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 8 vs MERV 11: Which is Better?

Jennifer Crowley
Jul 28, 2023
4 min read

Updated: Jul 9, 2024

Dirty furnace filter laying atop a clean filter — When comparing MERV 8 and MERV 11 filters, it is important to consider their particle capture capabilities, air flow rates, pressure drops, and costs.

MERV (Minimum Efficiency Reporting Value) is a rating system that measures the effectiveness of air filters in capturing particles of different sizes. The MERV rating scale ranges from 1 to 20, with higher numbers indicating greater efficiency in capturing smaller particles.

In this blog post, we will compare two standard MERV ratings for air filters: MERV 8 vs MERV 11. We will discuss the pros and cons of each type, their particle capture capabilities, air flow rates, pressure drops, and costs, as well as provide guidance on choosing between the two.

What is MERV 8?

MERV 8 filters can capture particles that are 3 to 10 microns in size, such as

Pollen
Dust mites
Pet dander

They are less expensive and allow for higher airflow rates than higher MERV-rated filters, making them a good choice for systems with high airflow needs. However, MERV 8 filters may not capture smaller particles that can cause respiratory problems, such as mould spores and bacteria. They must also be changed more frequently than higher-rated filters to maintain effectiveness.

What is MERV 11?

MERV 11 filters can capture particles that are 1 to 3 microns in size, such as:

Pollen
Dust mites
Mould
Large dust particles
Auto emissions
Fine air particles
Pet dander
Lint
Smoke
Smog
Bacteria

MERV 11 filters offer better filtration than MERV 8 filters and can help improve indoor air quality for people with allergies or respiratory problems. However, they are also more expensive and can restrict airflow more than MERV 8 filters. When the airflow is restricted, it can lead to discomfort, higher energy consumption, and faster wear and tear of HVAC equipment. Using an air filter with a MERV rating that exceeds your system’s recommended level can be particularly damaging, causing harm to the compressor, heat exchanger, and air conditioner coil.

Comparison of MERV 8 vs MERV 11

When comparing MERV 8 and MERV 11 filters, it is important to consider their particle capture capabilities, air flow rates, pressure drops, and costs.

Particle capture Comparison chart of MERV 8 Vs, MERV 11 — MERV 11 filters more effectively capture smaller particles than MERV 8 filters.

Particle capture capabilities: MERV 11 filters more effectively capture smaller particles than MERV 8 filters. This means they can help improve indoor air quality and reduce the risk of respiratory problems. However, MERV 8 filters may be sufficient for capturing larger particles like pollen and dust, which can also contribute to indoor air pollution.

Air flow rates: MERV 8 filters allow for higher airflow rates than MERV 11 filters. This means they may be a better choice for standard HVAC systems with high airflow needs, such as commercial buildings that don’t require superior air filtration. MERV 11 filters can restrict airflow more than MERV 8 filters, which can cause strain on HVAC systems and potentially increase energy costs.

Pressure drops: MERV 11 filters can cause higher pressure drops than MERV 8 filters, further straining HVAC systems and reducing energy efficiency. Choosing the right filter for the specific system is essential to avoid this issue.

Costs: MERV 11 filters are generally more expensive than MERV 8 filters, which can be a factor for those on a tight budget. Additionally, MERV 11-rated filters get clogged much more quickly than a MERV 8 filter and have to be replaced frequently.

How to Choose Between MERV 8 and MERV 11?

When choosing between MERV 8 and MERV 11 filters, it is crucial to consider the HVAC system’s and its occupants’ specific needs. Here are some factors to consider:

Types of particles present: If your system needs to capture smaller particles like mould spores or bacteria, a MERV 11 filter may be more appropriate. If it only needs to capture larger particles like pollen and dust, a MERV 8 filter may suffice.

Air flow rate required: If your system requires high air flow rates, a MERV 8 filter may be a better choice. However, if indoor air quality is a higher priority and airflow rates can be sacrificed, a MERV 11 filter may be a better choice.

Budget available: A MERV 8 filter may be more cost-effective if the budget is a concern. However, if indoor air quality is a higher priority and a higher initial cost can be justified, a MERV 11 filter may be a better choice in the long run.

Occupant health concerns: If occupants have allergies or respiratory problems, a MERV 11 filter may be more appropriate to help improve indoor air quality and reduce the risk of health issues. However, if no health concerns are present and your system only needs to capture larger particles, a MERV 8 filter may be sufficient.

In summary, MERV 8 and MERV 11 filters both have their advantages and disadvantages and choosing between them depends on your HVAC system’s specific needs and occupants. While MERV 11 filters offer better particle capture capabilities and improved indoor air quality, they also come with higher costs and potential airflow restrictions. On the other hand, MERV 8 filters allow for higher air flow rates and are more cost-effective but may not capture smaller particles that can cause health problems. It is important to weigh these factors carefully and consult a professional to determine the best filter for your system.

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