48. COVID, Aerosols, and Classrooms

Lately everyone has been debating the question of whether kids should return to in-person school this fall. It’s an impossible question, because we’re damned if we do and damned if we don’t.

This post isn’t intended to debate the logic or wisdom of reopening school. Rather, I’d like to discuss HVAC measures that can be taken to make schools safer if and when they do reopen. The WHO recently acknowledged that COVID19 probably aerosolizes, though the primary mode of spread is via larger respiratory droplets.

What is the significance of COVID19 aerosolizing? Although cloth and surgical masks help to block respiratory droplets, they won’t contain aerosols. Which means people who spend lots of time in poorly ventilated indoor spaces, like classrooms, can still share virus regardless of wearing masks. Furthermore, if virus is aerosolizing, remaining six feet apart won’t prevent all infections. While N95 masks are more effective than regular masks at blocking aerosols, they’re also more expensive and uncomfortable to wear for long periods of time. Short of everyone wearing N95 masks at all times, what else can we do to keep classrooms safe?

Specific HVAC measures can be taken, as I’ve outlined below:

  1. High-grade HEPA filters can and should be placed in every classroom.

During the COVID19 pandemic, Yale New Haven Hospital successfully used HEPA filters in patient rooms to reduce viral transmission. Free-standing HEPA units are absolutely capable of filtering coronavirus-sized particles. Relatively speaking, the units are cheap compared to other air purification systems. Modest freestanding units, for example, can be purchased on Amazon for about $120. My office bought a $500 HEPA filter for our central workspace, an area slightly larger than most grammar-school classrooms.

In June the state of Connecticut posted ventilation guidelines for reopening schools. Although the document recommends HEPA filters for rooms housing sick students prior to dismissal, it argues against using HEPA filters in classrooms.

Specifically, the guidelines state, “…we do not recommend separate, free-standing air cleaner or HEPA filter units for individual classrooms.  These units are highly variable in their effectiveness in larger open spaces such as classrooms and in general, any effect on indoor air quality is likely insignificant and greatly outweighed by the additional costs to school systems.”

Respectfully, I disagree. In my opinion, $500 per classroom is a small price to pay for protecting occupants. In many classrooms, HEPA filters may be the only affordable option for decontaminating air. In the absence of central air and/or adequate ventilation, students and teachers may be breathing close to 100% recirculated air, all day long. Will HEPA filters work perfectly for preventing infections in these environments? No, they won’t; nor will they replace masks and physical distancing as the first line of defense. But when it comes to mitigating risk from aerosolized virus, adding another layer of protection with HEPA filters is better than doing nothing.

  1. Upper-room germicidal UV lights can potentially be installed in ceiling spaces. These devices aren’t mentioned in the Connecticut guidelines, possibly due to high costs and/or misplaced safety concerns. Far UV light, which is safe for humans and deadly for microorganisms, is another up and coming modality that may prove useful for decontaminating air in school systems.
  2. For schools with heating/cooling systems that duct fresh air inside, air conditioning units can be programmed to provide more fresh-air exchanges per hour. In Connecticut, current building codes require six air exchanges per hour for medical offices and twelve for hospital rooms. Bringing more fresh air into classrooms will limit the inhalation of recirculated air. However, running units more quickly does require a greater expenditure of energy.

Central air conditioning systems can also be equipped with high-grade filters (MERV 14 or higher) capable of removing viral particles, similar to HEPA filters. But swapping one filter for another isn’t always a simple process due to pressure differentials, and every building has a unique airflow system. Hiring HVAC engineers to evaluate current airflow systems in our schools seems a wise investment.

For classrooms with baseboard heat that lack AC units, discussing airflow may be a moot point. Yet the importance of air purification devices such as HEPA filters and upper-room UV lights is potentially greater in spaces without central air, as decontamination options are limited.

  1. As stated in the Connecticut ventilation guidelines for schools, windows can potentially be left open to draw more fresh air into classrooms, weather permitting; but not all classrooms are equipped with windows that open.

COVID19 is forcing us to rethink indoor air spaces, and the HVAC measures described above can help to keep classrooms safer. The same goes for offices, stores, restaurants, hair salons and other indoor spaces where we collectively spend time. All of these measures cost money, but relatively small investments upfront can potentially offset misery and financial loss down the road.

Below is a link to the ventilation guidelines for schools released by the state of Connecticut.