Small Kills — How Inhaled Particles cause Health Risk

Indoors and outdoors, the air we breathe is full of particulate matter—a mixture of solid particles and liquid droplets. The size of the particulates is a critical factor in understanding its safety. The size of a particle determines its behavior in the air, as well as its effects within the human body.

Meet the Micrometer

The size of particulate meter is given in units ofmicrometers, abbreviated as µm. The length of a micrometer is so small as to be difficult to conceptualize because it falls into a range that the unaided human eye can no longer see. We can barely make out 1 millimeter and there are 1,000 micrometers in 1 millimeter. For reference here are objects and their size in micrometers:

  • 100 micrometers — the thickness of human hair
  • 10 micrometers — the size of a water droplet in a mist
  • 1 micrometer — the size of a small bacteria cell

Because of the focus on inhaled particulate matter, the environmental science focuses on debris that is 10 µm and smaller. For one thing, particulate matter larger than that cut-off size will tend to settle out of the air rapidly. Particulate matter in the 5µm range, or smaller, remain airborne indefinitely under most indoor conditions—unless there is removal from an air current. The particle size is therefore the most important determinant of its behavior.

There are three main weight classes the environmental literature uses to discuss particulate matter size. For reference:

  • PM10 refers to particulates 10 micrometers or smaller.
  • PM2.5 refers to fine particulates 2.5 micrometers or smaller.
  • PM0.1 are also known as nanoparticles and refers to superfine particulates 0.1 µm or smaller.

Because of the greater capacity for staying in the air and for adverse human health effects, the environmental literature focuses on PM2.5 levels for describing health risk.

Small Particle, Big Deal

Size is a key determinant for where particulate matter settles within the respiratory tract. Starting with the nose and mouth, the respiratory tract is the passageway for air that ends in the lungs. Particulate matter deposited anywhere along the respiratory tract, including the sinuses, may cause injury. But particulate matter that settles further down this pathway may be more entrenched and harder for the body to clear out.

In general, the smaller the particulate matter, the deeper it will go into the respiratory tract. Particulate matter 5µm and larger will rarely pass beyond the head because the larger size will allow the sinuses to capture the particle. As the particulate size diminishes, more of the particulate matter can progress past the head and into thelower respiratory tract, which includes the lungs. At 1µm, virtually all of the particulate matter will progress beyond the nose.

The smaller the particle, theharder it is for the respiratory tract to clear it, regardless of the site. The deepest a particulate can travel within the respiratory tract is the end air sacs of the lungs, known as the alveoli. Due to this particulate matter behavior, 6-12 µm debris will settle into the upper airways of the head and neck and only the particles smaller than 5µm will get into the lower respiratory tract.

Particulate matter has human sources. According to the World Health Organization, two thirds of PM2.5 in developed countries arises from human activity such as the combustion of fossil fuels, biomass burning, and ammonia emissions from agricultural operations. Natural sources of particulate matters includes airborne soil, pollen, and volcanic ash.

Particulate matter can also arise from biological sources. Molds, bacteria, and viruses can aerosolize into the air. As the COVID pandemic illustrated, airborne viral particles traveling on aerosolized droplets from breathing, sneezing, and coughing was the central mechanism for disease transmission.

Particulate Matter and Water-Damaged Buildings

Within water-damaged buildings, the particulate matter can rise above outdoor levels because the proliferation of mold and bacteria, along with the degeneration of building materials. In one environmental study, the PM2.5 of a water-damaged school was over three times higher than a control school, which had not suffered major water intrusion events.

Microbial pollution is small. The aerodynamic diameter—the length of the particle when in flight—of a Stachybotrys spore is about 4µm, which places it on the larger side for a spore in the fungal Kingdom. Actinobacteria, toxic bacteria that also grow under high dampness conditions, are smaller, with spores ranging from 0.57 to 1.28 µm. Under the 5µm size, the spores of nearly all microbes growing within a water-damaged building can rise and remain into the air.

But things get even smaller.

For every fungal spore in the air, there are about 300 to 500 fragments. Most of the fragments are of a submicrometer size, below 1µm. These fine and superfine particles represent the greatest harm because they stay air borne longer and, when inhaled, penetrate deeper into the respiratory tract…and beyond.

Particulate Matter Injury

In the respiratory tract, particulate matter can cause direct injury such as an inflammatory response, hyperreactivity of the airways, and impairment of defense mechanisms that protect against infection.

The small size of the particles allows them to travel beyond the respiratory tract and directly into the blood. From the circulation, the smallest particles can travel to and damage multiple organs—most notably, the brain, blood vessels, and digestive tract. This process of translocation explains how a particle inhaled via respiration can travel to virtually any part of the human body.

Translocation, the spread of inhaled particles beyond the respiratory tract, causes multiple injury mechanisms that include:

  • Systemic transport of cytokines — the spread of lung-generated inflammation into the blood
  • Coagulation properties — change in blood profile toward a tendency to clot that may increase the risks of stroke and heart attacks
  • Vascular dysfunction — injury to the blood vessels, particularly the delicate endothelial lining that, if chronic, begets atherosclerosis
  • Autonomic effects — particles or resultant chemicals from the particles disrupt the nervous system and affects the heart

According to the World Health Organization, the following health effects can result from particulate exposures:

  • Chronic Obstructive Pulmonary Disease (COPD)
  • asthma
  • cardiovascular disease
  • type 2 Diabetes
  • myocardial infarction (heart attack)
  • lung inflammation
  • systemic inflammation
  • endothelial and vascular dysfunction
  • atherosclerosis
  • infection
  • and respiratory cancer.

Particulate matter inhalation is therefore a critical component of, not only of outdoor pollution, but also in the world within.


Vesper, Stephen, et al. “Mold contamination in schools with either high or low prevelance of asthma.” Pediatric Allergy and Immunology 26.1 (2015): 49-53.

World Health Organization. Air quality guidelines: global update 2005: particulate matter, ozone, nitrogen dioxide, and sulfur dioxide. World Health Organization, 2006.

Górny, Rafał L., et al. “Fungal fragments as indoor air biocontaminants.” Applied and environmental microbiology 68.7 (2002): 3522-3531.

Fennelly, Kevin P. “Particle sizes of infectious aerosols: implications for infection control.” The Lancet Respiratory Medicine 8.9 (2020): 914-924.

Brown JH, Cook KM, Ney FG, Hatch T. Influence of particle size upon the retention of particulate matter in the human lung.Am J Public Health Nations Health. 1950;40:450–480.

Harper GJ, Morton JD. The respiratory retention of bacterial aerosols: experiments with radioactive spores. J Hyg. 1953;51:372–385.