Written by:

April 12, 2024

Updated on April 11, 2024

Navigating the world of air filters can be a daunting task. With acronyms like MERV, MPR, FPR, and CADR floating around, it’s easy to get lost in the specifics. However, understanding these ratings is crucial for choosing the right air filter for your needs, whether you’re looking to improve indoor air quality at home or in the workplace. In this article, we’ll dive deep into each rating system, discuss their reliability, and explore how they relate to HEPA filters.

MERV: The Standard Bearer

The Minimum Efficiency Reporting Value (MERV) is the most widely recognized rating system. Developed by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), MERV scores range from 1 to 20, with higher numbers indicating a filter’s superior ability to capture smaller particles. This system provides a straightforward approach to comparing filters. However, the comprehensive nature of MERV ratings means they encompass a broad spectrum of filter types and sizes, which can be both a strength and a weakness.

Understanding MERV Scores

At the heart of MERV ratings is a simple premise: the higher the score, the finer the particles a filter can trap. For instance, filters rated MERV 13 or above are effective against bacteria, tobacco smoke, and auto emissions, making them ideal for those with allergies or respiratory issues.

Let’s explore what each range of MERV ratings brings to the table and how they can impact the air quality in different environments.

MERV 1-4: The Basics

Starting at the base, MERV 1-4 filters capture particles like dust, pollen, and carpet fibers. Predominantly found in residential HVAC systems, they serve as the first line of defense, keeping the air free from visible pollutants and protecting the HVAC system.

MERV 5-8: The Residential Upgrade

Moving up, MERV 5-8 filters offer a more refined level of filtration. These are capable of entrapping finer dust particles, mold spores, and pet dander, making them a suitable choice for homes looking to enhance their air quality. Beyond residential use, these filters are the workhorses in commercial and industrial settings, balancing efficiency with airflow.

MERV 9-12: Commercial Precision

As we venture into MERV 9-12 territory, the filters become more specialized. Rare in homes but a staple in certain commercial spaces and hospital labs, these filters can capture automotive emissions and larger bacteria, among other pollutants. Their precision makes them ideal for environments where air quality directly impacts operational success or health outcomes.

MERV 13-16: Healthcare Heroes

In the realm of MERV 13-16, we find filters designed for the most demanding environments, such as hospital surgical centers. These filters excel in capturing a broad spectrum of bacteria, smoke particles, and even fine droplets from sneezes. Areas where air purity is non-negotiable, such as smoking lounges, rely on this range of filters to maintain a safe and clean atmosphere.

MERV 17-20: The Pinnacle of Purification

At the apex of the MERV scale, filters rated between 17 and 20 are engineered for ultra-clean environments, such as pharmaceutical manufacturing clean rooms and facilities handling sensitive electronics. These filters can trap viruses, carbon dust, and the finest smoke particles, ensuring the highest level of air purity necessary for producing critical components and medications.

The Testing Process

To calculate a filter’s MERV rating, a series of tests are conducted under controlled conditions. These tests introduce particles of specified sizes into an air stream passing through the filter. The sizes of particles tested range broadly, from 0.3 to 10 microns, to cover a variety of common airborne contaminants, such as pollen, dust mites, mold spores, bacteria, smoke, and smog.

The filter’s efficiency is measured by comparing the concentration of particles before and after they pass through the filter. This efficiency is determined for each particle size range, and the filter’s performance in these tests directly informs its MERV rating.

Grading on a Curve

A key aspect of MERV ratings is their range from 1 to 20, with higher numbers indicating a filter’s capability to trap smaller particles. Specifically, the ratings are derived from the filter’s lowest performance across the tested particle sizes. For example, if a filter performs exceptionally well in capturing larger particles but less effectively for smaller particles, its MERV rating will reflect its lower efficiency score. This approach ensures that the MERV rating provides a conservative estimate of a filter’s performance, emphasizing its weakest point.

The Limits of MERV

Despite their utility, MERV ratings have limitations. They don’t account for the filter’s lifespan or performance over time. Additionally, higher-rated filters may restrict airflow in some HVAC systems, potentially leading to increased energy costs and strain on the system.

MPR and FPR: Brand-Specific Ratings

While MERV ratings are widely accepted, some retailers and manufacturers use their proprietary systems, such as MPR and FPR. These ratings aim to differentiate products in a crowded market, but they can confuse consumers accustomed to the MERV standard.

MPR: Focused on the Fine Particles

3M’s Microparticle Performance Rating (MPR) hones in on a filter’s ability to capture particles between 0.3 to 1 micron. While MPR provides valuable insight into a filter’s performance against microparticles, the lack of a standardized testing method across all filter types means comparisons can be somewhat apples to oranges.

FPR: Home Depot’s Answer

The Filter Performance Rating (FPR), exclusive to The Home Depot, employs a color-coded system to simplify filter selection. However, like MPR, FPR ratings are not directly comparable to MERV scores without a conversion chart, potentially leading to confusion.

CADR: Measuring Air Purifiers

The Clean Air Delivery Rate (CADR) diverges from the rest, applying exclusively to air purifiers. It quantifies the volume of filtered air, providing a direct measure of a purifier’s effectiveness in real-world conditions. However, CADR ratings can be misleading. They don’t measure a purifier’s ability to eliminate gasses or odors, and high CADR numbers might not equate to overall better air quality.

Understanding CADR

The Clean Air Delivery Rate (CADR) rating of an air purifier is determined through standardized tests performed by the Association of Home Appliance Manufacturers (AHAM). During these tests, the air purifier is placed in a sealed chamber that has been filled with a specific concentration of airborne particulate matter, including pollutants such as pollen, dust, smoke, and mold spores. The device operates at its highest setting, and the reduction of particulate matter in the chamber is monitored over time. The CADR rating is calculated based on the speed at which the purifier removes these particles from the air. This process involves considering the volume of air cleaned and the type of particles removed, providing a comparative measure of the purifier’s effectiveness. The higher the CADR rating, the more efficient the air purifier is at cleaning the air of the tested particles, indicating its performance level in environments of certain sizes.

CADR’s Limitations

While CADR provides valuable insights into an air purifier’s performance, it shouldn’t be the sole criteria for selection. Important considerations like the placement of the purifier, the size of the room, and how these factors align with the purifier’s capacity can significantly impact its effectiveness. Additionally, CADR ratings do not account for the importance of source control or the role of ventilation in achieving optimal indoor air quality, underscoring the need for a more comprehensive approach to air purification.

HEPA Filters: The Gold Standard

HEPA filters stand out for their exceptional ability to trap at least 99.97% of airborne particles down to 0.3 microns. This high efficiency makes them ideal for environments requiring stringent air quality control and for individuals with allergies or respiratory issues. However, superior filtration comes with potential downsides. HEPA filters can restrict airflow more than less dense filters, possibly leading to increased energy use and greater strain on HVAC systems. To maintain their high performance, HEPA filters also require regular replacement, adding to maintenance considerations.

Breathing Easier: A Summary

Choosing the right air filter involves balancing filtration efficiency, airflow, and system compatibility. While MERV provides a broad benchmark, MPR and FPR offer insights into a filter’s performance against smaller particles. CADR ratings, though valuable, should be viewed in context with other air quality strategies. Ultimately, whether opting for a standard MERV-rated filter or investing in a HEPA system, understanding these ratings ensures you can make the best decision for your health and comfort. While higher ratings generally indicate better filtration, the best filter for you is one that fits your specific needs and your HVAC system’s capabilities. By staying informed about air filter ratings and understanding their limitations, you can ensure a healthier indoor environment. Whether it’s reducing allergens with a MERV 13 filter, capturing ultrafine particles with a HEPA filter, or using an air purifier with a high CADR rating, the right choice can lead to clearer air and better breathing at home or in the workplace.

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Lead Editor (IAQ), Clinical BacteriologistJessica Gunoskey

Jessie is the lead editor for Air Koality, overseeing all IAQ guides to ensure accuracy and quality. She holds a degree in Molecular & Infectious Disease Biology from Washington College in addition to working for multiple top-rated universities, such as UConn and UPenn. Not only does she write and edit, but she also is a STEM tutor and microbiologist!

Expertises: microbiology, infectious disease, biology, air quality

Air Filter Ratings Explained: MERV, MPR, FPR, CADR