Measuring the air in your home
An indoor PM2.5 monitor tells you whether your clean room is really clean. How to place it, record readings and read the trends.
Outdoor AQI tells you what is coming in. Only an indoor monitor tells you what you are breathing. It turns guesswork into feedback: is the purifier big enough, is the night speed too low, did sealing that window help?
What to buy is covered in the monitor spec guide. The short version is an optical PM2.5 sensor that shows µg/m³, not just a colour or an AQI number.
Where to put it
- In the room you are protecting, at about the height where people breathe (sitting or lying), such as a bedside table.
- Not right next to the purifier outlet, which reads falsely low, or next to the window, door or kitchen, which spike with every source.
- Leave it in one place for a few days so you are comparing like with like.
What to record
When testing a change, note the room, time, fan speed, indoor reading, the nearest outdoor station reading, and anything unusual (cooking, incense, door left open). A few days of notes show patterns quickly.
Reading the trends
| What you see | What to check |
|---|---|
| Falls and stays low after the purifier starts | It’s working. Repeat the check at the night-time fan speed. |
| Short spike when cooking or a door opens, then recovers | Expected. Reduce the source (exhaust, door closed) and watch how fast it recovers. |
| Stays high or keeps rebounding | Fan speed too low, purifier too small, filter clogged or badly seated, or a large leak. Compare with the outdoor trend. |
| Fine during the day, worse overnight | The sleep mode is too weak, or a door or window was opened. Try a higher steady speed. |
What “good” looks like
The WHO guideline for PM2.5 is 15 µg/m³ averaged over 24 hours and 5 µg/m³ averaged over a year. These are long-term averages, not limits for a single reading. In a Delhi winter, an achievable aim for a well-run clean room is a large drop from outdoor levels, ideally into the low tens. Keep the averaging period in mind when comparing numbers.
Know the limits of cheap sensors
Consumer monitors estimate PM2.5 by counting particles with a light beam. They are good at showing trends and relative changes, less reliable on the exact number.
- High humidity and fog make them over-read. Damp particles swell and look bigger, so readings on foggy winter mornings may be inflated.
- Different brands can disagree. Compare readings from the same device over time rather than across devices.