectively one is ventilating a space with fresh, outside air. It works on the concept that outside air is roughly 415 parts per million (ppm) of carbon dioxide with indoor spaces ranging from a highly ventilated 450 ppm to very poorly ventilated spaces reaching into the thousands of ppms of CO2.



Joseph G. Allen, the director of the Harvard Healthy Buildings program and an environmental health expert.

Ventilation is Important – Especially When the Virus is in the Air



What’s the owner of a wellness studio to do when an airborne virus turns into a pandemic? I’ve asked myself this question sooo many times, read through hundreds of research papers and articles, then sat down and crunched some serious math. I thought I’d share the evolution of my ventilation upgrades in the hopes that it is helpful for others trying to make sense of how to make indoor spaces safer.

Ventilation Matters

SARS-CoV-2 (COVID-19) travels with person-to-person contact and has been well-documented that microdroplet and aerosol transmission firmly puts this virus into the airborne category (1, 2, 3, 4, 5, 6). And while 2- and 3-ply masks do a good job of helping to reduce the amount of microdroplets and aerosolized secretions that make it into our breathing space (5, 7), but we don’t consider them to be a standalone too (hence the social distancing guidelines). Considering estimates say 40% of persons with COVID-19 present with no symptoms (8), it’s easy for me to see why ventilation needs to be an absolute priority for every indoor space right now.

In the New York Times article based on the Florida research that found virus in the air 7 and 16 feet away from COVID-19 patients in a local hospital (6), Several experts noted that the distance at which the team found virus is much farther than the six feet recommended for physical distancing. “We know that indoors, those distance rules don’t matter anymore,” Dr. Schofield said. It takes about five minutes for small aerosols to traverse the room even in still air, she added.”

Determining Ventilation Needs

First, I would need a few details about my room to determine my needs. I started with a tape measure, and three very helpful cats <insert eyeroll and a grin> to determine the square footage of the studio: 18′ (width) x 20′ (length) = 360 square feet. To determine the breathable space in the studio an additional number gets thrown in: ceiling height. So 18′ x 20′ x 8′ = 2,880 cubic feet of air to ventilate.

My Blue Pure 211+

This information helped me initially determine the size of air purifier I would need to purchase. I started out with the Blue Pure 211+ by Blueair as it was rated for five air changes an hour in a 540 square foot room.

After a while I wondered if five air changes an hour was enough for an airborne virus? An online search brought me to the CDC’s Ventilation Specifications for Health-Care Facilities, which is based on the American Institute of Architects’ Guidelines for Design and Construction of Hospitals and Health-Care Facilities, 2001. Having worked in the construction industry prior to going into the healing arts this felt like a good way to understand how buildings and HVAC systems are designed for healthcare facilities. The guidelines account for factors like air movement, air changes of outdoor air, and total air changes in a variety of spaces from emergency room waiting areas, to the OR, and even labor and delivery wards.

Drawing on my memories of helping determine HVAC needs for remodels and new construction I quickly realized more math would be needed here. Ha! Fortunately for someone like me with synesthesia around numbers, there’s a handy website calculator that helped determine how many air changes per hour (ACH) I could expect after purchasing a larger air purifier. The 211+ model cleans the air at 360 cubic feet per minute (CFM), and I purchased their next larger model, the 121, which cleans the air at 400 CFM, for a combined effort of 760 CFM . Based on my room size, 2,880 cubic feet, and a filtration rate of 760 cubic feet per minute my two air purifiers are capable of cleaning the air 15.83 times an hour, also known as an ACH of 15.83.

Next, I needed to determine how much outside air I was bringing into the room. While I have a window air conditioner in one window, the other window has a fan with dual-blades that can be run in exhaust, intake, or exchange (one fan blowing in, one blowing out). The CFM for my window fan is 272, which gives me an additional 5.67 ACH per hour.

Then I combined the two: 15.83 with the air purifiers + 5.67 with the window fan = 21.5 air changes per hour. Or every 2.8 minutes the room’s breathable air is being exchanged for clean air.

In November I swapped out the twin window fan with a 20″ Lasko box fan that has a CFM of 1,600 on low, 1,800 on medium, and 2,000 on the highest of the three fan speeds. This gives me the ability to vary my fresh air changes per hour (ACH) from 33 on low, 37.5 on medium, 41.6 on high. The math on that is spelled out as: 1,600 CFM x an hour (60 minutes) = 96,000. 96,000 / 2,880 cubic feet to ventilate = 33 ACH on low. When paired with the two air purifiers running I have 48.83 on low, 53.33 on medium, 57.83 on high.

Cleaning the Air Before and After Clients

The chart for Airborne Contaminant Removal shows an ACH of 20 will need 14 minutes for 99% removal of contaminants and 21 minutes for 99.9% removal. This guide is of particular help to know how long I need to budget to have the room empty on either side of an appointment so the air can be well cleaned. If my ACH gets smaller, I’ll need to budget longer for ventilation.

By adding the box fan on high I can achieve 99.9% efficiency of contaminant removal in 8 minutes.

In Closing

Overall this exercise in math, measuring with cats, and pouring through the available literature leaves me feeling quite pleased with my efforts. My ACH exceeds the CDC guidelines for ER waiting rooms, but it’s not anywhere near a negative pressure room like you’d expect in an isolation room in the ICU.

Other preventative measures I have in place to make in-person appointments and practicums safer, include:

  • Mask wearing for myself and clients/students;
  • Changing outer clothing between each client;
  • Using the list of EPA disinfectants approved for SARS-CoV-2 and ensuring they get proper dwell time to do their work on contact surfaces, equipment, flooring, and the bathroom (I use a fogger);
  • Small social bubble, in keeping with multiple government guidelines;
  • Keeping healthy by keeping a daily temperature log, restricting outings to essential errands and outdoor recreation, and wearing a face mask in public.

The adjustment period for us all has made 2020 particularly stressful but I feel confidant that steps like improving ventilation will go a long ways to making it safer for me to continue seeing folks in person and folks to continue their studies and/or receive wellness services.

References for this article:

  • Sundell, Jan, et al. “Ventilation rates and health: multidisciplinary review of the scientific literature.” Indoor air 21.3 (2011): 191-204.
Further reading:
  1. New study explores how coronavirus travels indoors, University of Minnesota, 7/28/20
  2. We Need to Talk About Ventilation, The Atlantic, 7/30/20
  3. Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission, The Lancet: Respiratory Medicine, 5/27/20
  4. Open windows while using air conditioning, experts say as WHO shifts stance on airborne coronavirus, The Telegraph, 7/11/20
  5. Reducing transmission of SARS-CoV-2, Science, 6/26/20
  6. Still Confused About Masks? Here’s the Science Behind How Face Masks Prevent Coronavirus, University of California San Francisco, 6/26/20
  7. 40% of People With COVID-19 Show No Symptoms, The CDC Estimates, Science Alert, 7/13/20