Two Different Pollutants, Two Different Problems
When people talk about indoor air quality, CO2 and PM2.5 are the two metrics that come up most often. While both indicate air quality problems, they measure fundamentally different things and require different solutions. Understanding each is key to a complete indoor air quality strategy.
What Is PM2.5?
PM2.5 refers to particulate matter smaller than 2.5 micrometres in diameter — roughly 30 times thinner than a human hair. Sources include:
- Cooking (especially frying and grilling)
- Candles and incense
- Outdoor pollution infiltrating indoors
- Wildfires
- Tobacco smoke
PM2.5 penetrates deep into the lungs and enters the bloodstream. Long-term exposure is linked to cardiovascular disease, respiratory illness, and premature death. The WHO recommends annual average exposure below 5 micrograms per cubic metre.
What Is CO2?
CO2 is a gas produced by human respiration and combustion. It is not toxic at typical indoor levels but serves as a powerful proxy for ventilation adequacy. High CO2 means air is not being replaced fast enough, which concentrates all indoor pollutants — not just CO2. Learn the specific thresholds in our PPM guide.
Key Differences
| Factor | PM2.5 | CO2 | |---|---|---| | Type | Solid particles | Gas | | Sources | Cooking, smoke, outdoor air | Human breathing, combustion | | Solution | Air purifier (HEPA filter) | Ventilation (fresh air) | | Health effect | Lung and heart damage | Cognitive impairment, poor sleep | | Measurement unit | Micrograms per cubic metre | Parts per million (ppm) |
Which Matters More?
Both matter, but they require different responses. High PM2.5 calls for filtration; high CO2 calls for ventilation. In fact, opening a window to fix CO2 could temporarily raise PM2.5 if outdoor air is polluted (during wildfire season, for example).
The ideal approach is a multi-sensor monitor that tracks both. See our best CO2 monitors guide for models that include PM2.5 sensing. You can also explore our CO2 monitors vs air purifiers article for guidance on when to ventilate versus filter.
Use the CO2 calculator to understand your ventilation baseline, then layer in PM2.5 monitoring for a complete picture.