How to Read Your Air Quality Monitor: Every Metric Explained
Learn what PM2.5, CO2, VOCs, humidity, and temperature readings mean on your air quality monitor, with healthy ranges and when to take action.
Table of Contents
- The Six Metrics Your Monitor Tracks
- PM2.5 (Fine Particulate Matter)
- PM10 (Coarse Particulate Matter)
- CO2 (Carbon Dioxide)
- VOCs (Volatile Organic Compounds)
- Temperature
- Humidity (Relative Humidity)
- Reading the Colors
- What to Do When a Number Spikes
- PM2.5 or PM10 Spiked
- CO2 Spiked
- VOCs Spiked
- Humidity Out of Range
- Understanding Trends vs. Snapshots
- When Your Monitor Readings Disagree with How You Feel
- Putting It All Together: Your Quick Reference Card
TL;DR
Your air quality monitor tracks several metrics. PM2.5 should stay below 12 µg/m³ (WHO ideal is under 5). CO2 below 800 ppm means good ventilation. VOCs below 250 ppb are fine. Humidity between 40% and 60% is the sweet spot. Temperature is personal but 68 to 72°F works for most people. If one metric spikes, the fix depends on which one: purifier for particles, ventilation for CO2, source removal for VOCs.
You turned on your air quality monitor, and now you are staring at a screen full of numbers, colors, and abbreviations. PM2.5 is at 8. CO2 reads 943. VOCs say "moderate." What does any of that actually mean, and when should you do something about it?
This guide breaks down every metric your monitor displays, gives you the exact thresholds for healthy versus unhealthy, and tells you what to do when a number goes red.
The Six Metrics Your Monitor Tracks
Most air quality monitors measure some combination of these six things. Not every monitor tracks all of them, but understanding each one helps you make sense of whatever your device shows you.
PM2.5 (Fine Particulate Matter)
PM2.5 refers to particles smaller than 2.5 micrometers in diameter. For perspective, a human hair is about 70 micrometers wide. These particles are so small they bypass your nose and throat, penetrate deep into your lungs, and can enter your bloodstream.
If you want to understand the science behind these particles in more detail, our explainer on what PM2.5 is and why it matters covers the health effects and sources thoroughly.
What the numbers mean:
| PM2.5 Level | Rating | What to Do |
|---|---|---|
| 0 to 5 µg/m³ | Excellent | WHO target. No action needed. |
| 5 to 12 µg/m³ | Good | EPA standard. Normal for most homes. |
| 12 to 35 µg/m³ | Moderate | Run your air purifier on medium. Identify the source. |
| 35 to 55 µg/m³ | Unhealthy for sensitive groups | Purifier on high. Close windows if outdoor air is the source. |
| 55+ µg/m³ | Unhealthy | Purifier on max. Limit the activity causing it. Ventilate once the source stops. |
Common indoor sources: Cooking, candles, incense, vacuuming, wildfire smoke infiltration, tobacco smoke, and dust disturbance.
The fix: A HEPA air purifier is the most effective tool for reducing PM2.5 indoors. A properly sized unit should cut PM2.5 by 50% within 15 to 20 minutes.
PM10 (Coarse Particulate Matter)
PM10 covers particles up to 10 micrometers. These are larger than PM2.5 and include dust, pollen, and mold spores. Your body's natural defenses (nose hairs, mucus membranes) catch most PM10 before it reaches your lungs, so it is less dangerous than PM2.5 but still relevant for allergy sufferers.
What the numbers mean:
| PM10 Level | Rating | What to Do |
|---|---|---|
| 0 to 20 µg/m³ | Good | Normal indoor level. |
| 20 to 50 µg/m³ | Moderate | Dust or vacuum more frequently. Check your HVAC filter. |
| 50+ µg/m³ | Poor | Something is kicking up dust or pollen. Identify and address the source. |
Common indoor sources: Sweeping, pet dander, pollen tracked in from outside, construction dust, and old HVAC filters releasing trapped particles.
The fix: Regular cleaning, HEPA purifiers, and keeping windows closed during high-pollen days.
CO2 (Carbon Dioxide)
CO2 is not a pollutant in the traditional sense. It is a natural byproduct of breathing. But indoor CO2 levels are the single best indicator of whether your space has adequate ventilation. When CO2 climbs, it means other pollutants (viruses, VOCs, humidity) are also accumulating because fresh air is not cycling through.
For a deep dive into CO2 thresholds and what they mean for your health and focus, see our complete guide to indoor CO2 levels.
What the numbers mean:
| CO2 Level | Rating | What to Do |
|---|---|---|
| 400 to 600 ppm | Excellent | Outdoor-like air quality. Well ventilated. |
| 600 to 800 ppm | Good | Normal for occupied rooms with decent ventilation. |
| 800 to 1,000 ppm | Moderate | Room is getting stuffy. Open a window or door soon. |
| 1,000 to 1,500 ppm | Poor | Cognitive performance drops measurably. Ventilate now. |
| 1,500+ ppm | Bad | Headaches, drowsiness, poor decision-making. Open windows immediately. |
Why it rises: Every person in a room exhales about 40,000 ppm CO2 with each breath. In a closed room, levels climb fast. A bedroom with two people and the door shut can exceed 2,500 ppm by morning.
The fix: Only fresh air lowers CO2. Open windows, run exhaust fans, or use a mechanical ventilation system (ERV/HRV). Air purifiers do absolutely nothing for CO2.
VOCs (Volatile Organic Compounds)
VOCs are gases released by household products, furniture, building materials, and cleaning supplies. Your monitor measures total VOCs (tVOC) and reports them in parts per billion (ppb) or as an index score. Consumer monitors cannot tell you which specific VOC is present, only the overall concentration.
Our VOCs explainer covers the common types and their health effects in detail.
What the numbers mean:
| tVOC Level | Rating | What to Do |
|---|---|---|
| 0 to 65 ppb | Excellent | Very clean air. |
| 65 to 220 ppb | Good | Normal indoor levels. No action needed. |
| 220 to 660 ppb | Moderate | Identify the source. Increase ventilation. |
| 660 to 2,200 ppb | Poor | Prolonged exposure is concerning. Remove the source and ventilate aggressively. |
| 2,200+ ppb | Unhealthy | Leave the room if possible. Ventilate thoroughly before returning. |
Common indoor sources: New furniture and carpets (formaldehyde), cleaning products, air fresheners, paint, nail polish, cooking oils at high heat, and personal care products like hair spray.
The fix: Source removal is the most effective approach. Stop using the product causing it, or move new furniture to a well-ventilated area to off-gas. Ventilation (open windows, fans) is the next best option. Activated carbon filters in air purifiers help but are slower.
Important caveat: Consumer VOC sensors are the least accurate metric on your monitor. Use the readings as a relative trend indicator ("higher than yesterday" or "spikes when I clean") rather than trusting the absolute number. If you suspect a serious VOC issue (new construction, chemical smell), invest in professional air quality testing.
Temperature
Temperature itself is not an air quality metric, but it affects both your comfort and how other pollutants behave. Higher temperatures increase VOC off-gassing from furniture and building materials. Extreme cold pushes people to seal up their homes, which raises CO2 and other indoor pollutants.
What the numbers mean:
| Temperature | Rating | Notes |
|---|---|---|
| 60 to 67°F (16 to 19°C) | Cool | Fine for sleeping. May feel cold while sitting. |
| 68 to 72°F (20 to 22°C) | Comfortable | Ideal for most people during the day. |
| 73 to 78°F (23 to 26°C) | Warm | Acceptable in summer. VOC off-gassing increases. |
| 79°F+ (26°C+) | Hot | Comfort drops. VOC emissions from materials accelerate. Ventilate more. |
Practical tip: If your VOC readings climb in summer without any obvious new source, the heat itself may be the culprit. Warmer temperatures cause building materials and furniture to release VOCs faster.
Humidity (Relative Humidity)
Humidity affects comfort, health, and how other pollutants behave. Too dry and your airways get irritated, viruses survive longer on surfaces, and static electricity increases. Too humid and you create ideal conditions for mold, dust mites, and bacterial growth.
What the numbers mean:
| Humidity | Rating | What to Do |
|---|---|---|
| Below 30% | Too dry | Use a humidifier. Common in winter with forced-air heating. |
| 30 to 40% | Slightly dry | Acceptable, but sensitive individuals may notice dry skin or airways. |
| 40 to 60% | Ideal | The sweet spot for health and comfort. |
| 60 to 70% | Slightly humid | Watch for condensation on windows. Consider a dehumidifier. |
| Above 70% | Too humid | Mold risk is high. Dehumidify immediately. Check for leaks. |
The connection to other metrics: High humidity can cause laser-based PM2.5 sensors to read higher than actual particle levels, because water droplets in the air scatter light similarly to particles. If your PM2.5 spikes but humidity is above 70%, the reading may be partially inflated.
Reading the Colors
Most monitors use a traffic-light color system. The exact thresholds vary by manufacturer, but the general pattern is consistent.
Green means the reading is in the healthy range. No action needed.
Yellow or orange means the reading is elevated. Pay attention and consider acting.
Red means the reading is in the unhealthy range. Take action now.
Some monitors, like the Airthings View Plus, add a small trend arrow showing whether a metric is rising, falling, or stable. This is often more useful than the absolute number, because a PM2.5 of 15 that is falling is very different from a PM2.5 of 15 that is rising.
What to Do When a Number Spikes
The right response depends entirely on which metric is elevated. Here is the decision tree.
PM2.5 or PM10 Spiked
- Check if there is an obvious source (cooking, candle, vacuuming, wildfire smoke outside).
- Turn on your HEPA purifier. Set it to high if the reading is above 35 µg/m³.
- If the source is outdoors (wildfire, construction), close windows and doors.
- If the source is indoors, ventilate after the source activity stops.
- A properly sized purifier should bring PM2.5 back to baseline within 30 to 60 minutes.
CO2 Spiked
- Open a window or door. Even a few inches makes a meaningful difference.
- Turn on exhaust fans (bathroom, kitchen range hood) to pull stale air out.
- If the room has many people, CO2 will keep climbing until you increase airflow or reduce occupancy.
- Do not rely on your air purifier. It filters particles, not CO2.
If you use a monitor alongside a purifier, our guide on pairing monitors with purifiers explains how to use the data to run your purifier more effectively.
VOCs Spiked
- Identify the source. Did you just clean, paint, open new furniture, or use aerosol products?
- Remove the source if possible. Move new furniture to a garage or porch to off-gas.
- Ventilate aggressively. Open windows on opposite sides of the room for cross-ventilation.
- An activated carbon filter helps but works slowly compared to ventilation.
- If VOCs remain high with no obvious source, check for hidden causes: new carpet, fresh paint in adjacent rooms, or a malfunctioning gas appliance.
Humidity Out of Range
- Too dry (below 30%): Run a humidifier. Evaporative models are more reliable than ultrasonic models, which can spike your PM2.5 readings with mineral mist.
- Too humid (above 60%): Run a dehumidifier or air conditioner. Check for leaks, poor drainage, or inadequate bathroom ventilation.
- In summer, AC naturally dehumidifies. In winter, forced-air heating naturally dries air out.
Understanding Trends vs. Snapshots
A single reading tells you what is happening right now. But the real power of an air quality monitor comes from watching trends over time.
Daily patterns. Most homes follow a predictable cycle. CO2 rises overnight in bedrooms, PM2.5 spikes during cooking, and VOCs peak in the morning when cleaning products are used. After a week of monitoring, you will start to see your home's rhythm.
Baseline vs. events. Your baseline is the typical reading when nothing unusual is happening, usually late at night when the house is quiet and no one is cooking or cleaning. Events are temporary spikes caused by specific activities. Knowing your baseline helps you distinguish between "something is wrong" and "dinner is on the stove."
Seasonal shifts. Winter tightens your home's envelope (closed windows, sealed doors), which pushes CO2 and VOCs up. Summer increases ventilation but may bring in outdoor pollutants like ozone and pollen. Wildfire season can spike PM2.5 for days or weeks even with windows closed.
Week-over-week comparison. If you made a change, like adding a purifier, sealing a drafty window, or switching cleaning products, compare this week's average to last week's. Monitors with app connectivity make this easy through historical charts.
When Your Monitor Readings Disagree with How You Feel
This happens more often than you might expect, and both the monitor and your body can be right.
Monitor says "good" but you feel bad. Possible causes: your monitor does not track the specific pollutant affecting you (some monitors skip VOCs or CO2), the monitor is poorly placed (too close to the purifier, too far from where you sit), or the issue is not airborne (dust on surfaces, allergens in bedding).
Monitor says "bad" but you feel fine. Possible causes: you have acclimated to poor air quality (especially common with elevated CO2), the reading is temporarily inflated (humidity affecting the PM2.5 sensor, or a VOC spike from a harmless source like cooking spices), or the monitor is next to an interference source.
If there is a persistent disconnect between your readings and how you feel, revisit your monitor placement before assuming either source is wrong.
Putting It All Together: Your Quick Reference Card
Keep these numbers in mind as your daily targets.
| Metric | Green (Good) | Yellow (Watch) | Red (Act Now) |
|---|---|---|---|
| PM2.5 | Below 12 µg/m³ | 12 to 35 µg/m³ | Above 35 µg/m³ |
| PM10 | Below 20 µg/m³ | 20 to 50 µg/m³ | Above 50 µg/m³ |
| CO2 | Below 800 ppm | 800 to 1,000 ppm | Above 1,000 ppm |
| tVOC | Below 220 ppb | 220 to 660 ppb | Above 660 ppb |
| Humidity | 40 to 60% | 30 to 40% or 60 to 70% | Below 30% or above 70% |
| Temperature | 68 to 72°F | 60 to 67°F or 73 to 78°F | Below 60°F or above 78°F |
The single most impactful thing you can do with your monitor is check it once in the morning and once in the evening for the first two weeks. You will quickly learn your home's patterns, identify the activities that cause the worst spikes, and know exactly which fix to reach for: purifier for particles, ventilation for CO2, source removal for VOCs.
Your monitor is a tool. Like any tool, it is only useful if you know how to read the output and act on it. Now you do.