"Indoor air quality" gets used as a vague wellness phrase, but it's a measurable, physical subject — and the numbers are worse than intuition suggests. The EPA's research finds indoor concentrations of many pollutants routinely run 2–5 times higher than outdoor levels — in a country where people spend roughly 90% of their time indoors. Here's the science, in plain English, and where your ductwork fits.
Factor one: particulate matter
Airborne particles are classified by size, and size is destiny. PM10 (10 micrometers and under — pollen, most dust) gets caught mostly in the nose and throat. PM2.5 (2.5 micrometers and under — combustion particles, smoke, fine dust) is the consequential class: small enough to reach deep lung tissue, per the EPA's PM primer. Indoors, cooking is a major PM2.5 source, along with candles, fireplaces, and smoke that infiltrates from outside — western Pennsylvania's industrial corridor and temperature inversions add a regional baseline on top.
Factor two: volatile organic compounds
VOCs are carbon-based gases that evaporate at room temperature from paints, adhesives, new furnishings, cleaning products, and air fresheners — that "new carpet smell" is off-gassing. The EPA finds indoor VOC levels consistently several times outdoor levels, spiking during and after renovation. VOCs are gases, so no filter or duct cleaning touches them directly — dilution through ventilation and source control are the levers — but renovation work also sheds enormous particle loads into ductwork, which is why post-renovation homes are one of the clearest cleaning cases.
Factor three: bioaerosols
The biological fraction — mold spores, bacteria, dust-mite debris, pollen, dander — behaves differently from inert dust because it can amplify: give mold a damp surface and a food source and it manufactures new spores continuously. Ductwork matters doubly here. It's a reservoir (debris layers hold allergens) and, when moisture is present, a growth site with a built-in distribution system. Households managing allergies or asthma should read our allergy & asthma guide for the applied version of this section.
Factor four: humidity, the master variable
Relative humidity governs how the other three factors behave. Below ~30%, airways dry out and particles stay airborne longer; above ~50%, dust mites flourish and condensation feeds mold — the 30–50% band is the standard target, consistent with ASHRAE's IAQ guidance. Around here, humid summers and damp basements push homes toward the high side; basement dehumidification through summer is one of the highest-leverage IAQ moves a local homeowner can make.
Where ductwork fits: reservoir plus distribution
Your duct system moves the whole house's air through itself several times an hour, which gives it a unique double role: everything airborne eventually deposits inside it, and everything inside it eventually goes airborne again. That's why duct condition amplifies — or quietly undermines — every other IAQ investment you make. Cleaning resets the reservoir to zero; filtration and humidity control keep it there. To see where your own home stands, the IAQ Score quiz turns these factors into a 0–100 rating in about a minute, and the energy waste calculator shows the efficiency side of the same airflow story.
Frequently asked questions
Why is indoor air often more polluted than outdoor air?
Concentration and confinement. Homes continuously generate particles and gases — cooking, cleaning products, furnishings, pets, people — inside a sealed envelope with limited fresh-air exchange. Modern, tightly built homes trade energy efficiency for air exchange, which lets concentrations build.
What is PM2.5 and why does it get so much attention?
Particulate matter 2.5 micrometers and smaller — about 1/30th the width of a hair. At that size, particles bypass the nose and throat's defenses and reach deep lung tissue, and the smallest can enter the bloodstream. It's the size class most consistently linked to health effects in EPA research.
Can duct cleaning alone fix poor indoor air quality?
No single measure can — IAQ is a system with four levers: source removal, ventilation, filtration, and humidity control. Duct cleaning is the source-removal lever for the distribution system itself; it makes the other three levers work on clean infrastructure instead of fighting a contaminated one.