HEPA, MERV, or ULPA? The Ultimate Guide to School Air Filters

Imagine that you walk into your school’s nurse’s office on a crisp fall afternoon. The heating system has just kicked on, and there’s a faint musty smell in the air. It’s not just the scent of “old building.” It may be a sign that your HVAC system plays host to excessive airborne contaminants and dust.

For school facility managers, moments like this raise a red flag. Cold and flu season is coming soon, and your air filtration directly impacts student attendance, teacher sick days, and even compliance inspections.

When students and staff are on the line, the question cannot be “if” your air quality is important. Instead, the question should be, “How can I best support my facility with powerful, effective air filtration systems?” There are a wide variety of options available on the market, but we’re breaking down the most common school air filters so you can choose confidently.

How Do Air Filtration Systems Work?

Air filtration systems capture and remove airborne particles before they circulate through your school’s HVAC system and back into occupied spaces like classrooms, gymnasiums, nurses' offices, and other areas. 

The filters work by pulling air through a dense mat of fibers, where they trap particles with a combination of mechanisms: interception, inertial Impaction, and diffusion.

• Interception: Medium-sized particles, like pollen or some bacteria, follow the airflow through the filter’s fibers but pass close enough to a filter fiber that they touch it and stick.
  
  
• Inertial Impaction: Larger, heavier particles like dust and larger allergens can’t change direction as the air moves around fibers, so they collide directly with the fibers and get trapped.
  
  
• Diffusion: The smallest particles, including many viruses and fine aerosols, move unpredictably. This random movement increases the chances they’ll bump into a fiber and be captured.

Why Does Filter Type Matter in School Facilities?

Not all school air filters are designed to capture the same size particles or serve the same purpose. 

The ability of a filter to capture particles depends on its micron rating, which is a measure of particle size, and its efficiency at removing particles of that size from the air. For context, a human hair is about 70 microns wide. Many bacteria are 1–5 microns. Viruses can be as small as 0.02 microns.

A filter that’s perfectly adequate for trapping dust in a hallway might do little to stop viruses in a nurse’s office. The type you choose influences everything from the range of particles it can remove to how well your HVAC system maintains airflow to how often the filter will need to be replaced. 

Higher-efficiency air filtration systems can create more resistance, which may strain a system that isn’t designed for them, while lower-efficiency options can leave high-risk areas unprotected. So while it’s important to select a filter that is neither underperforming or over-efficient for your specific school facility’s needs, it can be difficult to understand the differences on the market.

What Is a MERV Filter? And Where It Works in Schools

MERV stands for Minimum Efficiency Reporting Value, a scale from 1 to 16 that measures how well a filter captures particles of different sizes. 

For most school settings, MERV filters in the 8–13 range are common because they strike a balance between cost, efficiency, and HVAC compatibility. They can capture dust, pollen, and some bacteria, making them a solid choice for high-traffic but lower-risk areas like hallways, offices, and gymnasiums.

The limitation? Even at the higher end of the MERV range, these filters struggle to consistently capture particles smaller than 1 micron. That means fine aerosols, wildfire smoke, and many viruses can pass right through. 

When comparing MERV vs HEPA, MERV is dependable for everyday cleanliness, but it isn’t a substitute for high-efficiency HEPA air filtration in spaces where infection control is critical.

ULPA Filters: Strong, but Impractical for Schools

ULPA, Ultra-Low Particulate Air, filters push filtration even further, removing 99.999% of particles as small as 0.12 microns. This is the level of protection used in cleanrooms, laboratories, and certain medical environments where even the tiniest contamination can compromise safety or results.

While impressive, ULPA filters are rarely practical for schools. They create significant airflow resistance, require more frequent maintenance, and cost more to operate. Unless your facility includes a specialized lab, ULPA is usually more filtration than a school HVAC system needs or can handle efficiently.

Why HEPA Is the Gold Standard for School Air Filtration

What is a HEPA filter? True HEPA filters are engineered to remove 99.97% of airborne particles down to 0.3 microns. That’s precise enough to capture most bacteria, mold spores, and viruses that MERV filters miss. The CDC, EPA, and ASHRAE all recommend HEPA air filtration in schools, especially in areas with vulnerable populations or higher transmission risks.

In a school setting, that means spaces like nurses’ offices, special education classrooms, music rooms, and any location with frequent occupant turnover. HEPA air filtration in these areas can help reduce illness spread, cut down on absenteeism, and improve overall air quality during high-risk seasons.

The Case for Portable HEPA Filtration in Schools

The tradeoff is that HEPA air filtration can create higher resistance in HVAC systems not designed for it, making full-system changes or replacements challenging and expensive.

If your current HVAC system can’t accommodate HEPA air filtration, or if budget constraints make a large-scale retrofit unrealistic, portable HEPA units like the HEPACART® Classic offer a budget-friendly and flexible solution.

These standalone school air filters can be moved wherever they’re needed most: in nurses’ offices during flu season, classrooms with high occupancy, or spaces undergoing construction that stir up dust.

Portable HEPA air filtration units provide immediate, targeted air quality improvement without overhauling your existing system. For many schools, they’re the fastest, most cost-effective way to bring HEPA-level protection exactly where it’s needed.

Comparing MERV, HEPA, and ULPA: What You Need to Know

When it comes to school air quality, not all filters play the same role. The right choice depends on where it will be used, what you need to capture, and how your HVAC system is built. 

Here’s how MERV, HEPA, and ULPA filters stack up against one another:

MERV Filters

• Scale: Rated 1–16 (higher = better particle capture)
• Best for: General-use areas like classrooms, libraries, and hallways
• Strengths: Affordable, works with most HVAC systems, captures dust, pollen, and some bacteria
• Limitations: Less effective against very small particles like viruses and wildfire smoke

HEPA Filters

• Qualification: True HEPA filters remove at least 99.97% of particles ≥0.3 microns
• Best for: High-risk areas like nurse’s offices, special education rooms, and music classrooms
• Strengths: Captures most bacteria, mold spores, and viruses; recommended by CDC, EPA, and ASHRAE
• Limitations: Can strain HVAC systems not designed for them. Portable units are a good workaround

ULPA Filters

• Scale: Removes at least 99.999% of particles ≥0.12 microns
• Best for: Cleanrooms and labs
• Limitations: Overbuilt for schools; costly, high maintenance, and airflow restrictions

For most schools, the smartest approach is a layered one. MERV filters in common areas for everyday air quality, with HEPA filtration in high-risk zones for infection control. ULPA is powerful, but its cost and maintenance demands keep it best suited for specialized labs. Matching each filter type to its ideal application ensures you’re protecting health without overcomplicating your school’s system or your budget.

Making the Right Filtration Choices for Healthier Schools

In your school environment, air filtration is, yes, a technical issue, but more than that, it’s directly tied to student health, attendance, and your facility running smoothly. MERV, HEPA, and ULPA filters each have a place, but knowing when and where to use them makes all the difference.

You don’t have to overhaul your entire HVAC setup to make a big impact. Even starting with one high-risk space, such as a nurse’s office or a special education classroom, can improve school air filters' performance right away. From there, you can expand your strategy as needs and resources allow.

With the right filtration plan, you’re protecting kids from missing class, helping teachers stay healthy, and giving parents peace of mind that your school is using the best air filter for schools to create a safe place to learn every day.

Need more detailed guidance? Download the Ultimate Guide to Indoor Air Quality and see how HEPACART® can help you match the right filtration to the right space in your school.