How to Assess Your Home's Indoor Air Quality: Where to Start

Matching symptoms to root causes

Musty odor is almost always a moisture problem. In Maryland homes, the most common sources are biological growth on the evaporator coil (the coil sits wet between cycles in a humid climate), standing water in the condensate drain pan, ductwork that passes through unconditioned humid crawl spaces or attics, or return air drawing from a damp basement. The fix is identifying the moisture source — not a better filter or an air purifier.

Visible dust despite a clean filter points to duct leakage or insufficient HVAC runtime. When ductwork in an attic or crawl space leaks, it pulls unconditioned outdoor air — with insulation fibers, outdoor particulates, and whatever is in the attic — directly into the supply air stream, bypassing the return air filter entirely. Dust rings around supply registers are the diagnostic tell.

Allergy and respiratory flare-ups inside the home point to particulates (pollen, pet dander, mold spores), sustained high humidity enabling mold and dust mites, or VOCs from building materials or furnishings. Each has a different intervention. Particulates respond to MERV 11–13 filtration and adequate runtime. Humidity responds to dehumidification. VOCs respond to ventilation.

CO or gas odor concerns are in a different category from comfort-based IAQ issues. If you smell gas, leave immediately and call the gas company. For CO concerns in a home with combustion equipment — gas furnace, water heater, fireplace — a certified CO monitor handles ongoing detection. A combustion safety inspection by a qualified technician is appropriate if you have had any symptoms (headache, nausea, dizziness) while at home.

Humidity discomfort — feeling sticky or clammy despite the AC running — indicates either that the AC is oversized (short-cycling before it removes adequate moisture), that the system is undersized and can't keep up, or that the building envelope has significant infiltration. Measuring indoor RH with an inexpensive hygrometer gives you the number to act on.

• Musty odor → moisture source, not a filter or purifier problem.
• Visible dust despite good filter → duct leakage, check for dust rings at supply registers.
• Allergy flare-ups → particulates (filtration), humidity (dehumidification), or VOCs (ventilation).
• CO concern → combustion safety inspection plus CO monitor in every sleeping area.
• Humidity discomfort → measure RH first; if above 55%, AC sizing or dehumidification is the fix.

What to check before calling for IAQ service

Measure current relative humidity. An inexpensive digital hygrometer ($15–$25) gives you indoor RH. In a Maryland summer, indoor RH above 55% is a problem worth addressing directly. Above 60% and you have active mold growth conditions on any cool surface — especially the evaporator coil and areas near supply registers. If RH is consistently above 60% despite the AC running, that data point tells you more than any IAQ test would.

Check your MERV filter. Pull it out and look at it. A severely loaded filter restricts airflow, reducing both the system's ability to condition the space and its ability to capture particles. A MERV 8 filter in good condition outperforms a loaded MERV 13 on both counts. Also check that the filter is correctly seated — bypass around the edges (often visible as a clean channel around the filter frame) means air is circulating unfiltered.

Check for visible mold near supply registers or in the return air path. Small black or green spots near supply diffusers, or a musty smell strongest in the first few seconds of each cooling cycle, are signs of biological growth on the coil or in the air handler. This is a maintenance finding, not an IAQ product finding — it requires coil cleaning.

Check condensate drain function. A slow or clogged condensate drain allows water to stand in the drain pan, which is a direct mold growth site. The drain line typically exits the air handler through a PVC pipe and terminates outside the home or in a floor drain. Pour a cup of water into the drain pan and verify it drains within a minute or two.

• Measure indoor RH: above 55% in summer requires direct action, not IAQ products.
• Check filter condition and seating: a loaded or bypassed filter explains most dust complaints.
• Inspect near supply registers for visible mold — a maintenance finding, not a product gap.
• Verify condensate drain flow: standing water in the drain pan is an active mold site.

When to get professional IAQ testing vs. when symptoms are enough to act

CO testing: a UL-listed CO monitor with digital display handles ongoing detection in any home with combustion equipment. If you have had symptoms consistent with CO exposure (headache, nausea, dizziness that resolve when you leave the home), a combustion safety inspection is the appropriate professional intervention — not a CO test kit, which can't differentiate between a chronic low-level source and intermittent equipment malfunction.

Radon testing: Maryland has moderate radon risk, and Frederick County sits in a Zone 2 area. The EPA recommends testing every two to five years and after any major renovation. Short-term test kits ($15–$25 at hardware stores) are accurate enough for initial screening. If a short-term test returns above 4 pCi/L, a long-term test or mitigation assessment is warranted.

Mold testing: when visible mold exists, testing is rarely useful — the mold is there, and the intervention is remediation, not identification. When mold is suspected but not visible (persistent musty odor with no identified source after checking the coil, drain pan, and ductwork), professional air sampling can confirm the presence of elevated mold spore counts and help guide where to look. Testing does not replace finding and fixing the moisture source.

VOC testing: relevant for new construction, recent major renovation (flooring, cabinetry, paint), or specific concerns about products used in the home. Volatile organic compounds off-gas over time — most new construction VOC levels drop substantially within 6–12 months with adequate ventilation. If a specific product or material is suspected, professional testing can identify the compound.

• CO: certified monitor for every home with combustion equipment; combustion safety inspection if you have had symptoms.
• Radon: test every 2–5 years; short-term kits are adequate for initial screening.
• Mold: when visible, remediate without testing; when suspected but not visible, air sampling helps guide where to look.
• VOCs: test for new construction or specific material concerns; increase ventilation while off-gassing resolves.