Despite the Claims of Safety, Spending Three Hours in an Indoor Classroom Entails Significant COVID Risk

Initial theories about the spread of COVID-19 attributed the majority of cases to exposure to large droplets which are exhaled by people who are infected and then tend to drop rather quickly to the ground. This is why very close exposure was thought to be critical for infection to occur and why the six foot rule was widely accepted as conveying protection.

However, more recently, there has emerged a compelling body of research demonstrating that COVID-19 can also spread through aerosols, much smaller particles that tend to rise rather than fall and can spread much longer distances as well as enter ventilation systems and continue to provide potential exposure for much longer periods of time.

For example, a study by Shen et al. concluded: "To inform the knowledge gap and investigate potential transmission routes of COVID-19, we present the result from an outbreak investigation of COVID-19 among lay Buddhists worshiping in a temple. The attack rate on an enclosed bus containing an individual with COVID-19 was 34.3% while no one on a second similar bus was infected. The natural experiment provided convincing evidence that the disease transmission largely occurred during the bus rides. Through further epidemiologic investigation, we determined that the possibility of transmission by direct contact on the exposed bus was low. The absence of a significantly increased risk in the part of the bus closer to the index case suggested that the risk was fairly evenly distributed throughout, and thus, airborne spread of the virus may, at least partially, explain the extraordinarily high attack rate observed. This finding has important public health significance and future efforts at prevention and control should consider the potential for airborne spread of COVID-19, particularly in enclosed spaces with re-circulating air conditioning systems."

A study published in Lancet Respiratory Medicine also concluded that small droplet aerosols, especially in poorly ventilated spaces, can transmit COVID-19.

An article in Science makes the same point: "Why coronaviruses cluster so much more than other pathogens is “a really interesting open scientific question,” says Christophe Fraser of the University of Oxford, who has studied superspreading in Ebola and HIV. Their mode of transmission may be one factor. SARS-CoV-2 appears to transmit mostly through droplets, but it does occasionally spread through finer aerosols that can stay suspended in the air, enabling one person to infect many. Most published large transmission clusters “seem to implicate aerosol transmission,” Fraser says."

The Wall Street Journal described another study with similar findings: "the new coronavirus can also be transmitted through aerosols, or minuscule droplets that float in the air longer than large droplets. These aerosols can be directly inhaled. That’s what may have happened at a restaurant in Guangzhou, China, where an infected diner who was not yet ill transmitted the virus to five others sitting at adjacent tables. Ventilation in the space was poor, with exhaust fans turned off, according to one study looking at conditions in the restaurant. Aerosolized virus from the patient’s breathing or speaking could have built up in the air over time and strong airflow from an air-conditioning unit on the wall may have helped recirculate the particles in the air, according to authors of the study, which hasn’t yet been peer-reviewed."

Another article in Science concluded that: "a large proportion of the spread of coronavirus disease 2019 (COVID-19) appears to be occurring through airborne transmission of aerosols produced by asymptomatic individuals during breathing and speaking. Aerosols can accumulate, remain infectious in indoor air for hours, and be easily inhaled deep into the lungs."

There is already a case report of three teachers who shared a classroom and remained more than six feet apart, with masks on, at all times. One was apparently infected and both of the others became infected.

Joseph Allen, the director of the Healthy Buildings program at Harvard T.H. Chan School of Public Health wrote about the importance of airborne aerosol transmission of COVID-19 in an op-ed piece in the Wall Street Journal:

"If you’ve been following advice about covid-19 from the Centers for Disease Control and Prevention or the World Health Organization, chances are you’ve heard a lot about how the coronavirus can travel through large droplets via coughing and sneezes. You’ve also probably heard about the virus being transmitted through surfaces. But you probably haven’t heard anything about airborne transmission, which many organizations have largely ignored. That’s a mistake. Airborne transmission — caused by small particles that can linger in the air for extended periods of time, unlike droplets from coughs, which settle quickly — is key to understanding why this disease spreads so rapidly in certain circumstances. It’s also key to figuring out how best to reopen our country. I’ve been warning about airborne transmission of covid-19 since early February. The explosive transmission on the Diamond Princess cruise ship, as well as other coronavirus outbreaks, constituted telltale signs that airborne transmission was happening. Close contact transmission was likely happening on that cruise ship, but the disease had spread far more quickly than non-airborne diseases typically spread."

An article published in JAMA also concluded that airborne spread of COVID-19 through aerosols is a significant factor in the spread of the virus.

Another factor that must be considered is that ventilation levels in BU classrooms have been measured and the investigating scientists concluded that they pose significant COVID risks because of poor ventilation.

Finally, what has been largely ignored by the School and University is that duration of exposure, not just level of exposure, is a critical component of the dose of exposure and therefore of the likelihood of infection. In fact, the duration of exposure may be the most critical factor determining the risk of infection. And for SPH classes, the duration of exposure is substantial. Most of our classes are 3 hours long.

There is some evidence that the risk of infection may be exponentially related to the duration of exposure, not linearly related. But even if it is linearly related, this means that if we set the risk of a 10-minute class at 1, then the risk of a 3-hour class is 18 times higher.

Chemistry professor Jose-Luis Jiminez at the University of Colorado has created a COVID-19 aerosol transmission estimator that estimates the probability of infection in a classroom given various parameters. Assuming a classroom that is 25 feet by 20 feet, containing 10 students, of whom one is infected and assuming all students are wearing masks, the program estimates that the probability that one student becomes infected over the course of the semester is 11% and that the probability that one student requires hospitalization over the course of the semester is 2%. The probability that a death occurs is 0.1%. The ratio of this death risk compared to that of dying in a car accident is 2,000.

This is clearly not an acceptable level of health risk, especially for students, faculty, or staff who have conditions that put them at increased risk of severe disease if they become infected. In no way can the School suggest that it is protecting the health of the BUSPH community under these conditions.

Moreover, the testing protocol at the School is inadequate to control infection spread. According to a study published by researchers at Yale School of Public Health, testing would need to be conducted every other day to be effective in preventing an outbreak. BUSPH's plan to test students once per week is not even close to adequate.

The bottom line is that the best available evidence suggests that six-foot spacing does not provide adequate protection from exposure to coronavirus in a classroom setting and that even with mask wearing, the exposure should be considered high risk because of the existence of widespread airborne transmission, poor ventilation in many BU classrooms, and the immense duration of exposure.