Air quality science

What the research says about the air in your home

The air inside a North Texas home is shaped by four things: what gets in, what gets made indoors, what the HVAC system captures, and how much moisture it holds. Here is what the research says about each, and where professional service helps and where it doesn’t.

~90%

of our time is spent indoors, where some pollutants run 2–5× higher than outdoors.

U.S. EPA

25.7%

of U.S. adults have a diagnosed seasonal allergy.

CDC National Center for Health Statistics

MERV 13

is the minimum filter rating EPA recommends for central systems, where the system can accommodate it.

U.S. EPA

35–50%

relative humidity is the allergist-recommended range to limit dust mite growth.

AAAAI/ACAAI Practice Parameter: Dust Mite Exposure Control (2013)

Animated guide, about 3 minutes

How a central air system works

Press play, or step through the chapters. Each stage lights up the parts involved.

Transcript and video

Overview

How a central air system works

A split-system air conditioner moves heat; it doesn’t make cold. In ten short chapters, here is the path the air and the refrigerant take through a typical North Texas home.

Why indoor air matters

EPA estimates that Americans spend about 90% of their time indoors, where concentrations of some pollutants are often two to five times higher than typical outdoor levels. The most susceptible people, including young children, older adults and those with heart or lung disease, tend to spend even more time inside.

Indoor particles come from outside (pollen, ozone-driven smog, construction dust) and from daily life (cooking, pets, candles, cleaning products, and moisture that feeds mold). Because a home recirculates the same air thousands of times a year, the HVAC system acts as the home’s lungs: it either captures particles or keeps moving them.

Sources: U.S. EPA, Report on the Environment: Indoor Air Quality

Allergens: what they are and where they hide

About one in four U.S. adults has a diagnosed seasonal allergy. The common indoor triggers are dust mite allergen, pet dander, mold spores and pollen carried in from outside. They differ greatly in size, which decides how long they stay airborne and which filter catches them.

Larger particles such as pollen and dust mite allergen settle within minutes and live in bedding, carpet and upholstery, so source control and HEPA vacuuming matter most. Smaller particles such as pet dander, mold fragments and smoke stay suspended for hours and are what HVAC filtration and air cleaners remove best.

Particle sizes shown as typical ranges. Larger particles settle quickly into bedding and carpet; smaller ones stay airborne for hours, which is where filtration helps most. Sources: EPA, AAAAI.

Sources: CDC National Center for Health Statistics, Data Brief 460 (2023) · AAAAI/ACAAI Practice Parameter: Dust Mite Exposure Control (2013) · U.S. EPA, Guide to Air Cleaners in the Home

Filtration: what physicians and engineers recommend

Filter efficiency is rated by MERV under ASHRAE Standard 52.2. EPA recommends a filter rated at least MERV 13, or as high as the system fan and filter slot can accommodate; MERV 13 filters must remove at least 50% of the smallest particles tested. The American College of Allergy, Asthma & Immunology recommends MERV 11 to 13 for whole-home filtration and HEPA units for individual rooms.

The clinical evidence is strongest for filtration as part of a combined plan. In a two-year controlled study of 937 inner-city children with asthma cited by the AAAAI Indoor Allergen Committee, an environmental intervention that included bedroom HEPA filtration reduced asthma symptoms and bedroom allergen levels. A 2024 meta-analysis in Indoor Air reports that HEPA filtration can lower allergen concentrations by roughly 65–90%, with airflow and timely filter changes as important as the rating itself.

One practical caution from engineering research: residential system fans typically run less than 20% of the hours in a year, so a filter only cleans air while the fan runs. That is why EPA pairs MERV 13 with frequent changes, and why we check the filter slot and the system’s airflow before recommending a higher-rated filter, so the upgrade doesn’t starve the system of air.

Minimum particle capture by filter rating
Filter	0.3–1 µm smoke, bacteria, fine dander	1–3 µm dander, mold fragments	3–10 µm mold spores, dust
MERV 8	not rated	≥ 20%	≥ 70%
MERV 11	≥ 20%	≥ 65%	≥ 85%
MERV 13 EPA’s minimum recommendation	≥ 50%	≥ 85%	≥ 90%
HEPA	99.97%	99.97%+	99.97%+

Minimum removal efficiency by ASHRAE Standard 52.2 test range. HEPA is tested at 0.3 µm and needs a dedicated unit; most home systems can’t push air through a true HEPA filter. Sources: EPA, ACAAI, ASHRAE.

Sources: U.S. EPA, Guide to Air Cleaners in the Home · American College of Allergy, Asthma & Immunology, Air Filters · AAAAI Indoor Allergen Committee, “Air filters and air cleaners,” J Allergy Clin Immunol, 2010 · Shih et al., “Effectiveness of Air Filters in Allergic Rhinitis: Systematic Review and Meta-Analysis,” Indoor Air, 2024 · Harriman, Stephens & Brennan, “New Guidance for Residential Air Cleaners,” ASHRAE Journal, Sept. 2019

Humidity, mold and dust mites

Allergists recommend keeping indoor relative humidity between 35% and 50% to limit dust mite growth, a strong recommendation in the AAAAI/ACAAI practice parameter. EPA’s guidance on mold is just as direct: the key to mold control is moisture control.

In North Texas, air conditioners do most of the dehumidifying. A dirty evaporator coil, a clogged condensate drain or an oversized system that short-cycles can leave a home cool but damp, which is when musty odors, coil biofilm and mold growth appear.

Relative humidity target from the AAAAI/ACAAI dust mite practice parameter. EPA: the key to mold control is moisture control.

Sources: AAAAI/ACAAI Practice Parameter: Dust Mite Exposure Control (2013) · U.S. EPA, Mold and Moisture

UV-C light: what it does, and what it doesn’t

Ultraviolet germicidal irradiation (UV-C, typically 254 nm) inactivates bacteria, viruses and mold by disrupting their DNA. In HVAC systems its best-documented use is keeping the wet cooling coil free of biofilm. ASHRAE’s handbook notes that a clean coil has lower air-side pressure drop and better heat transfer, which can reduce fan and cooling energy.

ASHRAE-funded field research in a hot, humid climate measured improvements of about 15% in both heat transfer and airflow resistance after coil irradiation, and found the lamps did not remove existing buildup, so the coil should be cleaned first. Oak Ridge National Laboratory’s 2022 review describes mixed results overall but agrees the application makes the most sense in humid climates where coils stay wet, which describes a Dallas summer.

UV-C is a complement to filtration and coil cleaning, not a replacement. Lamps must be ozone-free, sized and aimed at the coil, and replaced on schedule; EPA warns against devices that intentionally produce ozone.

UV-C works best on the coil, a continuously wet surface where biofilm forms. Clean the coil first; use ozone-free lamps. Sources: ASHRAE Handbook, ASHRAE RP-1738.

Sources: ASHRAE Handbook—HVAC Applications, Ch. 62: Ultraviolet Air and Surface Treatment · ASHRAE RP-1738, UVC cooling coil irradiation field study (Bahnfleth & Firrantello) · Oak Ridge National Laboratory, UVGI for HVAC: Literature Review (2022) · U.S. EPA, Ozone Generators That Are Sold as Air Cleaners

Duct cleaning: the honest pros and cons

EPA’s position is that duct cleaning has never been shown to prevent health problems, that a light amount of household dust in ducts poses no known risk, and that ducts should be cleaned as needed rather than routinely. A 2010 scientific review in Indoor Air reached a similar conclusion and noted that poorly controlled cleaning can temporarily raise indoor particle levels.

Where the evidence supports cleaning is specific: visible mold growth, rodent or insect contamination, excessive debris released from registers, and post-construction dust. Cleaning the components inside the air handler matters most for performance. In an EPA–NADCA field study of nine homes, source-removal cleaning effectively removed particulate and fiber contamination, and the estimated heat removed by the cooling coil rose 14% and 23% in the two homes measured.

Where cleaning helps

Removes visible mold, rodent and insect contamination at the source (an EPA-listed reason to clean)
Clears construction dust after remodels and new builds
Coil and blower cleaning can restore airflow and cooling capacity
Before-and-after photos document what was actually in the system

What it won’t do

Not shown to prevent health problems on its own (EPA)
Cleaning only the ducts, without the coil and blower, shows little efficiency benefit
Uncontained cleaning can stir particles into the home; negative pressure and HEPA collection prevent that
Routine chemical biocides and ozone treatments are not recommended

That is why we inspect and photograph first, clean the coil and blower along with the ducts, keep the system under negative pressure with HEPA collection, and tell you when cleaning can wait.

Sources: U.S. EPA, Should You Have the Air Ducts in Your Home Cleaned? · Zuraimi, “Is ventilation duct cleaning useful? A review of the scientific evidence,” Indoor Air, 2010 · U.S. EPA / NADCA pilot field study of residential HVAC cleaning (9 homes) · NADCA, ACR: The NADCA Standard, 2025 Edition

North Texas specifics

Dallas–Fort Worth is designated a severe nonattainment area for the 2008 federal ozone standard, so outdoor smog days are part of life here, and they reach indoors whenever windows open or a home leaks air. Pollen arrives in waves: mountain cedar in winter, trees in spring, grasses through summer and ragweed into fall, with mold spores rising after rain.

That calendar is why we recommend filter upgrades before spring, coil and drain service before summer, and dryer vent and chimney service before winter.

A typical North Texas pattern. Exact timing shifts with weather; check daily counts in season.

Sources: Texas Commission on Environmental Quality, DFW Attainment Status · CDC National Center for Health Statistics, Data Brief 460 (2023)

What this means for your home

Keep humidity between 35% and 50%; a clean coil and clear drain are what make that possible in summer.
Use the highest-rated filter your system handles (MERV 13 if it fits) and change it on schedule.
Consider UV-C at the coil in our humid summers, after a coil cleaning.
Clean ducts when there’s a reason: mold, pests, construction dust or debris at the registers.
Avoid ozone generators and routine chemical treatments.

We’ll measure, photograph and explain what your system actually needs, and tell you when something can wait.

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Questions about air quality

Will air duct cleaning help my allergies?

Not by itself. EPA states duct cleaning has never been shown to prevent health problems. What helps most, according to allergist guidance, is a combined plan: source control, humidity at 35–50%, high-efficiency filtration (MERV 11–13 whole-home, HEPA in bedrooms) and a clean coil and drain. Duct cleaning is warranted when there is visible mold, pests or excessive debris, and we’ll show you photos either way.

What filter should I use?

EPA recommends at least MERV 13, or the highest rating your system can accommodate. Most systems handle MERV 13 if it’s changed on schedule. Because a denser filter can restrict airflow, we check your system before recommending one and set a change schedule for your home: often every 1–3 months for a 1-inch filter.

Are UV lights worth it?

For the coil, often yes in Dallas. ASHRAE research shows UV-C keeps wet cooling coils free of biofilm, which helps airflow and heat transfer, and the benefit is greatest in hot, humid climates. The coil should be cleaned first, the lamp must be ozone-free, and UV complements filtration rather than replacing it.

What humidity should my home be?

Between 35% and 50% relative humidity, per allergist guidance for dust mites; EPA ties mold control directly to moisture control. An inexpensive hygrometer will tell you where you are. Persistent readings above 50% in summer usually point to an AC, drainage or airflow problem worth checking.

Are ozone air purifiers safe?

EPA advises against ozone generators sold as air cleaners. Ozone is a lung irritant at concentrations these devices can produce. Choose filtration, ventilation and ozone-free UV instead.

How does DFW’s outdoor air affect my home?

The 10-county DFW area is a severe nonattainment area for the 2008 ozone standard, and pollen seasons run nearly year-round. Outdoor air enters through leaks, doors and ventilation, which is why sealing, filtration and well-timed maintenance matter here.

References

This page summarizes public research for general education. It isn’t medical advice; talk with your physician or allergist about symptoms.