Flu Season Meets School Season: How Smarter Air Quality Keeps Classrooms Healthy

Every fall, classrooms fill with students—and viruses. Discover how smarter air quality strategies like low-resistance filtration, ventilation, and HEPA keep schools healthier, reduce absences, and support better learning outcomes.

Ava Montini

Aug 19, 2025

Written by

Published on

The scene every September

Every September, the school bell rings and hallways come alive again. But as backpacks and lunch boxes make their way back into classrooms, another unwelcome guest tends to sneak in too: flu season.

Teachers know it all too well. The cough that spreads from desk to desk, the hand sanitizer bottles running low by mid-morning, the spike in absenteeism that leaves lesson plans hanging. Parents know it when the inevitable call from the school office comes: “Your child has a fever, please come pick them up.”

It’s a cycle we’ve come to accept as part of the school year. But what if healthier air could help change that story?

Why flu season and school season collide

Respiratory viruses (including influenza) spread more readily indoors, where exhaled particles accumulate. That’s not speculative; CDC/NIOSH is unambiguous that better indoor ventilation reduces occupants’ overall exposure to airborne viruses. CDC

We also know influenza isn’t only about big droplets from a sneeze. People exhale infectious virus in fine aerosols during normal breathing and speaking, which can linger and travel within a room. That was demonstrated in a landmark study that detected infectious influenza virus in exhaled breath from symptomatic adults, no cough required. PNAS Nature

The drier, colder air from the fall and winter cause low humidity, helping influenza survive and transmit more efficiently. Put simply: when we bring students back into dry, tightly sealed buildings, small airborne particles build up and stay infectious longer. That’s the fixable part.

Think of clean classroom air as a budget with three line items:

Dilute what’s in the room (ventilation/outdoor air)
Remove what’s in the room (filtration/air cleaning)
Disable what’s in the room (UVGI where appropriate)

The key is using them together, sized to the space, and tuned to the school day.

What the standards now say and why it matters

Before the pandemic, most schools designed ventilation systems mainly for comfort—things like controlling odours or keeping CO₂ levels down—not for stopping the spread of illness.

That changed with ASHRAE’s new Standard 241, which focuses specifically on infection control. ASHRAE’s Standard 241: Control of Infectious Aerosols changes the target by introducing Equivalent Clean Airflow (ECA)—a flexible, additive way to hit a per-person clean air goal using any combination of ventilation, filtration, and proven air cleaning. That means a classroom can meet its target by mixing outdoor air with high-efficiency filters, HEPA units, and/or UVGI, rather than relying on outdoor air alone. ASHRAE+1

In parallel, CDC/NIOSH and EPA emphasize practical steps for schools: keep systems maintained, upgrade to MERV-13 or better where equipment allows, and supplement with portable HEPA when central systems can’t carry the whole load. CDC+1 Environmental Protection Agency

The evidence that this keeps kids in class

In a study of 162 California elementary school classrooms, illness-related absences dropped by 1.6% for every extra 1 l/s‑person of ventilation. Increasing ventilation to meet the state standard (7.1 l/s‑person) from the average (4 l/s‑person) could reduce absences by 3.4%, gain $33 million annually in attendance-based funding, while costing just $4 million more in energy.
A study across Washington and Idaho found that a 1,000 ppm increase in indoor CO₂ correlated with a 0.5–0.9% drop in average daily attendance, translating into a 10–20% rise in student absences.
In controlled environments, each 500 ppm rise in CO₂ resulted in 1.4–1.8% slower response times, along with a 2.1–2.4% lower throughput on cognitive tasks.
Harvard’s COGfx study revealed that building occupants in green-certified, well-ventilated environments scored, on average, 101% higher in cognitive tests than those in conventional buildings.

“Will MERV-13 break my units?” (The energy/airflow reality)

The honest answer: it depends on the filter you pick and your fan capacity. Research on rooftop units shows that moving from MERV-8 to MERV-13/14 can raise cooling-mode energy use by a few percent if the filter adds a lot of resistance, or it can reduce airflow if the fan can’t keep up. That’s why filter selection matters as much as efficiency.

Not all MERV-13 filters are created equal. Traditional pleated designs often create a higher pressure drop, forcing HVAC systems to work harder and sometimes leading to performance issues. But newer filtration technologies (explicitly engineered for low resistance at high efficiency, like Blade Air's Pro Filter,) are changing that equation. By combining advanced media with optimized form factors, these filters deliver MERV-13 (and higher) performance without the heavy airflow penalty.

California’s Title 24 research reinforces this point: Many modern low-pressure MERV-13 options can maintain pressure drops under 0.20 in. w.c., keeping systems within safe operating ranges. That means schools can improve air quality, meet public health guidance, and stay compliant without sacrificing system efficiency or longevity.

When you factor in the bigger picture—fewer student absences, better cognitive performance, and improved overall school operations—the ROI clearly tilts toward upgrading. Healthier air doesn’t just protect occupants; it protects the bottom line.

How this translates into a classroom target (the ECA idea)

ASHRAE 241’s Equivalent Clean Airflow lets you add up all the ways you’re cleaning air—outdoor air, central filtration, HEPA, UVGI—until you reach the per-occupant target for your space type. It’s flexible, measurable, and avoids unrealistic demands for 100% outdoor air in cold snaps. ASHRAE

A practical approach:

Estimate your current outdoor air (from design or testing).
Add the “clean air” from MERV-13 upgrades (using published efficiencies) and from each HEPA unit’s clean air delivery rate.
If the sum doesn’t meet the ECA target, add another portable unit or rethink your filtration strategy. ASHRAE

What about measurement and transparency?

CO₂ for ventilation

Track a few representative rooms across grade levels and building wings. Persistently high readings during class point to areas needing a fix (dampers, schedules, or supplemental air cleaning). Health Canada’s 1000 ppm residential benchmark is a useful anchor for conversations with families and staff. Canada.ca

PM₂.₅ for smoke days

A couple of low-drift sensors at kid-height in hallways or problem rooms can confirm your filtration strategy keeps indoor levels below outdoors during wildfire events. Health Canada and EPA both recommend this principle. Canada.ca

Bottom line

Flu season doesn’t have to mean higher absence rates and strained HVAC systems. The most effective path is a consistent program: keep ventilation tuned, use filters that balance efficiency with low resistance, and supplement with portable HEPA or UVGI where it makes sense.

MERV 13 vs HEPA Filter: How Are They Different?

Jennifer Crowley
Jul 28, 2023
3 min read

Updated: Jul 9, 2024

Cropped images of a pleated filter (MEV 13) and a HEPA filter, one on top of the other, to create one image — Both MERV 13 and HEPA filters are designed to remove pollutants from the air; however, there are some key differences between them including cost, fit and airflow.

Air filters are a vital component in maintaining good indoor air quality. Two of the most commonly used air filters are MERV 13 filters and HEPA filters. Both types of filters are designed to remove pollutants from the air; however, there are some key differences between them.

Illustration comparing the particle capture of MERV , 10 and 13 filters — The higher a filter’s MERV rating, the more effective it is at capturing airborne particles.

What are MERV 13 filters?

All traditional air filters are differentiated according to their MERV (Minimum Efficiency Reporting Value) rating. The higher a filter’s MERV rating, the more effective it is at capturing airborne particles. MERV 13 and below are considered HVAC-system-grade filters for residential, commercial and general hospital use.

MERV 13 filters can filter particles closer to 0.3 microns in size, which includes contaminants such as pollen, mould, dust, bacteria, pet dander, smoke and virus carriers. MERV 13 filters are typically used in residential and commercial HVAC systems to improve indoor air quality.

What are HEPA filters?

On the other hand, HEPA filters are considered the most efficient air filters available. HEPA stands for High-Efficiency Particulate Air and has a rating of MERV 17 or higher. HEPA filters are designed to remove 99.97% of particles that are 0.3 microns or larger in size; This includes particles such as smoke, bacteria, and viruses.

Since HEPA filters are so efficient, they cause a higher pressure drop than filters with MERV ratings. Given their high efficiency, HEPA filters are best suited for rooms where air quality is a concern, such as in hospitals, laboratories, and cleanrooms.

Many ventilation systems are not designed for HEPA, but these filters are available as portable air cleaners or vacuum cleaners and can be used in homes to improve indoor air quality, particularly for people with allergies or respiratory issues.

MERV 13 vs HEPA Filter: How Are They Different?

14 Pleated HVAC filters standing side by side as if on a bookshelf — MERV 13 filters can capture particles as small as 0.3 microns.

Pros of MERV 13 Filters:

Cost: Affordable and easy to find.
High Efficiency: MERV 13 filters can capture particles as small as 0.3 microns, making them highly effective at removing pollutants such as dust, pet dander, pollen, and even some bacteria and viruses from the air.

Cons of MERV 13 Filters:

Filter Efficiency: Not as effective as HEPA filters in removing the smallest and most harmful particles.
Maintenance: MERV 13 filters may need to be replaced more frequently than lower MERV-rated filters, which can add to the overall cost and maintenance of the HVAC system.
Compatibility: MERV 13 filters may not be compatible with all HVAC systems, and using a filter that is too efficient for a system can cause damage. Before using a MERV 13 filter, it is important to check if the HVAC system can handle the filter’s efficiency and pressure drop.

Photo of a free standing HEPA Filter — HEPA filters are extremely effective at removing pollutants, including smoke, bacteria, and viruses.

Pros of HEPA Filters:

High efficiency: HEPA filters are highly efficient at trapping small particles, such as dust, pollen, and pet dander. Extremely effective at removing pollutants, including smoke, bacteria, and viruses.
Long lifespan: HEPA filters are designed to last a long time, so they do not need to be replaced as frequently as other filters.

Cons of using a HEPA filter in an HVAC system include:

High cost: HEPA filters can be more expensive than other types of filters, which can make them a less cost-effective option for some people.
Decreased airflow: HEPA filters can reduce airflow through a standard HVAC system, which can make it less energy efficient and increase the amount of time it takes to heat or cool a room.
Need for professional installation: HEPA filters are typically larger and more complex than other types of filters, which means they may need to be installed by a professional.

In conclusion, both MERV 13 and HEPA filters effectively improve indoor air quality, but they have different pros and cons depending on the setting and the level of air purity required. MERV 13 filters are more affordable and easy to find and effectively remove a wide range of pollutants. HEPA filters are extremely effective at removing pollutants, but they are more expensive and can be more difficult to find.

It’s also important to consult a professional to determine the most appropriate filter for your setting, MERV 13 vs HEPA, as the filter type and frequency of replacement can vary depending on the size and usage of the space, as well as the type of pollutants present in the air.

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