MERV, HEPA, UV: what does each actually do?
MERV, HEPA, and UV are three different indoor-air tools that marketing loves to blur: MERV rates how well a filter captures particles; true HEPA is a capture standard so fine (99.97% at 0.3 microns) it needs purpose-built equipment; and UV lights don't capture anything — they inhibit biological growth on wet coils. Matching the technology to your actual complaint — dust, smoke, must, humidity — is the whole game, not buying the highest number on the shelf.
The quick answer
- MERV = a filter's particle-capture rating (residential range: 8–13)
- HEPA = 99.97% capture at 0.3 microns — needs dedicated equipment, not a furnace-filter slot
- UV lights target mold and biological growth, mainly on the indoor coil — not dust, smoke, or dander
- First move is diagnosis: dust, allergies, smoke, odors, and humidity each point to different tools
Match the tool to the complaint
Dust and pet dander: filtration — a better media filter usually moves the needle most per dollar. Allergies and asthma: higher-capture filtration, possibly a HEPA layer for bedrooms. Wildfire smoke: media depth plus room HEPA (see the smoke playbook). Musty smells or a damp coil smell when the AC starts: that's biological territory where UV on the coil earns its keep. Persistent humidity problems: that's a moisture conversation, not a filter one.
What doesn't work: buying technology at random because a duct-cleaning ad was scary. Start from the symptom, and be suspicious of any pitch where one gadget cures everything.
In Seattle's smoke seasons, homeowners often arrive with a MERV 13 1-inch filter that collapsed their airflow — then wonder why the house feels stuffy. Depth (a 4–5-inch media cabinet) beats density in a 1-inch slot almost every time in our housing stock.
MERV, honestly
MERV ratings run 1–16; residential systems live between 8 (basic dust) and 13 (fine particles including much smoke and many allergens). Higher capture means denser media, which resists airflow — the practical ceiling for your system depends on the blower and the filter's surface area. This is why a deep-pleat 4–5-inch media cabinet at MERV 11–13 usually beats a 1-inch filter of the same rating: vastly more surface area, far less restriction, changed twice a year.
HEPA, honestly
True HEPA captures 99.97% of 0.3-micron particles — but that density can't sit in a normal furnace-filter slot; your blower would choke. Whole-home HEPA runs as a bypass unit with its own fan, or you deploy portable HEPA machines room by room (which is what most households actually need). Beware 'HEPA-like' and 'HEPA-type' labels — those are marketing, not the standard.
UV, honestly
The well-supported residential use is a coil-mounted UV lamp: the indoor coil is cold, wet, and dark all summer — ideal for biological growth that fouls performance and smells musty. UV keeps the coil clean, which protects airflow, efficiency, and odor. In-duct 'air-sterilizing' claims are weaker science: air moves past a lamp in a fraction of a second, and dose = intensity × time. We'll be straight with you about which UV claims hold up.
In Puget Sound's damp shoulder seasons, coil fouling happens even when the AC runs modest hours — must when the system first starts in spring is often a biological film on the coil, not 'mystery mold in the ducts.' UV on the coil targets that specific failure mode.
Where IAQ money gets wasted
Stacking gadgets on an undiagnosed problem — a $900 gizmo can't fix what a $40 humidity meter would have identified as a bathroom-fan problem. Jamming max-MERV 1-inch filters into an unmeasured system and quietly strangling airflow. Paying for 'HEPA' that isn't. And replacing filtration thinking when the real issue is source control: the crawl space, the unvented bathroom, the shoes-in-the-house dog.
IAQ ozone generators deserve a special mention: ozone is a lung irritant. Nothing that intentionally produces ozone belongs in an occupied home.
Before smoke season, confirm your blower can handle the filter you plan to run — a static-pressure check during a tune-up beats discovering airflow collapse the week the AQI hits purple. That's a standard part of honest filter advice in our market, not an upsell.
Questions Seattle homeowners ask
Is a MERV 13 filter always better than MERV 8?
Better at capture, yes; better for your system, only if the blower and filter geometry can maintain airflow through it. Restricted airflow costs efficiency, comfort, and eventually parts. The honest sequence: check what your system can take, then buy the highest capture within that limit — or add a media cabinet and stop compromising. Smoke season is when Puget Sound homeowners discover their blower can't pull through a maxed 1-inch filter.
Do UV lights kill viruses in my home's air?
In-duct air passes a lamp too fast for a reliable dose, so treat air-sterilization claims skeptically. Where UV demonstrably helps is the coil: keeping that cold, wet surface free of biological growth — a real benefit to airflow, efficiency, and that musty-AC smell when you first start cooling in a damp Seattle spring.
What actually helps allergies most?
Usually: upgrade central filtration to the best your system supports (ideally a deep media cabinet at MERV 11–13), add a true-HEPA portable in the bedroom, change filters on schedule, and control sources (humidity, bedding, pets' zones). That stack beats any single silver-bullet device — and in Puget Sound's long indoor season, source control (bathroom exhaust, crawl-space moisture) matters as much as the filter rating.
Do I need duct cleaning?
Sometimes — after construction dust, rodent issues, or visible growth. As a routine annual upsell, the evidence is thin: filtration and coil hygiene matter more, and a scare-ad quoting one price on the phone and another in your hallway is a known pattern. Ask for photos and a specific reason. In Greater Seattle's mid-century stock, duct sealing often fixes the dust problem cleaning was supposed to solve — by stopping crawl-space air from being pulled in after the filter.
Continue exploring
How we know this
Capture standards per EPA and ASHRAE filtration guidance; UV assessment reflects published residential evidence and Glendale's service experience with coil-mounted installations across Greater Seattle.
Reviewed by the Glendale Heating & Air Conditioning team · Last updated 2026-07-15
Ready for comfort you can count on?
Whether it's a furnace that quit on a cold morning or a whole new system, the Glendale team is ready. Honest pricing, expert work, and 87+ years behind every visit.
Hours & Emergency
- Mon–Sat: 8:00 AM – 7:00 PM
- Sun: Emergency On-Call Only
- Emergency line: (206) 243-7700