Indoor air can be two to five times more polluted than outdoor air, according to the U.S. Environmental Protection Agency ([EPA, Indoor Air Quality](https://www.epa.gov/indoor-air-quality-iaq)). That's the core reason IAQ monitors have exploded in popularity. The problem isn't the devices — it's that most people get a wall of numbers and no context for what's good, bad, or worth fixing.
The EPA frames indoor air quality around three strategies: source control, improved ventilation, and air cleaners ([ANSI Blog on ASHRAE 62.2](https://blog.ansi.org/ansi/ansi-ashrae-62-2-2022-ventilation-residential-air/)). A monitor tells you when you need the second or third one. Here's what the readings mean.
PM2.5 — the one to watch first
PM2.5 is fine particulate matter under 2.5 micrometers — small enough to reach deep into the lungs and enter the bloodstream. The EPA's National Ambient Air Quality Standards set a primary annual limit of 9.0 µg/m³ and a 24-hour limit of 35 µg/m³ ([EPA NAAQS Table](https://www.epa.gov/criteria-air-pollutants/naaqs-table)). Indoors, you want to stay well below those. Common indoor sources: cooking, candles, smoking, and infiltration of outdoor wildfire or traffic smoke.
What to do when PM2.5 spikes.If it jumps during cooking, run the range hood on high and crack a window. If it's elevated all day, the issue is usually filtration — upgrade to a MERV 13 or better filter if your equipment can handle the static pressure.
CO₂ — the ventilation report card
Carbon dioxide isn't dangerous at typical indoor levels, but it's the best proxy for how well a space is ventilated. Outdoor air is roughly 400–420 ppm. The common guideline is that indoor CO₂ above 1,000 ppm signals underventilation, and levels below 800 ppm are preferred ([Illinois Dept. of Public Health IAQ Guidelines](https://dph.illinois.gov/topics-services/environmental-health-protection/toxicology/indoor-air-quality-healthy-homes/idph-guidelines-indoor-air-quality.html)).
If CO₂ climbs past 1,000 ppm in a bedroom overnight or a living room during a gathering, the fix is more outside air — either by opening a window, running an exhaust fan, or improving whole-house mechanical ventilation.
VOCs — useful, but read with caution
Volatile organic compounds come from paint, cleaning products, new furniture, and many building materials. Most consumer monitors report TVOC — total VOCs — as a single number. The catch: there's no regulatory consensus on a safe TVOC threshold, and sensors can't distinguish a harmless emission from a harmful one. Treat TVOC as a trend indicator, not an absolute. A spike after cooking or cleaning is normal; a persistently high baseline points to a source you should track down and remove.
The thresholds, in one table
| Pollutant | Good / target | Watch / act | Source authority |
|---|---|---|---|
| PM2.5 (24h) | < 12 µg/m³ | > 35 µg/m³ | EPA NAAQS |
| CO₂ | < 800 ppm | > 1,000 ppm | IDPH / ASHRAE guidance |
| CO | < 5 ppm | > 9 ppm | EPA / IDPH |
| Relative humidity | 30–50% | < 30% or > 60% | EPA / IDPH |
| TVOC | Baseline trend | Persistent spikes | No formal threshold |
What we'd actually track
- PM2.5 — most directly tied to health risk, and the easiest to improve with filtration.
- CO₂ — the single best indicator of whether your home is ventilated enough.
- Humidity — keep it 30–50% in winter, under 60% in summer to limit mold and dust mites.
- CO — not all monitors include it, but if you have combustion appliances, it's non-negotiable.
A monitor is not a safety device.A CO₂ or PM2.5 monitor won't warn you about carbon monoxide unless it has a dedicated CO sensor. If you have a gas furnace, boiler, or water heater, install a UL-listed CO alarm — that's a different device with a different job.
The bottom line
An IAQ monitor is a diagnostic tool, not a solution. The value is in spotting patterns — cooking spikes, overnight CO₂ buildup, humidity creep — and then acting with source control, ventilation, or filtration. Track PM2.5 and CO₂ first; treat TVOC as a trend; and never substitute a monitor for a real carbon monoxide alarm.
Want help interpreting your readings or sizing filtration and ventilation for your home? Get in touch — we'll look at the actual numbers, not sell you a gadget.