Indoor Air Quality Assessment Checklist — Test and Improve Your Home's Air (2026)

Place a digital hygrometer ($10–$20) in the living area, a bedroom, and the basement or lowest level. Record readings at different times of day. Target range is 35–50% relative humidity. Below 30% causes dry skin, cracked lips, and increased respiratory infections. Above 55% promotes mold growth, dust mite reproduction, and musty odors. Basements often read 10–20% higher than upper floors.

Morning condensation on windows indicates indoor humidity is too high for the current outdoor temperature. Light fogging is normal on very cold mornings; persistent dripping indicates a problem. Check behind furniture on exterior walls — condensation there causes hidden mold growth. If you find condensation, improve ventilation before it becomes a mold remediation project.

Hold a tissue near the exhaust fan grille while it runs — it should pull the tissue against the grille firmly. A weak draw means a clogged duct, failed motor, or disconnected duct in the attic (common). Bathroom fans should run during and 20 minutes after showers. Kitchen range hoods should vent outdoors, not recirculate — recirculating hoods just filter grease, not moisture or combustion gases.

Pull the current filter and note the MERV rating on the frame. MERV 8 is the minimum worth using — it catches pollen, dust mite debris, and mold spores. MERV 11–13 catches finer particles including pet dander, smog, and bacteria. If your filter has no MERV rating printed on it, it's likely MERV 1–4 (fiberglass) and catches almost nothing useful. Replace with at least MERV 8, or MERV 11–13 if your system handles it.

A filter with gaps around the edges is worse than useless — air bypasses it entirely while giving false confidence that you're filtering. The filter should fit snugly in the housing with no visible gaps. If the housing is oversized or bent, buy a filter with a cardboard frame and seal edges with aluminum foil tape. A 1-inch gap around a MERV 13 filter makes it perform like a MERV 4.

Look at where the return duct connects to the air handler. Gaps, disconnected duct sections, or missing filter doors allow unfiltered air to reach the blower and coil. This is surprisingly common — especially in attics and crawlspaces where connections work loose over time. Seal any gaps with mastic sealant or metal tape (not cloth duct tape, which degrades).

Symptoms of inadequate ventilation: persistent stuffiness even with clean filters, cooking odors that linger for hours, moisture problems, and CO₂ headaches. Homes built or renovated after 2000 with tight construction often need mechanical ventilation. A CO₂ monitor ($30–$80) is the easiest way to test — readings consistently above 1,000 ppm indicate insufficient fresh air exchange.

Disconnected ducts in attics and crawlspaces pull unconditioned, unfiltered air into the system. Check accessible duct connections visually and by feel — run the system and hold your hand near joints to feel for air leaks. The Department of Energy estimates typical duct systems lose 20–30% of conditioned air through leaks. Sealing ducts with mastic is the single highest-ROI IAQ improvement in most homes.

A clogged or disconnected dryer vent pushes lint, moisture, and heat into the house (or attic/crawlspace). Check the outdoor vent flap while the dryer runs — you should feel strong airflow. If weak, the duct is partially clogged. Professional dryer vent cleaning costs $80–$150 and should be done annually. This is both an air quality and fire safety issue.

Radon is the #2 cause of lung cancer after smoking and is present in every state. It seeps through foundation cracks from naturally occurring uranium in soil. You cannot smell, see, or taste it. Short-term test kits cost $15–$30 at hardware stores. Place in the lowest regularly occupied level, wait 48–96 hours, and mail to the lab. If results exceed 4 pCi/L, install a radon mitigation system ($800–$1,500). One in 15 homes tests above the action level.

Gas furnaces, water heaters, and boilers produce CO during normal combustion. A cracked heat exchanger, blocked flue, or backdrafting chimney can leak CO into living spaces. Verify you have CO detectors within 15 feet of every gas-burning appliance and on every sleeping level. Test them monthly. If a CO detector has alarmed even briefly, call for an immediate HVAC inspection — don't assume it was a false alarm.

Volatile organic compounds off-gas from pressed wood furniture, new carpet, paint, adhesives, cleaning products, and vehicle exhaust from attached garages. Walk through each room and note recent additions or strong chemical smells. The attached garage is often the biggest VOC source — combustion exhaust seeps through shared walls and doors. Weatherstrip the garage-to-house door and keep the garage ventilated.

Look inside supply registers by removing the grille and using a flashlight. Black staining, fuzzy growth, or rodent droppings indicate contamination that gets blown into every room. Light dust is normal; visible mold or pest activity warrants professional duct cleaning ($300–$500 for a whole house) or remediation. Do not attempt to clean mold-contaminated ducts yourself — improper cleaning spreads spores.