Reading the EPA AQI
The EPA Air Quality Index (AQI) runs from 0 to 500. Bands are 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, and 151–200 Unhealthy. The AQI is built for outdoor air, but it is the fastest daily signal for whether to open windows.
When the AQI is above 100, closing windows and running a HEPA purifier is reasonable. The US DOE HEPA definition requires capturing at least 99.97% of particles of 0.3 micrometres. When the AQI is below 50, ventilation is usually the better move.
The AQI does not measure indoor sources. Cooking, candles, and cleaning can push indoor PM2.5 above the outdoor level even on a Good AQI day. That is why an indoor PM2.5 sensor adds information the AQI cannot provide.
A short worked example: outdoor AQI is 40 (Good), but you fry bacon indoors. Your indoor PM2.5 sensor reads 60 µg/m³. The AQI said open windows; the indoor sensor says run the vented hood. Both are correct for their scope.
Consumer laser PM2.5 sensors
Consumer laser PM2.5 sensors count particles by light scattering. They report a mass concentration in µg/m³. They are not reference-grade monitors; the US EPA has tested many low-cost sensors and publishes performance targets.
Use them for trends, not for compliance. A sensor that reads 5 µg/m³ indoors on a closed-window day is plausible. A sensor that reads 0 µg/m³ is likely under-reporting. The WHO 2021 24-hour guideline for PM2.5 is 15 µg/m³, a useful comparison point.
Placement matters. Keep the sensor away from direct sunlight, drafts, and cooking fumes. A sensor on a kitchen counter will spike during cooking; that is real, but it is not the room average. For a bedroom reading, place it at breathing height near the bed.
Calibration drift is real. Many low-cost PM2.5 sensors use a factory calibration that assumes a specific aerosol. Humidity above 60% can bias readings. The US EPA recommends co-locating a sensor with a reference monitor if you need accuracy.
NDIR CO2 sensors and the 420/1000 ppm bands
Non-dispersive infrared (NDIR) CO2 sensors are the reliable consumer option for CO2. They report ppm directly. Outdoor air holds about 420 ppm of CO2. Many ventilation guidelines treat about 1000 ppm indoors as the upper comfort band.
A bedroom that climbs past 1000 ppm overnight is under-ventilated. That is a signal to increase fresh air, not necessarily to buy a purifier. ASHRAE Standard 62.1 and Standard 62.2 define ventilation rates for acceptable indoor air quality.
NDIR sensors need fresh air for reference. Most consumer units use a baseline calibration algorithm that assumes the sensor sees outdoor-level CO2 periodically. If the sensor never sees fresh air, it can drift. Follow the manufacturer's calibration instructions.
CO2 is not a perfect proxy for all indoor pollutants. It tracks human occupancy and ventilation, not cooking particles or VOCs. Pair an NDIR CO2 sensor with a PM2.5 sensor for a fuller picture.
Short-term vs long-term radon testing
Short-term radon tests run 2 to 90 days. Long-term tests run more than 90 days. The US EPA recommends a short-term test first, then a follow-up if the result is near the action level. The US EPA action level is 4 pCi/L, about 148 Bq/m³.
The WHO reference level is 100 Bq/m³. That is lower than the US EPA action level, so some homes act sooner. Both numbers come from the same physical measurement: radon concentration in air.
Place the kit in the lowest lived-in level. Keep windows and doors closed as much as possible during the test. Avoid bathrooms, kitchens, and laundry rooms because of moisture and drafts.
Long-term tests give a better annual average because radon varies with season and weather. A short-term test is a screening tool. If the short-term result is at or above 4 pCi/L, a long-term test or a second short-term test is the standard follow-up.
Key figures at a glance
| PM2.5 particle size | 2.5 micrometres or smaller | Roughly 30 times finer than a human hair. |
|---|---|---|
| US DOE HEPA definition | 99.97% of particles of 0.3 micrometres | US Department of Energy definition for HEPA filtration. |
| Radon test durations | Short-term: 2–90 days; long-term: >90 days | US EPA testing guidance. |
| WHO 2021 PM2.5 24-hour guideline | 15 µg/m³ | Global Air Quality Guidelines, World Health Organization, 2021. |
How to place an indoor air quality monitor
Pick the room that matters
Choose the room where you spend the most time, often a bedroom. If you cook often, also monitor the kitchen. The US EPA recommends testing the lowest lived-in level for radon.
Set the height
Place the monitor at breathing height, roughly 1 to 1.5 metres above the floor. For a bedroom, that is near the bed. For a kitchen, keep it away from the stove to avoid cooking spikes.
Avoid interference
Keep the monitor away from direct sunlight, drafts, and exterior walls. Humidity above 60% can bias some PM2.5 sensors. An NDIR CO2 sensor needs periodic fresh air for baseline calibration.
Log for a week
Record readings for at least a week before drawing conclusions. Compare PM2.5 to the WHO 2021 24-hour guideline of 15 µg/m³. Compare CO2 to the 420 ppm outdoor baseline and the 1000 ppm indoor comfort band.
Act on the pattern
If CO2 stays above 1000 ppm overnight, increase ventilation. If PM2.5 spikes during cooking, use a vented range hood. If radon is at or above 4 pCi/L, consider a long-term test and mitigation.
Frequently asked questions
How accurate are consumer PM2.5 sensors?
Consumer laser PM2.5 sensors are not reference-grade monitors. The US EPA has tested many low-cost sensors and publishes performance targets. Use them for trends, not for compliance. Humidity above 60% can bias readings.
Do I need an NDIR CO2 sensor or will any monitor do?
NDIR CO2 sensors report ppm directly. Some low-cost monitors estimate CO2 from other signals. For ventilation decisions, an NDIR sensor is the reliable choice. Outdoor air holds about 420 ppm of CO2; about 1000 ppm indoors is the common comfort band.
How long should a radon test run?
Short-term radon tests run 2 to 90 days. Long-term tests run more than 90 days. The US EPA recommends a short-term test first. If the result is near the 4 pCi/L action level, follow up with a long-term test.
Where should I put my air quality monitor?
Place it at breathing height in the room where you spend the most time. Keep it away from direct sunlight, drafts, and exterior walls. For radon, place the kit in the lowest lived-in level. Follow the manufacturer's placement instructions.
What does the EPA AQI tell me about indoor air?
The EPA AQI runs from 0 to 500 and is built for outdoor air. Bands are 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, and 151–200 Unhealthy. It does not measure indoor sources like cooking or candles. Use it as a daily outdoor signal.