How Viruses Stay Airborne and How to Reduce Exposure>>>

By INDEX Editorial Team | Based on peer-reviewed research –

Introduction: Why Airborne Transmission Matters Now

You’ve probably heard the phrase “the air in here feels stuffy.” But when respiratory viruses are circulating in your community, that stale feeling may signal something more serious than poor ventilation — it may mean the air is carrying infectious particles.

We now know with confidence that many common respiratory viruses — including influenza, RSV, and SARS-CoV-2 (the virus that causes COVID-19) — can remain suspended in indoor air for extended periods. As the EPA states plainly: “Particles from an infected person can move throughout an entire room or indoor space. The particles can also linger in the air after a person has left the room — they can remain airborne for hours in some cases.”

This isn’t alarmism; it’s a well-documented physical reality. The question is: What can you actually do about it? This article breaks down the science of airborne virus behavior and presents a criteria-based framework for reducing exposure — whether you’re a homeowner, a facility manager, or someone who simply wants to breathe more safely indoors.

Part 1: The Science — How Respiratory Viruses Become and Stay Airborne

What “Airborne” Actually Means

When an infected person breathes, speaks, coughs, or sneezes, they release respiratory fluids in the form of droplets and aerosol particles. These particles exist across a wide size range — from large, visible droplets to microscopic aerosols far smaller than the width of a human hair.

The critical distinction is this:

• Large droplets (greater than 5–10 microns) fall to the ground within seconds to minutes, typically traveling no more than a few feet.
• Small aerosols (less than 5 microns) can remain suspended in the air for hours, travel throughout a room, and accumulate over time if not removed. [Update: modern physics indicates the “airborne threat” actually includes particles a bit larger than the traditional 5-micron cutoff.]

According to the comprehensive 2024 review by Marr and Samet in Environmental Health Perspectives (published by the National Institutes of Health), the scientific community has now reached a firm consensus: airborne transmission via small aerosols is a primary route for many respiratory viruses, not just a theoretical possibility.

Why Indoors Changes Everything

Outdoors, aerosol particles are rapidly diluted by the enormous volume of moving air. Indoors, the physics shift dramatically. Without sufficient ventilation, exhaled aerosol particles build up in the room — much like cigarette smoke accumulates in a sealed car.

The key factors that determine airborne virus concentration indoors include:

1. Ventilation rate — How much outdoor air is being brought in to dilute and remove particles.
2. Occupant density — How many people are sharing the air in a given space.
3. Activity level — Talking loudly, singing, or exercising generates significantly more aerosol particles than quiet breathing.
4. Duration of exposure — The longer you share air with an infected person, the greater your potential inhaled dose.
5. Filtration and air cleaning — Whether particles are physically removed or inactivated.

A landmark study published in Nature in 2025 found that indoor air quality was directly associated with classroom transmission rates, confirming that inadequate ventilation creates conditions where viruses thrive.

Real-World Evidence You Should Know About

Laboratory and observational studies have confirmed infectious virus in the air of hospital rooms, cars, and residences. One chamber study showed that increasing ventilation from roughly 1 to 6 air changes per hour (ACH), or running a portable HEPA air cleaner, resulted in measurably lower concentrations of airborne virus.

Perhaps most compelling: a study of elementary schools in Georgia found that COVID-19 incidence was 35% lower in schools that adopted ventilation strategies, and 48% lower in schools that improved both ventilation and filtration.

This isn’t a niche finding. The evidence across dozens of studies converges on the same conclusion: managing indoor air is one of the most powerful tools we have for reducing the spread of airborne viruses.

Part 2: The Exposure Reduction Framework — 4 Evidence-Based Strategies

Rather than chasing quick fixes, INDEX recommends a layered approach based on the EPA’s Clean Air in Buildings Challenge and CDC ventilation guidance. No single measure is sufficient on its own, but together they create meaningful risk reduction.

Strategy 1: Increase Ventilation with Outdoor Air

This is the most fundamental intervention. Bringing in outdoor air dilutes the concentration of virus particles indoors.

What to do at home:

• Open windows and doors when weather and outdoor air quality permit (check AirNow.gov for local conditions).
• Run bathroom and kitchen exhaust fans that vent outdoors.
• If you have a window air conditioner, open the vent control to bring in outside air.
• Operate ceiling fans to improve air mixing.

What to do in commercial or institutional settings:

• Ensure HVAC systems are delivering the maximum amount of outdoor air the system can handle.
• ASHRAE Standard 241 recommends a minimum of 10 liters per second per person for infection control — a higher standard than typical comfort-based ventilation.
• The CDC now recommends a target of 5 equivalent air changes per hour (ACH) to reduce disease transmission.
• Consider consulting an HVAC professional to evaluate your system’s actual delivered ventilation rate.

Strategy 2: Improve Air Filtration

Ventilation alone may not be sufficient, especially in older buildings or during extreme weather when windows must remain closed. Filtration provides a second line of defense.

What to look for in a filtration solution:

Independent data suggests that effective filtration requires attention to several criteria:

• Filter efficiency: Look for filters rated MERV 13 or higher. A properly installed HEPA filter captures at least 99.97% of particles at 0.3 microns — and even higher efficiency for smaller and larger particles.
• Clean Air Delivery Rate (CADR): For portable air cleaners, choose a unit with a CADR appropriate for the room size. EPA recommends a CADR of at least two-thirds of the room’s square footage.
• System compatibility: For HVAC upgrades, ensure the system fan can handle the pressure drop of a MERV 13 filter. Not all residential systems are designed for higher-efficiency filters.
• Device placement: Portable air cleaners work best when placed in the room where people spend the most time, away from walls and obstructions.

Important consideration: Bipolar ionization and photocatalytic oxidation devices have been marketed as air cleaning solutions, but large-scale evaluations by the EPA have found their real-world performance significantly below marketing claims. A 99% reduction in a lab chamber after 60 minutes translates to a CADR of only 22 CFM — roughly one-tenth that of a typical portable HEPA air cleaner.

Strategy 3: Control Humidity

Research shows that humidity levels between 40% and 60% can reduce the infectivity of some airborne viruses. Both very dry air (common in heated winter spaces) and very humid air may create conditions more favorable to virus survival.

What independent data suggests:

• Use a hygrometer to monitor indoor relative humidity.
• In dry conditions, a humidifier can help maintain the 40–60% target range.
• In humid conditions, dehumidification and proper HVAC operation are key.

Strategy 4: Source Control and Occupant Management

Reducing the amount of virus entering the air in the first place is the most direct intervention.

Practical measures include:

• Staying home when sick and isolating from household members when possible.
• Wearing well-fitted masks or respirators during periods of high community transmission.
• Reducing occupancy in enclosed spaces — moving activities outdoors when feasible.
• Shortening the duration of indoor gatherings.

As the CDC notes, ventilation interventions reduce transmission risk but “will not eliminate risk completely,” especially for people in very close (face-to-face) contact.

Part 3: Practical Pathways — What to Implement Based on Your Situation

For Homeowners and Families

The most cost-effective steps you can take right now require no purchase at all: open windows, run exhaust fans, and use ceiling fans to improve air circulation. If your budget allows, adding a portable HEPA air cleaner to the most-used room (especially the bedroom or living area) provides an additional layer of protection.

A note on “natural ventilation”: Simply opening windows is surprisingly effective. A classroom study found that opening windows in naturally ventilated rooms produced some of the highest air change rates observed across all building types.

For Facility Managers and Building Operators

The COVID-19 pandemic highlighted a troubling reality: we lack data on ventilation in the vast majority of the nation’s more than 6 million commercial buildings and 140 million dwellings. The Government Accountability Office estimates that 41% of school districts need to update or replace HVAC systems in at least half of their schools.

Priority actions based on peer-reviewed evidence:

1. Commission a professional ventilation audit of your building.
2. Upgrade HVAC filters to MERV 13 or the highest your system can accommodate.
3. Operate HVAC fans continuously (the “On” setting rather than “Auto”).
4. Consider installing portable HEPA air cleaners in high-occupancy spaces like conference rooms, break rooms, and classrooms.
5. Use carbon dioxide (CO₂) monitoring as a screening tool — while CO₂ is not a direct measure of infection risk, sustained readings above 800–1,000 ppm often indicate inadequate ventilation relative to occupancy.

For Those Considering Technology Upgrades

Beyond ventilation and filtration, emerging technologies offer additional options. Far-UVC light (222 nm) has shown promise in laboratory studies for inactivating airborne pathogens while minimizing risks to human skin and eyes. However, the 2024 National Academies workshop noted that more research is needed on long-term health effects and potential formation of secondary air pollutants like ozone from some UV systems.

The principle to follow: Prioritize established, tested technologies (ventilation, MERV 13+ filtration, HEPA air cleaners) before considering newer, less-studied solutions.

Assess Your Indoor Air Quality Risk

How confident are you that the air in your home or building is working in your favor? Most people don’t know until they measure.

Take our free IAQ Risk Calculator — a science-based tool that evaluates your space across key risk factors including ventilation adequacy, filtration status, occupancy patterns, and humidity control. Get a personalized risk score in under five minutes.

→ Take the IAQ Risk Calculator Now

Your results are based on criteria developed from peer-reviewed research and EPA/CDC guidelines. No purchase necessary.

Criteria-Based Product Considerations

If you’re considering an air cleaner or HVAC upgrade, independent data suggests evaluating options against these criteria:

8 things to consider when evaluating an air cleaner for virus protection:

1. CADR rating — Look for a clean air delivery rate appropriate for your room size (at least two-thirds of the room’s square footage).
2. Filter type — True HEPA or MERV 13+ for particle removal.
3. Noise level — Consider whether the unit is quiet enough for continuous operation in a bedroom or office.
4. Energy use — Continuous operation is important; check energy consumption.
5. Ozone emissions — Avoid devices that intentionally generate ozone. The California Air Resources Board (CARB) certifies devices that meet ozone limits.
6. Filter replacement cost — Factor in ongoing costs — a cheap unit with expensive replacement filters may cost more over time.
7. Room size matching — Oversizing is generally fine; undersizing defeats the purpose.
8. Third-party testing — Look for verification from AHAM (Association of Home Appliance Manufacturers) or similar independent bodies.

Practical options that meet these criteria include portable HEPA air cleaners from established manufacturers with independently verified CADR ratings and CARB certification. For whole-building solutions, consult an HVAC professional about upgrading to MERV 13 filters or higher, provided your system can accommodate the increased static pressure.

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The Bottom Line

The science is clear: respiratory viruses can remain airborne indoors for extended periods, and the concentration of virus in the air you breathe is something you can influence. The tools exist — ventilation, filtration, humidity control, and source reduction — and they are backed by decades of research that predates and extends beyond the COVID-19 pandemic.

The biggest barrier to reducing airborne transmission is no longer a lack of scientific knowledge but rather a gap between what we know and what we implement. Ventilation rates in the vast majority of buildings remain unmeasured and undocumented. HVAC systems go unmaintained. Portable air cleaners are underused.

The opportunity is substantial: improved indoor air management doesn’t just reduce virus transmission — it also lowers rates of asthma attacks, improves cognitive function, and reduces absenteeism from school and work.

The question is not whether airborne viruses are a threat. The question is whether we choose to do something about the air we share.

References

• EPA. Indoor Air and Coronavirus (COVID-19). Updated February 17, 2026.
• EPA. Ventilation and Respiratory Viruses. Updated September 8, 2025.
• CDC/NIOSH. Ventilation Mitigation Strategies. October 3, 2024.
• Marr LC, Samet JM. Reducing Transmission of Airborne Respiratory Pathogens: A New Beginning as the COVID-19 Emergency Ends. Environ Health Perspect. 2024;132(5):055001.
• ASHRAE Standard 241. Control of Infectious Aerosols. 2023.
• Gettings J, et al. Mask Use and Ventilation Improvements to Reduce COVID-19 Incidence in Elementary Schools — Georgia. MMWR. 2021;70(21):779–784.
• Buonanno G, et al. Increasing Ventilation Reduces SARS-CoV-2 Airborne Transmission in Schools. Front Public Health. 2022;10:1087087.

By the INDEX Editorial Team. This article is based on peer-reviewed research and guidance from EPA, CDC, NIOSH, and ASHRAE. INDEX is a 501(c)(3) nonprofit dedicated to improving indoor environmental health through independent, science-based information.