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.

How Winter Affects Indoor Air Quality: Tips to Improve IAQ and Save Energy

Ava Montini
Nov 22, 2024
5 min read

As snow blankets the ground and the warmth of home becomes a retreat from the frigid air outside, another less visible element takes center stage: indoor air quality (IAQ). Winter is a season of contrasts—while we cherish cozy nights and festive gatherings, the environmental trade-offs of sealed windows, cranked-up heaters, and reduced ventilation can quietly affect our health and comfort.

The air indoors becomes a silent participant in our daily lives, influencing everything from how well we breathe to how much energy we consume. Understanding how winter habits impact IAQ is essential to creating healthier, more sustainable spaces.

1. The Winter Environment and Indoor Air Quality Challenges

During the winter months, we naturally seal windows and doors to retain heat, but this significantly reduces fresh air exchange. According to the Environmental Protection Agency (EPA), indoor pollutant levels can be two to five times higher than outdoors, and winter often amplifies this imbalance.

Common Winter IAQ Issues:

Trapped Pollutants: Dust, pet dander, and volatile organic compounds (VOCs) from cleaning products and materials accumulate in sealed homes.
Dry Air: Heating systems lower indoor humidity levels, often below the recommended 30-50%, causing respiratory discomfort and allowing airborne viruses to spread more quickly.
Carbon Dioxide (CO2): Poor ventilation increases CO2 levels, leading to fatigue, poor concentration, and even health risks.

For example, cooking hearty meals—a winter staple—releases fine particles and nitrogen dioxide into the air, particularly if ventilation systems are inadequate. Add to this candles, which emit particulates, and cleaning sprays with VOCs, and you get a cocktail of pollutants that linger in the air during winter.

2. The Link Between Poor IAQ and Winter Sickness

It’s no coincidence that cold and flu season coincides with winter. Poor IAQ creates an environment where airborne viruses thrive. Research from the Harvard T.H. Chan School of Public Health shows that higher levels of particulates increase the risk of respiratory infections.

Consider schools during winter: crowded classrooms with inadequate ventilation often experience higher absenteeism due to illness. Similarly, in workplaces, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) reports that poor IAQ reduces cognitive function by up to 15%.

Even at home, children and older adults are particularly vulnerable. However, addressing IAQ with solutions like air filtration and humidity control can reduce these risks, keeping households healthier through the season.

3. Heating Efficiency and Environmental Impact

Winter heating is essential for comfort and health, but it comes with both financial and environmental costs. According to the U.S. Department of Energy, heating accounts for nearly 42% of a household’s energy consumption during the colder months, making it the single largest energy expense. However, the impact of heating goes beyond energy bills—inefficient systems can strain resources, contribute to higher carbon emissions, and even affect indoor air quality (IAQ).

How HVAC Systems Influence IAQ and Efficiency

Heating, ventilation, and air conditioning (HVAC) systems are central to indoor comfort, but their efficiency depends on regular maintenance. Dirty or clogged air filters reduce airflow, forcing HVAC systems to work harder and use more energy. This inefficiency increases costs and leads to uneven heating, worsened air circulation, and reduced system lifespan.

Replacing filters is a simple yet effective solution. As noted by the U.S. Department of Energy, a clean filter can reduce energy consumption by 5-15% while also helping maintain better IAQ by trapping dust, allergens, and other pollutants.

Environmental Costs of Inefficient Heating Systems

Beyond individual households, inefficient heating systems have a broader environmental impact. Residential heating contributes significantly to greenhouse gas emissions, especially when fueled by natural gas, propane, or oil. Poorly maintained systems exacerbate this by burning more fuel to compensate for inefficiencies.

The materials used in air filters also contribute to environmental sustainability. Millions of disposable filters are sent to landfills annually, where they add to long-term waste. Many are made of non-recyclable synthetic materials, adding to the ecological burden. Opting for reusable or recyclable filter options helps reduce this waste while supporting a more sustainable heating system.

Practical Steps for Improving Heating Efficiency

To minimize environmental impact and reduce costs while maintaining good IAQ, consider these strategies:

Replace Filters Regularly: Dirty filters strain the system and reduce airflow, leading to inefficiencies.
Seal Ducts: Leaks in ductwork can cause up to 30% of heated air to be lost, forcing the HVAC system to work harder.
Upgrade Insulation: Properly insulating attics, walls, and floors helps retain heat and reduces the workload on heating systems.
Invest in Smart Thermostats: These devices allow precise temperature control, optimizing energy use during the day and night.
Schedule Routine Maintenance: Regular professional check-ups ensure that HVAC systems operate efficiently and identify potential issues early.

4. Sustainable Solutions for Winter IAQ

Improving indoor air quality (IAQ) during winter is a balancing act between maintaining health, reducing energy consumption, and minimizing environmental impact. The good news is that modern technologies and sustainable practices make achieving all three more accessible than ever.

Innovative IAQ Technologies

Emerging technologies offer smarter and more efficient ways to tackle winter IAQ challenges:

High-Efficiency Particulate Air (HEPA) Filters: These filters capture 99.97% of particles as small as 0.3 microns, including allergens, mold spores, and some bacteria, making them an excellent choice for improving air quality in homes with sensitive occupants.
Electromagnetic Filters: Unlike traditional pleated filters, electromagnetic filters use charged surfaces to attract and trap particles. This innovative design reduces resistance, allowing for better airflow and greater energy efficiency.
Energy Recovery Ventilators (ERVs): ERVs exchange stale indoor air for fresh outdoor air while retaining heat, ensuring good ventilation without sacrificing warmth. These systems are particularly effective in homes that prioritize energy conservation during winter.

The Role of Sustainability in IAQ Solutions

Sustainable solutions for IAQ extend beyond individual technologies. For instance, many traditional air filters are disposable and contribute significantly to landfill waste. By switching to reusable or recyclable filters, households and businesses can reduce their environmental impact while maintaining effective air filtration.

Additionally, some manufacturers are adopting circular economy practices, designing filters with biodegradable components or creating programs to refurbish and reuse old filters. These innovations align with the growing demand for eco-friendly solutions that minimize waste while addressing IAQ challenges.

Practical Steps for Adopting Sustainable IAQ Solutions

Incorporating sustainable IAQ practices doesn’t require a complete home or HVAC system overhaul. Small, intentional choices can make a significant difference:

Choose Long-Lasting Filters: Opt for high-efficiency or reusable filters that reduce waste and provide better performance.
Invest in Smart Ventilation Systems: Modern systems like ERVs automatically balance fresh air intake and heat retention, making them energy-efficient for maintaining IAQ during winter.
Reduce Source Pollutants: Limit the use of products that emit volatile organic compounds (VOCs), such as certain paints and cleaning agents.
Monitor IAQ in Real-Time: Smart air quality monitors provide data on pollutant levels, helping you identify and address issues proactively.

Winter brings its own unique challenges, but it’s also an opportunity to take control of your indoor environment. By addressing IAQ through sustainable choices, regular maintenance, and innovative technologies, you can create a home or workplace that supports well-being while minimizing environmental impact.

The air you breathe indoors this winter doesn’t just keep you warm—it’s a foundation for health, energy savings, and a greener future. Let’s make winter air work for everyone.

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