A “good” indoor air quality reading means major indicators sit in ranges associated with minimal short‑term discomfort and adequate ventilation: EPA AQI in the Good band (0–50), PM2.5 concentrations at or below the EPA Good breakpoint, CO2 generally under ~1,000 ppm, low TVOC levels, and relative humidity roughly 30–50%. If one metric is poor, act on that specific cause—ventilation for CO2, filtration for particles, source control for VOCs.
Why these numbers matter
These targets combine public‑health benchmarks with practical measures most consumer monitors report. The EPA’s Air Quality Index (AQI) compresses multiple pollutants into a simple band — “Good” (0–50) indicates pollutant concentrations that are unlikely to cause adverse effects for the general population. PM2.5 (particles smaller than 2.5 µm) is the particle fraction most associated with indoor/outdoor exposure concerns; keeping PM2.5 near the EPA Good breakpoint reduces acute exposure.
CO2 isn’t a pollutant at the concentrations typically found indoors; it’s valuable as a proxy for ventilation and occupant‑generated bioeffluents. When CO2 stays below about 1,000 ppm in occupied spaces, ventilation is usually adequate for comfort and to limit accumulation of human‑sourced contaminants. Relative humidity affects both comfort and biology: too low is drying, too high encourages mould and dust‑mite growth. VOCs (volatile organic compounds) are a broad class; there isn’t a single universal “safe” number, so aim to avoid persistent or large spikes and remove identified sources.
Common IAQ measures and how to read them
| Metric | Practical “good” range | Interpretation & common action |
|---|---|---|
| AQI (composite) | 0–50 (Good) | No action usually needed for most occupants; maintain normal ventilation and check outdoor AQI before opening windows during wildfires or heavy traffic. |
| PM2.5 (µg/m³) | Near or below EPA Good breakpoint (refer to device/AQI conversion) | If consistently above Good, reduce indoor sources (cooking, smoking), increase ventilation when outdoor air is clean, and use filtration (HEPA). |
| CO2 (ppm) | <1,000 ppm | Above ~1,000 indicates inadequate ventilation for occupancy; open windows, boost mechanical ventilation, or add scheduled air exchange. |
| TVOCs (ppb or index) | As low as reasonably achievable; avoid persistent high readings | Identify and remove sources (cleaning products, new furniture, paints); ventilate until levels drop. Consumer sensors report relative values—watch trends and spikes. |
| Relative humidity (%) | 30–50% | Below 30% can feel dry; above 60% raises mould risk. Use humidifiers or dehumidifiers to maintain the range. |
Recommended target ranges (practical shorthand)
- AQI: 0–50 (Good).
- PM2.5: near or below the EPA Good breakpoint for PM2.5 (use your monitor’s AQI conversion if available).
- CO2: aim <1,000 ppm; investigate if regularly >1,000 ppm.
- Relative humidity: ~30–50% indoors.
- TVOCs: avoid persistent high values or repeated large spikes; ventilate and remove sources.
Exceptions, caveats and common misunderstandings
Sensors vary. Low‑cost monitors may report PM1, PM2.5 or PM10 differently, give TVOC indices rather than specific chemical concentrations, and can drift over time. Use trends and relative changes rather than single‑value readings from an unfamiliar device. Short spikes—cooking, vacuuming, candle use—are common; focus on averages over 15–60 minutes when deciding whether to act.
Outdoor conditions matter. If outdoor AQI is poor (wildfire smoke, heavy urban pollution), outdoor air may worsen indoor PM levels if you ventilate with windows open. In those situations rely on filtration and close windows. Vulnerable populations (young children, older adults, people with chronic respiratory or cardiovascular conditions) may benefit from stricter targets; consult clinicians for individualized guidance rather than relying solely on consumer thresholds.
What to do next — practical steps
- Identify which metric is out of range. That determines the intervention: ventilate for high CO2, filter for high PM2.5, remove sources for VOCs, control moisture for humidity.
- Ventilate intelligently. If outdoor air is cleaner than indoor, open windows or run mechanical ventilation. If outdoor air is polluted, keep windows closed and rely on filtration.
- Use filtration where appropriate. HEPA filtration reduces particles; activated carbon helps absorb some VOCs and odors. If you want a combined device that senses and responds automatically, see our guide to Best Smart & Wi‑Fi Air Purifiers (/best/smart-air-purifiers/).
- Address sources. Stop indoor smoking, allow off‑gassing items to air out, choose low‑VOC products, and avoid using strong cleaners or aerosols in occupied rooms when possible.
- Check sensor placement and calibration. Place monitors in representative occupied areas, away from direct stove or shower plumes. If values seem inconsistent with outdoor reports or symptoms, consider cross‑checking with another monitor or professional testing.
Example product note: some consumer purifiers combine HEPA, activated carbon and built‑in sensing. For instance, the GermGuardian HEPA unit offered on AirPurifierFAQ includes HEPA + activated carbon filtration, an Intellisense PM2.2 sensor, optional UV‑C, and is CARB/ETL certified; its current listed price is $129.99. Use product features and independent certifications to match a purifier to your needs—look for HEPA filtration for particles and activated carbon for odors/VOCs. (See full product listing for details.)
Related questions
How do I interpret PM2.5 readings on my monitor?
Check whether the monitor reports mass concentration (µg/m³) or the device’s AQI conversion. Short spikes are common; focus on 15–60 minute averages and on whether values map into AQI bands. If PM2.5 regularly places indoor AQI above 50, reduce sources and run a HEPA filter. See also our glossary for PM2.5 terms (/glossary/).
What CO2 level is concerning?
CO2 below ~1,000 ppm generally indicates acceptable ventilation for most indoor activities. Values consistently above 1,000 ppm suggest increased accumulation of exhaled air and a need to increase ventilation or reduce occupancy. Very high CO2 (several thousand ppm) warrants immediate ventilation and possibly professional assessment.
Can I trust cheap IAQ sensors?
Cheap sensors can be useful for trends and detecting events, but absolute accuracy varies. Use them to spot when levels change, not necessarily for exact compliance with health thresholds. For critical decisions, cross‑check with a calibrated device or consult professionals. Our specs and testing primers explain sensor types and reliability (/faq/specs/).
How often should I check IAQ readings?
During normal occupancy a continuous monitor is best so you can observe patterns. Check alerts or hourly averages rather than single instantaneous values. Reassess after activities that commonly change levels—cooking, cleaning, painting, or changes to ventilation.
Sources
- U.S. Environmental Protection Agency — Indoor Air Quality (IAQ) basics: https://www.epa.gov/indoor-air-quality-iaq
- U.S. EPA — Particulate Matter (PM) basics: https://www.epa.gov/pm-pollution/particulate-matter-pm-basics
- AirNow — AQI Basics: https://www.airnow.gov/aqi/aqi-basics/
- Association of Home Appliance Manufacturers (AHAM): https://www.aham.org/
- ASHRAE — Ventilation and indoor air quality resources: https://www.ashrae.org/
- World Health Organization — WHO guidelines for indoor air quality: https://www.who.int/publications/i/item/9789240054303
- Pope CA, Dockery DW. Health effects of fine particulate air pollution: lines that connect. J Air Waste Manag Assoc. 2006;56(6):709–742. PubMed: https://pubmed.ncbi.nlm.nih.gov/16988655/
