PM10 is particulate matter 10 microns across or smaller; PM2.5 is the finer fraction, 2.5 microns or smaller. PM10 is mostly dust and mostly lodges in the nose and throat, while PM2.5 is mostly smoke and combustion particles and can travel deep into the lungs. Indoors, PM2.5 is the number that matters more, and a HEPA purifier captures both.
What the Numbers Mean
“PM” stands for particulate matter: tiny solid or liquid particles floating in air. The number is a size cut-off in microns (a micron is a thousandth of a millimetre).
- PM10 includes every particle 10 microns or smaller.
- PM2.5 includes every particle 2.5 microns or smaller.
- PM1.0 includes every particle 1 micron or smaller.
These are nested, not separate. All of PM2.5 is also counted inside PM10, and all of PM1.0 is inside PM2.5. So when a monitor shows PM10 higher than PM2.5, that is expected: the PM10 figure includes the fine particles plus the coarser ones between 2.5 and 10 microns. Both are far too small to see individually; a human hair is many times wider than either.
Where Each Comes From in Indian Cities
| Coarser particles (PM10 side) | Fine particles (PM2.5 side) | |
|---|---|---|
| Main outdoor sources | Road dust, construction and demolition, unpaved ground, wind-blown soil | Vehicle exhaust, stubble and crop-residue burning, wood and waste burning, industry, firecrackers |
| Main indoor sources | Sweeping, dusting, shaking bedding, fans lifting settled dust | Frying and tadka, agarbatti and dhoop, mosquito coils, smoking, candles |
| How they behave | Settle out of the air relatively quickly; come back up when disturbed | Stay suspended for a long time; travel far and slip through gaps |
| When they peak | Dry, windy days; near construction; summer dust | Still, cold evenings and nights; October to January in north India |
This is why a dusty house in summer and a smoky winter evening feel different. Dust you can often see on surfaces; our guide on why your house gets so dusty covers that side. Winter smog is mostly the fine fraction, which you cannot see settling anywhere.
Why PM2.5 Does More Harm
The body’s airways act like a series of filters. Larger particles are mostly caught in the nose and throat and cleared by mucus, sneezing and coughing. The smaller the particle, the further it gets:
- Coarse particles in PM10 mostly irritate the nose, throat and upper airways. That is the sneezing, itchy throat and dust-allergy side of air pollution.
- PM2.5 can reach the small airways and air sacs deep in the lungs, where the body’s clearing mechanisms are slower.
- Fine particles from combustion also tend to carry the products of burning with them, which is part of why smoke is treated as more harmful than plain dust.
That is why the health limits for PM2.5 are much stricter than for PM10, and why public health agencies focus on PM2.5. If smog season brings on throat burning or watery eyes, see our page on throat irritation and burning eyes in smog; for anything that persists or affects breathing, see a doctor.
In one line: PM10 is mostly the dust you can sweep; PM2.5 is mostly the smoke you cannot see. Indoors in winter, watch PM2.5 first.
PM1.0: The Smallest Number on Your Monitor
Some monitors and purifiers also report PM1.0. It is the finest slice of PM2.5 and is dominated by fresh combustion particles: vehicle exhaust, burning, incense and cooking smoke. There is no Indian standard for PM1.0, so it is best used as a trend signal. If PM1.0 jumps while you are cooking or lighting dhoop, the source is in the room; if it climbs slowly in the evening with the windows shut, it is likely seeping in from outside.
What Each Kind of Sensor Can See
Most home air-quality monitors and purifier sensors use a laser particle sensor. A small fan draws air past a laser beam, the sensor counts the flashes of scattered light, and software estimates the mass in each size range.
- Laser particle sensors report PM1.0, PM2.5 and PM10. They measure particle size, not what the particle is. They cannot tell pollen from smoke from dust.
- Gas sensors (TVOC, eCO₂, NO₂) measure different things altogether: volatile chemicals, an estimate of carbon dioxide build-up, and a traffic-related gas. They do not see particles.
- Older or very cheap “dust” sensors often report a single vague number and struggle with the finest particles. A monitor that shows only one unlabelled number is not telling you much.
For a full comparison of monitors, apps and what each can measure, read how to check air quality at home.
Boon Airvana’s AtmoScan sensor reads PM1.0, PM2.5 and PM10 with a laser, plus TVOC, eCO₂ and NO₂, every second, so you can see which kind of pollution is in your room.
See Boon Airvana →How a HEPA Purifier Handles Both
A HEPA filter is a dense mat of fine fibres. It does not work like a sieve with holes of a fixed size; particles are caught by several mechanisms at once. Large particles slam into fibres, mid-sized ones are intercepted as they pass close to a fibre, and the very smallest ones zig-zag randomly and stick when they touch a fibre. There is a size in between, around 0.3 microns, that is hardest to catch, and HEPA filters are rated at that hardest size.
- PM10: the pre-filter catches the largest dust and hair; the HEPA layer catches the rest of the coarse fraction easily.
- PM2.5 and PM1.0: captured by the HEPA layer. An H13 grade filter captures 99.95% of particles in a single pass at its rated test size.
- Gases and smells: not captured by HEPA at all. That is the job of an activated carbon layer, and even carbon only helps with some gases. Our guide on HEPA vs ionizer vs UV purifiers explains why HEPA is the technology to rely on for particles.
The filter grade is only half the story. How much air passes through the filter each hour (the CADR) decides how quickly the room’s particle level actually falls.
Indian Limits and Which Number to Watch
| Standard | PM2.5 | PM10 |
|---|---|---|
| India NAAQS, 24-hour | 60 µg/m³ | 100 µg/m³ |
| India NAAQS, annual | 40 µg/m³ | 60 µg/m³ |
| WHO 2021 guideline, 24-hour | 15 µg/m³ | Not covered here |
| WHO 2021 guideline, annual | 5 µg/m³ | Not covered here |
For daily decisions in an Indian home: in winter, watch PM2.5 and keep it under 60 µg/m³ at the very least, as close to 15 as you can manage in the bedroom. In dusty months, PM10 tells you when to close up and when the sweeping has stirred things up. PM1.0 tells you when something in the room is burning.
Frequently Asked Questions
What is the difference between PM2.5 and PM10?
PM10 is all particulate matter 10 microns or smaller; PM2.5 is the finer part, 2.5 microns or smaller. PM2.5 is counted inside PM10. PM10 in Indian cities is dominated by road dust and construction, while PM2.5 comes mostly from combustion: vehicles, stubble and waste burning, firecrackers, and indoor smoke from cooking and incense.
Which is more harmful, PM2.5 or PM10?
PM2.5 is generally considered more harmful because the particles are small enough to reach deep into the lungs, where they are cleared more slowly. Coarser PM10 particles are mostly trapped in the nose and throat. That is why PM2.5 limits are much stricter. Both are worth reducing, but indoors in winter PM2.5 is the number to watch first.
Why is my PM10 reading always higher than PM2.5?
Because PM10 includes PM2.5. The PM10 figure counts all the fine particles plus the coarser ones between 2.5 and 10 microns, so it is always equal to or higher than PM2.5. A big gap between the two means a lot of coarse dust; readings close together mean the air is mostly fine particles, typical of smoke and winter smog.
Does a HEPA air purifier remove both PM2.5 and PM10?
Yes. A HEPA filter captures both coarse and fine particles, and an H13 grade filter captures 99.95% of particles in a single pass at its rated size. How quickly the room clears depends on the purifier’s CADR relative to the room size and on keeping the door closed. HEPA does not capture gases or odours.
What is PM1.0 and should I worry about it?
PM1.0 is particles 1 micron or smaller, the finest part of PM2.5. It mostly comes from fresh combustion such as vehicle exhaust, incense and cooking smoke. India has no separate standard for it, so treat it as a signal: a sudden PM1.0 spike usually means something is burning in or near the room.
Can an air quality sensor tell if particles are pollen or smoke?
No. Laser particle sensors measure particle size and estimate mass in each size range; they cannot identify what a particle is made of. You can make reasonable guesses from patterns, for example a PM1.0 spike while cooking, but a home sensor cannot confirm pollen, mould spores or smoke specifically.
See PM1.0, PM2.5 and PM10 in your room separately, every second. Boon Airvana pairs a laser particle sensor with an H13 HEPA filter.
See Boon Airvana →