Most particle pollution research and air quality regulations are focused on PM2.5. However, there is growing evidence that ultrafine particles, which are sometimes also referred to as nanoparticles, are a particular concern.

What are ultrafine particles?

Particulate matter is classified by size. Fine particles, or PM2.5, are 2.5 μm (micrometers, or microns) in diameter and smaller. 2.5 μm is about 30 times smaller than the diameter of an average human hair.  

Ultrafine particulate matter, or PM0.1, includes particles that are 0.1 μm, or 100 nm (nanometers), in diameter and smaller. PM0.1 is a subset of PM2.5.

A majority of wood smoke particles are in the ultrafine size range.

As with other combustion mixtures, such as diesel and tobacco smoke, fresh wood smoke contains a large number of ultrafine particles…

Naeher, et al. Woodsmoke health effects: A review.

Less mass, but greater numbers and surface area

For the same amount of mass, ultrafines have a much higher particle number and surface area than larger particles.

For example, a million ultrafine particles have the same mass as just one PM10 particle, and 100 times the surface area.

More reactive than larger particles

Ultrafines have a higher surface to volume ratio than larger particles, which allows them to more easily absorb organic compounds. This makes them more reactive than larger-sized particles.

Bypassing the blood/brain barrier

Once inhaled, ultrafine particles can enter the bloodstream, and they can become embedded in virtually any cell or organ of the body.

They can also travel up through the nose and, rather than passing down into the lungs, be delivered directly into the brain and central nervous system via the olfactory nerve, bypassing the body’s protective blood/brain barrier.

Ultrafines not considered with regulatory air monitoring

Regulatory air quality monitoring is based on the particles’ mass.

Since ultrafines have very little mass but occur in greater numbers, PM2.5 readings that are based on particle mass likely don’t fully reflect the associated health hazard of air pollution sources that, like wood smoke, contain a large number of ultrafines.