Air quality is measured using a standardized index that converts pollutant concentrations into a simple 0-to-500 scale, where anything under 50 is considered good and anything above 150 is unhealthy for everyone. Whether you’re checking a government monitoring station, setting up a consumer sensor at home, or interpreting readings from a portable device, the fundamentals are the same: detect specific pollutants, measure their concentration, and compare those numbers against health-based thresholds.
The Six Pollutants That Define Air Quality
The EPA bases its Air Quality Index on six “criteria pollutants” that pose the greatest widespread risk to health:
- Particulate matter (PM2.5 and PM10), the tiny particles from combustion, dust, and smoke that penetrate deep into your lungs
- Ground-level ozone, formed when vehicle exhaust and industrial emissions react with sunlight
- Carbon monoxide, a colorless gas from burning fuel
- Sulfur dioxide, released primarily by power plants and industrial facilities
- Nitrogen dioxide, produced mainly by vehicle engines
- Lead, now mostly from industrial sources after its removal from gasoline
Of these, PM2.5 and ozone are the two most people encounter in daily AQI reports. PM2.5 refers to particles smaller than 2.5 micrometers, roughly 30 times thinner than a human hair. These are small enough to pass through your lungs into your bloodstream, which is why the EPA tightened its annual PM2.5 standard in February 2024, lowering the safe threshold from 12 to 9 micrograms per cubic meter.
How the AQI Scale Works
The AQI translates raw pollutant concentrations into a number between 0 and 500. Each pollutant gets its own AQI value, and the highest one becomes the overall reading for that location. For PM2.5, the breakpoints over a 24-hour average work like this:
- Good (0–50): 0 to 9.0 µg/m³
- Moderate (51–100): 9.1 to 35.4 µg/m³
- Unhealthy for sensitive groups (101–150): 35.5 to 55.4 µg/m³
- Unhealthy (151–200): 55.5 to 125.4 µg/m³
- Very unhealthy (201–300): 125.5 to 225.4 µg/m³
- Hazardous (301–500): 225.5 to 325.4 µg/m³
The jump between “moderate” and “unhealthy for sensitive groups” is the one that matters most for everyday decisions. If you have asthma, heart disease, or young children, an AQI above 100 is your cue to limit prolonged outdoor exertion.
Government Monitoring vs. Consumer Sensors
Federal reference monitors are the gold standard. These are large, expensive instruments operated by trained technicians at fixed stations. They use precise methods, such as weighing particles collected on filters, to produce high-confidence data. This is what feeds official AQI maps and regulatory decisions.
Consumer-grade sensors are far cheaper and more portable, but their accuracy varies significantly. A Government Accountability Office report found that these sensors cannot meet federal monitoring standards and that their reliability depends heavily on the target pollutant. Light-scattering sensors for PM2.5 tend to produce more reliable results than those measuring PM10 or gases. Some vendors don’t disclose how they calibrated their devices or under what conditions, making it hard to know exactly how much to trust the numbers.
That said, consumer sensors are useful for tracking relative changes. If your PM2.5 sensor reads 15 one day and 60 the next, the absolute number may be slightly off, but the trend is meaningful. Co-locating a consumer sensor near a government reference monitor and comparing readings is the most reliable way to calibrate it and understand its margin of error.
How Sensors Actually Detect Pollutants
Most affordable particulate matter sensors use laser scattering. Air flows into a small dark chamber where a laser beam hits each passing particle. A photodetector positioned to the side picks up the scattered light, and the intensity of that light is proportional to the particle’s size. The sensor counts particles, estimates their volume by assuming they’re spherical, and converts that into a mass concentration in micrograms per cubic meter. This spherical assumption is a known limitation, since real-world particles from traffic or wildfires come in irregular shapes, which can introduce error.
Gas sensors work differently depending on the target. CO2 monitors typically use infrared absorption: they shine infrared light through an air sample and measure how much light gets absorbed at CO2-specific wavelengths. The more absorption, the higher the concentration. For toxic gases like carbon monoxide or nitrogen dioxide, electrochemical sensors are common. The gas diffuses through a membrane and reacts with an internal solution, generating a tiny electrical current proportional to the gas concentration.
Volatile organic compounds (VOCs), the chemical off-gassing from paints, cleaning products, and building materials, are usually detected with metal oxide semiconductor sensors. These work by measuring changes in electrical resistance when VOC molecules react with the sensor surface. They’re sensitive to formaldehyde, acetone, and benzene, though most consumer devices report only a combined “total VOC” number rather than identifying individual chemicals.
Where to Place an Indoor Sensor
Placement makes a surprising difference in your readings. The EPA recommends positioning indoor sensors at breathing zone height, between 3 and 6 feet off the ground. Keep the sensor away from direct pollution sources like toasters, stoves, or candles, and equally away from air purifiers, which would give you an artificially clean reading.
Avoid placing sensors near windows, exterior doors, or HVAC vents. These locations introduce rapidly changing temperature and humidity that can throw off readings, and the air there is more influenced by outdoor conditions than by the actual quality of air in your living space. The sensor should have free airflow around it, so don’t tuck it behind furniture or into a corner where air stagnates.
Measuring Indoor Air With CO2
CO2 concentration is one of the simplest and most useful things to measure indoors. It’s not toxic at typical indoor levels, but it serves as a reliable proxy for ventilation. The standard recommendation from ASHRAE (the organization that sets ventilation guidelines) is to keep indoor CO2 below 1,000 ppm.
A study from the Harvard School of Public Health put this into sharper focus. Researchers exposed participants to CO2 levels of 600, 1,000, and 2,500 ppm in a simulated office. At 1,000 ppm, performance dropped measurably on six out of nine decision-making tasks. At 2,500 ppm, a level that’s common in crowded conference rooms with poor ventilation, seven of nine measures declined substantially. A CO2 monitor costing under $100 can tell you in real time whether your home office or classroom needs a window opened.
Keeping Your Sensor Accurate Over Time
All sensors drift. The chemical and physical components inside gradually change, causing readings to shift even when actual air quality stays constant. Temperature swings, humidity, and simple aging of the sensor elements all contribute. This is why regular calibration matters, even for sensors marketed as maintenance-free.
The most reliable calibration method is comparing your sensor’s readings against a known reference, either by placing it next to a government monitoring station or by exposing it to a gas of known concentration. How often you need to do this depends on the sensor and your environment, but checking every few months is a reasonable starting point for consumer devices. If your readings start looking consistently different from nearby government monitoring data (available free on AirNow.gov), it’s likely time to recalibrate or replace the sensor.
Free Tools for Checking Air Quality Now
You don’t need to buy anything to start measuring air quality. AirNow.gov provides real-time AQI readings from government monitors across the United States, searchable by zip code. PurpleAir’s online map shows readings from thousands of community-operated sensors, giving you hyperlocal data that government networks often can’t match. The IQAir platform aggregates data globally if you’re tracking conditions in other countries.
For indoor air, a dedicated monitor is the only way to get readings since outdoor networks can’t tell you what’s happening inside your home. Devices that measure PM2.5, CO2, temperature, and humidity together typically cost between $80 and $250 and give you a solid baseline picture. Look for models that have been evaluated against reference instruments, as some manufacturers publish performance data or have participated in EPA sensor evaluation programs.