Indoor air particle sizes: what air purifiers actually remove

Why particle size matters for filtration

Not all particles are the same size, and not all filters catch them equally. Size is measured in microns, where one micron is one millionth of a meter. A human hair is about 75 microns thick, which means most airborne particles are invisible without a microscope.

A filter works by trapping particles as air passes through. Larger particles are easier to catch because they cannot slip between the fibers. Smaller particles are harder to catch because they can follow the airflow around obstacles. There is a worst-case size called the Most Penetrating Particle Size, or MPPS, where a filter is at its least effective.

For HEPA filters, the MPPS is around 0.3 microns. This is why HEPA standards specify 99.97 percent efficiency at exactly this size: it is the point where the filter works hardest. Particles smaller or larger than 0.3 microns are actually easier to capture.

What each size range means

• Particles larger than 10 microns settle quickly on surfaces and are easiest for filters to remove.
• Particles from 1 to 10 microns stay airborne longer and require stronger airflow to capture.
• Particles smaller than 1 micron, including cooking smoke and wildfire smoke, stay suspended longest and demand maximum filter efficiency.

The difference between size and health impact

Particle size affects how deep into your lungs it travels. Large particles like pollen stop in your nose and throat. Medium particles like dust mite allergen reach the main airways. Very small particles like smoke and bacteria penetrate deep into the alveoli, where gas exchange happens.

This is why wildfire smoke is harder on people with asthma than pollen: the smaller the particle, the deeper it can reach. A purifier that removes small particles will also remove large ones, because the same filter medium works on both. The challenge is that removing small particles requires a filter dense enough to slow airflow, which is a trade-off in any unit.

How HEPA standards measure performance at different sizes

HEPA stands for High-Efficiency Particulate Air, and the standard is defined in EN 1822 for Europe and ASME B29.100 for the United States. The test uses a monodisperse aerosol, meaning all particles are exactly one size.

The test runs at six sizes: 0.3, 0.5, 1, 2, 3, and 5 microns. The filter is rated at whichever size shows the lowest efficiency. For most HEPA filters, that worst case is 0.3 microns, which is why 99.97 percent at 0.3 microns is the standard definition.

Real-world air is polydisperse: it contains a mix of sizes all at once. A filter tested at 0.3 microns will perform better at other sizes, so the 99.97 percent efficiency is the floor, not the ceiling. This standard is what allows HEPA filters to be the reference point for any air purifier claim.

Why not all particles are filtered out

Even a perfect HEPA filter removes 99.97 percent, which means 0.03 percent of particles pass through. In a room with continuous pollution from cooking or smoking, new particles enter faster than the filter can remove existing ones. The goal of air change rate is to run enough cycles to remove most particles, not all.

The other reason particles remain is that filters cannot catch particles that are not in the airflow. A filter in the corner removes only air flowing past it. If a source is on the opposite side of the room, that air has to travel, mix, and eventually reach the intake. This is why air purifier placement matters and why most people need more than one unit in a large home.

Monitor readings and what they measure

Air quality monitors usually report PM2.5 and PM10 concentrations. PM2.5 means particulate matter 2.5 microns and smaller, measured in micrograms per cubic meter of air. PM10 is 10 microns and smaller.

A PM2.5 reading of 35 micrograms per cubic meter is moderately unhealthy. The same reading at PM10 tells you nothing, because it includes pollen and dust that do not penetrate your lungs as deeply. This is why PM2.5 is the standard for indoor air quality guidance, and why many monitors show only PM2.5.