Introduction
Your smart air quality monitor is beeping. The display shows a confusing array of numbers—PM2.5: 35, CO2: 1200, TVOC: 0.8. Should you open a window? Call an electrician? Panic?
If you’ve ever stared at an air quality monitor wondering what all those acronyms actually mean, you’re not alone. The world of indoor air quality is filled with technical jargon that manufacturers assume everyone understands. But most of us just want to know one thing: Is the air in my home healthy or not?
This guide decodes the most common air quality measurements, explains what they mean for your health, and gives you practical thresholds you can use to interpret your monitor’s readings.
The Big Picture – Why Indoor Air Quality Matters
Before diving into individual pollutants, it’s worth understanding why this matters. We spend approximately 85-90% of our time indoors . The air in our homes can be significantly more polluted than outdoor air—in fact, indoor concentrations of some pollutants can be 2-5 times higher than outdoors .
Poor indoor air quality has been linked to a wide range of health effects, from immediate symptoms like headaches and fatigue to long-term risks including respiratory disease and cancer . The good news? These are problems you can do something about—once you understand what you’re measuring.
The Key Measurements – A Practical Glossary
Carbon Dioxide (CO2)
What it is: Carbon dioxide is a colourless, odourless gas that we exhale with every breath. It’s the primary greenhouse gas emitted through human activity and a major contributor to climate change .
Where it comes from indoors: Mostly us. Human respiration is the primary indoor source. In poorly ventilated spaces, CO2 builds up throughout the day as people breathe .
Why it matters: CO2 itself at typical indoor levels isn’t toxic, but it’s an excellent proxy for ventilation. High CO2 means stale air—and where CO2 accumulates, so do other pollutants, including viruses and bio-effluents .
At elevated levels, CO2 can cause:
- 1000-1500 ppm: Drowsiness, reduced concentration, stuffy air
- 1500-2000 ppm: Headaches, fatigue, perceived poor air quality
- >2000 ppm: Significant health concerns, prolonged exposure risky
What the numbers mean:
| Level (ppm) | Meaning | Action |
|---|---|---|
| < 800 | Excellent ventilation | No action needed |
| 800-1000 | Moderate ventilation | Monitor |
| 1000-1500 | Poor ventilation | Open windows briefly |
| >1500 | Very poor ventilation | Ventilate immediately |
The 1000 ppm benchmark: The most common indoor CO2 limit is 1000 ppm . This threshold originated from studies on body odour perception rather than direct health effects—at 1000 ppm, most people find the air acceptably fresh . Many guidelines use this as a target for acceptable ventilation.
One-minute summary: CO2 is your ventilation tracker. High numbers mean you need fresh air—not just for the CO2 itself, but for all the other pollutants building up with it.
Particulate Matter (PM2.5 and PM10)
What it is: Particulate matter refers to tiny particles suspended in the air. The numbers refer to their size in micrometres (µm). For perspective, a human hair is about 70µm in diameter.
- PM10: Coarse particles up to 10µm (inhalable)
- PM2.5: Fine particles up to 2.5µm (can penetrate deep into lungs)
- PM1.0: Ultrafine particles up to 1.0µm (can enter bloodstream)
Where it comes from indoors: Cooking (especially frying), candles, fireplaces, vacuuming, and even just moving around. Outdoors, traffic, industry, and construction contribute—and outdoor PM infiltrates indoors .
Why it matters: PM2.5 is particularly concerning because these tiny particles can penetrate deep into the lungs and even enter the bloodstream . They contain various toxic substances including heavy metals, bacteria, and viruses .
Health effects include:
- Respiratory issues
- Cardiovascular problems
- Exacerbation of asthma and other conditions
- Long-term exposure linked to premature death
What the numbers mean (UK targets):
| Level (µg/m³) | PM2.5 Meaning | PM10 Meaning |
|---|---|---|
| < 10 | Excellent | Excellent |
| 10-25 | Good | Good |
| 25-35 | Moderate | Acceptable |
| > 35 | Poor | Action needed |
The World Health Organization recommends annual average PM2.5 concentrations below 10 µg/m³ and PM10 below 20 µg/m³ . The UK’s national targets are slightly less stringent.
One-minute summary: PM2.5 and PM10 measure tiny particles you can’t see but can breathe deep into your lungs. High readings often come from cooking, candles, or outdoor pollution seeping in.
Volatile Organic Compounds (VOCs) and TVOC
What they are: Volatile Organic Compounds are a large group of chemicals that vaporise at room temperature. They’re often human-made and found in countless household products .
TVOC (Total Volatile Organic Compounds) is a combined measurement—think of it as the “sum total” of all VOCs detected.
Where they come from indoors:
- Building materials and new furniture (off-gassing)
- Paints, varnishes, and solvents
- Cleaning products and air fresheners
- Cooking and burning activities
- Cosmetics and hobby supplies
Indoor VOC concentrations are typically 2-5 times higher than outdoors, and can spike dramatically during activities like painting or cleaning .
Why it matters: Health effects vary by specific compound, but common VOC exposure can cause:
- Eye, nose, and throat irritation
- Headaches and nausea
- Difficulty breathing
- Long-term exposure linked to more serious conditions
Some individual VOCs of particular concern:
| VOC | Common Source | Health Concern |
|---|---|---|
| Benzene | Tobacco smoke, stored fuels | Carcinogenic |
| Formaldehyde (HCHO) | Particle board, MDF furniture | Irritation, suspected carcinogen |
| Toluene | Paints, nail polish | Neurological effects |
| Styrene | Plastics, insulation | Headaches, fatigue |
What the numbers mean (TVOC):
| Level (mg/m³) | Meaning | Action |
|---|---|---|
| < 0.3 | Excellent | No action |
| 0.3-0.5 | Good | Monitor |
| 0.5-1.0 | Moderate | Identify source, ventilate |
| > 1.0 | Poor | Take action |
One-minute summary: TVOC measures chemical fumes from household products, furniture, and materials. High readings often happen after renovations, new furniture delivery, or heavy use of cleaning products.
Formaldehyde (HCHO)
What it is: A specific VOC that deserves special attention. Formaldehyde is a colourless gas with a strong odour, classified as a human carcinogen.
Where it comes from indoors: Pressed wood products (MDF, particle board), some insulation, tobacco smoke, and combustion sources .
Why it matters: Formaldehyde is particularly concerning because it’s so common in modern furniture and building materials. Health effects include:
- Eye, nose, and throat irritation
- Respiratory issues
- Classified as carcinogenic to humans
What the numbers mean:
| Level (ppm) | Meaning | Action |
|---|---|---|
| < 0.03 | Excellent | No action |
| 0.03-0.05 | Good | Monitor |
| 0.05-0.08 | Moderate | Identify source |
| > 0.08 | Poor | Take action |
The 0.08 ppm threshold is a common guideline—exceed this and you should identify the source and increase ventilation .
Pollen
What it is: Fine powder from plants, trees, and grasses that becomes airborne. While technically a biological particle, many monitors specifically track pollen as a separate category.
Where it comes from indoors: Primarily from outdoors via open windows, doors, and on clothing.
Why it matters: Pollen is a major trigger for seasonal allergies and asthma. For allergy sufferers, knowing when indoor pollen is high helps you decide when to close windows and run purifiers.
What the numbers mean: Pollen counts vary dramatically by season and location. Your monitor’s pollen reading is most useful for tracking relative changes—watching it spike after you open windows, for example.
Understanding the Units
Air quality monitors use several different units. Here’s what they mean:
How These Pollutants Interact
The Indoor/Outdoor Connection
About 80% of indoor PM2.5 comes from outdoor sources . This means even if you never cook or light candles, your indoor air quality is still affected by traffic, industry, and other outdoor pollution sources.
The Ventilation Trade-off
Here’s the dilemma: opening windows dilutes CO2 and VOCs but lets in outdoor PM2.5 and pollen. Closing windows keeps out outdoor pollutants but allows indoor pollutants to build up.
This is why smart air quality monitoring is so valuable—you can make informed decisions based on current conditions. When outdoor PM2.5 is low but indoor CO2 is high, open windows. When pollen counts are high outdoors, run an air purifier instead.
What “Good” Air Quality Looks Like
A truly healthy indoor environment typically shows:
| Measurement | Target Range |
|---|---|
| CO2 | < 1000 ppm |
| PM2.5 | < 10 µg/m³ |
| PM10 | < 20 µg/m³ |
| TVOC | < 0.5 mg/m³ |
| HCHO | < 0.08 ppm |
| Humidity | 40-60% |
Practical Actions Based on Readings
The Bottom Line
You don’t need to memorise all these numbers. The beauty of smart air quality monitors is that they do the hard work for you—flashing red when something’s wrong and staying green when everything’s fine.
But understanding what you’re measuring helps you take the right action. High CO2? Open a window. High PM2.5? Check your cooking habits and consider an air purifier. High TVOC? Maybe that new furniture needs to off-gas in a well-ventilated room.
The single most important takeaway: If your monitor is showing elevated readings, don’t panic. Ventilation solves most problems. Open windows for 5-10 minutes and watch the numbers drop. Then identify the source to prevent recurrence.
Your air quality monitor isn’t just a fancy gadget—it’s a tool that gives you the power to create a genuinely healthier home. Now you know exactly what it’s telling you.
Quick Reference: Air Quality at a Glance
| Pollutant | Symbol | Common Sources | Ideal Level | Action Level |
|---|---|---|---|---|
| Carbon Dioxide | CO2 | Human breath | < 800 ppm | > 1000 ppm |
| Fine Particles | PM2.5 | Cooking, candles, outdoors | < 10 µg/m³ | > 25 µg/m³ |
| Coarse Particles | PM10 | Dust, pollen | < 20 µg/m³ | > 50 µg/m³ |
| Total VOCs | TVOC | Furniture, cleaners, paints | < 0.3 mg/m³ | > 0.5 mg/m³ |
| Formaldehyde | HCHO | MDF, particle board | < 0.03 ppm | > 0.08 ppm |
| Humidity | RH | Breathing, cooking, washing | 40-50% | > 60% |
