Understanding residential air quality and HVAC filtration requires cutting through dense engineering acronyms, proprietary marketing labels, and misleading performance metrics. This master glossary provides authoritative, technically precise definitions for the 22 most critical terms in residential air filtration.
Quick Term Directory
1. MERV (Minimum Efficiency Reporting Value)
Standard: ASHRAE Standard 52.2.
MERV is the universally recognized North American independent rating standard evaluating how effectively an air filter captures airborne particles ranging from 0.3 to 10.0 microns across a standardized test duct. The scale runs from 1 to 16 for residential/commercial filters (with ratings 17 to 20 reserved for cleanrooms and HEPA applications):
- MERV 1–4: Coarse fiberglass utility filters. Protects blower motor from carpet fibers and large hair; captures <20% of residential dust.
- MERV 8: Baseline residential standard. Captures >70% of mold spores and pollen (3.0 to 10.0 microns), but <20% of fine particles under 1.0 micron.
- MERV 11: Intermediate residential protection. Captures 65%+ of 1.0–3.0 micron pet dander and auto emissions.
- MERV 13: Premium residential balance. Captures 85%+ of 1.0–3.0 micron particles and 50%+ of sub-micron particles (0.3–1.0 microns), including wildfire smoke, bacteria, and virus-carrying respiratory droplets.
2. MPR (Microparticle Performance Rating)
Proprietary System: 3M Filtrete.
MPR is 3M's proprietary rating scale measuring a filter's ability to capture microscopic airborne particles between 0.3 and 1.0 microns in size (the particulate fraction most likely to penetrate deep into human lung alveoli). Unlike MERV, which weighs three separate particle size bins (E1, E2, E3), MPR focuses strictly on the smallest E1 fraction:
- MPR 300 / 600: Approximately equivalent to MERV 8.
- MPR 1000 / 1200: Approximately equivalent to MERV 11.
- MPR 1500 / 1900: Directly equivalent to MERV 12 and strong MERV 13.
- MPR 2200 / 2800: High-efficiency MERV 13 and MERV 14 capture.
3. FPR (Filter Performance Rating)
Proprietary System: The Home Depot (developed with independent testing labs).
FPR is a color-coded 1 to 10 point rating system used exclusively by The Home Depot to simplify consumer filter purchasing across brands like Honeywell and HDX:
- FPR 4–5 (Good - Green): Equivalent to MERV 8. Captures pollen, dust, and pet dander.
- FPR 6–7 (Better - Red): Equivalent to MERV 10–11. Captures smoke, smog, and mold spores.
- FPR 8–9 (Best - Blue): Equivalent to MERV 12. Captures bacteria and fine microscopic allergens.
- FPR 10 (Premium - Black): Equivalent to MERV 13. Captures aerosolized viruses and ultrafine smog.
4. CADR (Clean Air Delivery Rate)
Standard: ANSI/AHAM AC-1.
CADR is the gold-standard independent metric used by the Association of Home Appliance Manufacturers (AHAM) to evaluate standalone room air purifiers. Measured in Cubic Feet per Minute (CFM), CADR quantifies the volume of filtered air delivered free of three specific particulate contaminants:
- Smoke CADR (0.09 to 1.0 micron): Smallest, most hazardous combustion particles.
- Dust CADR (0.5 to 3.0 microns): Fine household airborne dust.
- Pollen CADR (5.0 to 11.0 microns): Large seasonal botanical allergens.
Rule of Thumb: Under AHAM's 2/3 rule, an air purifier's Smoke CADR should equal at least two-thirds of the room's floor area in square feet (assuming standard 8-foot ceilings) to deliver 4.8 air changes per hour.
5. ACH (Air Changes per Hour)
ACH defines how many times the total volume of air within a closed room or structure is completely cycled through a filtration device or ventilation system in 60 minutes:
ACH = (CFM × 60) ÷ (Length × Width × Ceiling Height in feet)
- 1 to 2 ACH: Typical residential background infiltration (insufficient for active allergy or illness reduction).
- 4 to 5 ACH: The medical benchmark for allergy, asthma, and airborne virus control in occupied rooms.
- 6 to 12 ACH: Healthcare isolation and clinical cleanroom standard.
6. TESP (Total External Static Pressure)
Units: Inches of water gauge (in. w.g.) or Pascals (Pa).
TESP is the total resistance to airflow exerted on a central HVAC blower by the ductwork, evaporator coil, and air filter. Residential furnaces and air handlers are typically engineered to operate within a design maximum of 0.50 in. w.g. (or up to 0.80 in. w.g. on modern high-efficiency variable-speed units). If an excessively dense or dirty air filter adds 0.35 in. w.g. to a system that already has 0.30 in. w.g. coil resistance, total static pressure exceeds safe engineering limits, causing blower motor overheating, reduced CFM, and potential heat exchanger shutdown or AC coil freeze-up.
7. True HEPA vs. HEPA-Type
True HEPA: A rigid manufacturing standard defined by the U.S. Department of Energy (DOE-STD-3020) requiring a continuous micro-glass fiber mat to capture at least 99.97% of airborne particles as small as 0.3 microns in diameter (the Most Penetrating Particle Size, or MPPS).
HEPA-Type / HEPA-Style: An unstandardized, non-certified marketing term used for lower-cost residential filters that typically achieve only 85% to 95% efficiency and leak around unsealed cardboard frames. Always demand certified True HEPA or EN 1822 H13 ratings.
8. VOCs (Volatile Organic Compounds)
VOCs are carbon-based chemical gases emitted at room temperature by household furnishings, paints, building materials, adhesives, cleaning products, and cooking appliances (e.g., formaldehyde, benzene, toluene, and acrolein). Mechanical HEPA and MERV filters cannot trap VOCs because gas molecules are thousands of times smaller than HEPA fiber pores. VOCs can only be removed through chemical adsorption using activated carbon, potassium permanganate, or zeolite.
9. NDIR (Non-Dispersive Infrared) CO2 Sensor
An optical spectroscopic sensor used to measure ambient carbon dioxide (CO2) concentrations in parts per million (ppm). NDIR sensors pass a specific wavelength of infrared light (4.26 microns) through an air sample chamber; because CO2 molecules absorb light at this exact band, the sensor calculates exact CO2 levels without chemical depletion or thermal drift, unlike low-cost metal oxide (MOX) estimate sensors.
10. PM Fractions: PM1.0, PM2.5, and PM10
Particulate matter classified by aerodynamic diameter in micrometers (microns):
- PM10 (<10 microns): Inhalable coarse dust, mold spores, and pollen. Trapped in nasal passages and upper airways.
- PM2.5 (<2.5 microns): Fine respirable particles from wildfire smoke, combustion, and vehicle exhaust. Penetrates deep into lung bronchioles.
- PM1.0 (<1.0 micron): Ultrafine particles, viruses, and smog aerosols. Capable of passing directly through lung tissue into the bloodstream.
11. Electret Media (Electrostatic Charge)
Non-woven synthetic polymer fibers (such as polypropylene) that carry permanent quasi-electrostatic dipole charges. Electret media captures sub-micron particles through electrostatic attraction (similar to dust clinging to a TV screen) without requiring tightly packed physical fibers, thereby achieving high MERV ratings with significantly lower initial static pressure drop.
12. Face Velocity
Units: Feet Per Minute (FPM).
The speed at which air travels through the perpendicular face area of a filter frame. Standard residential ASHRAE 52.2 filter testing is conducted at 295 to 300 FPM. Higher face velocity increases static pressure drop and can degrade filtration efficiency on electret media by shearing particles past electrostatic charges.
13. Initial Static Pressure Drop vs. Final Resistance
Initial Static Drop: The clean resistance of an unused filter out of the box when operating at nominal CFM (typically 0.10 to 0.25 in. w.g. for residential filters).
Final Resistance: The maximum allowable pressure drop across a loaded, dirty filter before replacement is mandatory (typically 0.50 in. w.g. for 1-inch filters or 1.0 in. w.g. for deep commercial media). Running a filter beyond its final resistance starves the furnace blower of cooling airflow.
14. Air Bypass
Unfiltered air that leaks around the perimeter frame of an air filter due to loose filter racks, bent grilles, or missing gaskets. Because air takes the path of least resistance, a 1/4-inch gap around a high-MERV filter can allow up to 25% of total circulating air to completely bypass the media, depositing dirt directly onto the evaporator coil.
15. Adsorption vs. Absorption
Adsorption (with a 'd'): The physical surface adhesion of gas or vapor molecules onto the internal pore walls of a microporous solid (such as activated carbon). Air purification carbon operates strictly through adsorption.
Absorption (with a 'b'): The dissolution of a fluid into the bulk volume of a liquid or solid (like water soaking into a sponge).
16. Deep Pleat Media (4-Inch to 5-Inch Filters)
Thick residential media cartridges designed for dedicated filter cabinets (e.g., Aprilaire 413 or Honeywell F100). By increasing pleat depth from 1 inch to 4 or 5 inches, media surface area expands from ~4 sq. ft. to over 28–32 sq. ft. This dramatic expansion lowers face velocity across the media fibers, allowing MERV 13 or 16 filtration at a lower static pressure drop than a 1-inch MERV 8 filter, while extending service intervals to 9–12 months.
17. CARB Compliance (Ozone Emission Limit)
Regulation enacted by the California Air Resources Board (CARB) requiring all portable indoor air cleaning devices sold in California to produce an ozone emission concentration of no more than 0.050 parts per million (ppm) under ANSI/UL Standard 867 or Standard 2998. Mechanical-only HEPA purifiers are inherently zero-ozone and exempt from testing, whereas ionizers, plasma generators, and PCO devices must undergo rigorous chamber certification.
18. AHRI Standard 680
The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) performance standard for residential central air cleaners. Standard 680 establishes certified testing procedures for dust holding capacity, initial resistance, and fractional particle efficiency under realistic HVAC airflow conditions.
19. PSC (Permanent Split Capacitor) Blower Motor
The traditional single-phase induction motor found in older and builder-grade furnaces. PSC motors operate at fixed rotational speeds (Low, Med, High). When high-resistance filters are installed on PSC blowers, the motor cannot compensate; CFM airflow drops precipitously, elevating temperature rise in heating mode and risking coil freeze in cooling mode.
20. ECM (Electronically Commutated Motor)
Modern brushless DC blower motors equipped with onboard microprocessors that modulate rotational speed to maintain constant CFM airflow regardless of static pressure variations. While ECM blowers protect home comfort when filters load, high static pressure forces ECM motors to ramp up wattage, substantially increasing electricity consumption and blower motor noise.
21. Water Panel / Evaporative Pad
The aluminum mesh or porous ceramic matrix used in whole-house bypass and fan-powered evaporative humidifiers (e.g., Aprilaire #35 or #10). Coated with a porous ceramic clay slurry to distribute water evenly across airflow, water panels collect mineral scale (calcium, magnesium) and must be replaced annually to prevent mold and water overflow.
22. Dust Holding Capacity (DHC)
Units: Grams (g) of standardized test dust.
The total mass of synthetic test dust an air filter can capture and retain before reaching its manufacturer-defined final static pressure limit. High-pleat-count and deep 4-inch filters feature 300% to 500% higher DHC than flat panel filters, explaining why they last four times longer before restricting HVAC airflow.