How to Measure Indoor Air Quality at Home: A Step-by-Step Plan

What to measure and with what

Benchmarks from the agency named in each row.

Hazard	Tool	Benchmark	Source
Radon	Short-term test (2 to 90 days), then long-term (over 90 days) or a digital radon monitor	Fix at 4 pCi/L or more; consider fixing from 2 to 4	EPA
Carbon monoxide	CO alarm on every level and outside sleeping areas	Any alarm: get to fresh air, call 911	CPSC
PM2.5	Monitor with a laser particle sensor	Annual outdoor standard 9.0 µg/m³; AQI Good to 9.0 (24-hour)	EPA
CO2	Monitor with an NDIR sensor	A ventilation indicator; no ASHRAE indoor limit	ASHRAE
Humidity	Hygrometer or monitor	30 to 50%, below 60%	EPA
VOCs	Monitor with a VOC sensor (relative trend)	No federally enforceable standard for homes	EPA
Mold	Visual inspection and smell	Visible growth: sampling usually unnecessary	EPA

Step 1: Test for radon

Radon is the one hazard you cannot see, smell or infer from other readings, so test first. EPA's Citizen's Guide says to place the test in the lowest lived-in level (the basement if it is used often, otherwise the first floor), in a room used regularly but not the kitchen or bathroom, at least 20 inches above the floor and away from drafts, high heat, high humidity and exterior walls. For a short test of 2 or 3 days, close windows and outside doors at least 12 hours beforehand and keep them closed.

If the first short-term result is 4 pCi/L or higher, EPA says to follow up with a long-term test or a second short-term test; if it is more than twice the 4 pCi/L action level, take a second short-term test immediately. Our radon detector guide covers kits and digital monitors, and the best radon detectors page has picks.

Step 2: Fit carbon monoxide alarms

CPSC advises CO alarms on every level of the home and outside sleeping areas, interconnected if possible so that when one sounds they all sound, with battery operation or battery backup, and with the batteries checked regularly. CPSC says more than 200 people in the US die every year from accidental non-fire CO poisoning linked to consumer products. Consumer air quality monitors are not CO alarms; even one that reports other gases does not replace a listed CO alarm.

Step 3: Run a monitor for a week

A monitor that logs PM2.5, CO2, humidity and temperature shows your home's daily pattern. A week is enough to see cooking spikes, how high CO2 climbs in a closed bedroom overnight, and whether the basement humidity stays above 60%. Move it between rooms if you have one monitor, keeping it at breathing height and away from windows, vents and the stove; see where to place a monitor.

Choose one with an NDIR CO2 sensor and, ideally, an independent particle result from South Coast AQMD's AQ-SPEC program. The best air quality monitors page compares the options.

Step 4: Look for moisture and mold

EPA's position on mold testing is direct: in most cases, if visible mold growth is present, sampling is unnecessary, and there are no EPA standards for airborne mold. Walk the house looking for water stains, condensation on windows, musty smells and visible growth, especially in basements, bathrooms and under sinks. A hygrometer reading above 60% points to the cause. See the humidity level that prevents mold.

Reading the results

• PM2.5 spikes when cooking, then falls: normal. Use the range hood vented outside, as EPA advises, and a purifier nearby.
• PM2.5 high all day: look for a constant source (smoking, a leaky wood stove, outdoor smoke) and size a purifier by its smoke CADR. See PM2.5 explained.
• CO2 climbs overnight: the bedroom needs more outdoor air. See why CO2 matters at home.
• Humidity above 60%: dehumidify and fix water leaks; below 30%, consider a humidifier. See the ideal indoor humidity level.
• VOCs up after painting or new furniture: ventilate; EPA recommends more ventilation when using VOC-emitting products.

When to call a professional

• Radon at or above the action level. EPA's Citizen's Guide says lowering high radon levels requires technical knowledge and special skills, and advises a contractor trained to fix radon problems; your state radon office can name qualified or certified contractors.
• Mold sampling. EPA says sampling should be done by professionals with specific experience, and is rarely needed when growth is visible.
• Fuel-burning appliances. EPA advises having a professional inspect, clean and tune up central heating systems every year.
• Any CO alarm. Get to fresh air immediately and then call 911, as CPSC advises.

What consumer tests cannot tell you

Monitors measure a handful of things. They do not identify which VOC is present, cannot detect asbestos or lead dust, and their particle readings are estimates. If a monitor says the air is fine but someone in the home has persistent symptoms, talk to a doctor and consider a professional indoor air quality assessment. Our guide to monitor accuracy explains what the numbers can and cannot support.