Ragweed now, cedar in December. What MERV rating actually helps allergies, why the highest is the wrong answer, and the humidity range to aim for in DFW.
North Texas has two allergy seasons that most of the country doesn’t: ragweed from late August through the first hard freeze, and mountain cedar from December into February — the one that sends people to urgent care convinced they have the flu. Both arrive when the house is closed up, which makes your HVAC system the thing deciding what you breathe indoors for months at a time.
The standard fiberglass filter that comes with most systems exists to protect the equipment, not you. It catches the debris that would foul the blower and coil, and pollen passes straight through it.
| Filter Rating | What It Catches | Notes |
|---|---|---|
| MERV 1–4 | Large debris only | Equipment protection, not air quality |
| MERV 8 | Most pollen, mold spores, dust | Sensible default for most DFW homes |
| MERV 11–13 | Fine particulate, pet dander, smoke | Best allergy result — if the system can take it |
| HEPA | Nearly everything | Needs a dedicated bypass unit, not a filter slot |
A denser filter restricts airflow. Dropping a MERV 13 into a system with a single undersized return — which describes a great many DFW homes — starves the blower, freezes the evaporator coil in summer, overheats the heat exchanger in winter, and drives up your bill in both.
The way to get high filtration safely is more filter area, not denser filter: a 4- or 5-inch media cabinet holds a MERV 11–13 across far more surface, so it filters better and restricts less than the 1-inch version of the same rating. If someone recommends jumping to MERV 13 in a 1-inch slot without looking at your return sizing, they’re giving you a problem instead of a solution.
Dust mites and mold need humidity above roughly 50% to thrive. North Texas swings hard: humid enough in spring and early summer to feed both, then dry enough in a January cold snap to crack sinuses and wood floors alike.
The target band is 35–50%. An oversized air conditioner is the most common reason a house can’t hold it in summer — it cools fast, shuts off before it dequmidifies, and leaves the house at 73°F and clammy. That’s a sizing problem showing up as an air quality problem.
Standalone plug-in ionizers and ozone generators are widely sold and poorly supported by evidence; ozone at concentrations that do anything to contaminants is a respiratory irritant in its own right. Duct cleaning is not an allergy treatment either — unless there’s visible contamination or you’ve just finished a renovation, sealing the ducts does far more for what you breathe than cleaning them.
December is when North Texas cedar peaks, and the useful work — return sealing, a properly sized media cabinet, a humidity check — is all better done in the fall.
Call 214-900-2533 or see our indoor air quality services. If your dust returns days after cleaning, start with leaky attic ducts.
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Kenny founded Absolute Comfort Mechanical after more than 20 years in the Dallas–Fort Worth HVAC trade, because he was tired of watching homeowners get sold equipment they didn't need. He still runs diagnostics himself and reviews the technical advice published on this site.
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