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Taking a Look at Airborne Particulate Matter: Inhalable, Thoracic, and Respirable Fractions

Updated: Nov 18, 2022

Particle pollution refers to the combination of tiny solid and liquid particles in the air. It can be dangerous to inhale because particle pollution may trigger asthma, hospitalization, and premature death.

Particulate matter is one of the major air pollutants since it is composed of inhalable particles that penetrate the thoracic region of the respiratory system, leading to extremely adverse health effects. Breathable, thoracic, and respirable particulate matters are air pollutants that act as major air quality stressors.

What are the sources of airborne particulate matter?

Even if you can’t see it, there are many air pollutants in the air you inhale. Particulate matter can form through primary sources as well as secondary sources.

Primary PM Sources: derived from both human and natural activities

  • Human (anthropogenic) activities such as agricultural operations, industrial processes, combustion of wood and fossil fuels, and construction and demolition

  • Natural (biogenic) activities include windblown dust and wildfires

Secondary PM Sources: air contaminant sources formed in the atmosphere by the transformation of gas emissions that form or help form PM

  • SOx (sulfur oxides), NOx (nitrogen oxides), VOCs (volatile organic compounds), ammonia

Tobacco smoke, wood dust, gas stoves, cleaning products, pets, mold, and respirable crystalline silica (a natural substance found in the earth, bricks, and concrete leading to fine airborne dust) all contribute to airborne particulate matter as well.

What are the effects of particle pollution exposure?

Research has also linked exposure to particle pollution to:

  • difficulty breathing and increased allergy symptoms

  • increased mortality in infants and early deaths

  • increased risk of cardiovascular disease, including heart attacks, strokes, and ischemic heart disease

  • increased development and the severity of asthma among children

  • worsening of COPD (chronic obstructive pulmonary disease) in adults

  • slowed lung function growth in children and teenagers

  • cause cancer harm, nervous system harm (reduced brain volume, cognitive effects), and respiratory harm (e.g., worsened asthma, worsened COPD, inflammation)

Inhalable, Thoracic, and Respirable Fractions:

The word, “aerosol,” is used to describe the suspension of particles in the air. The behavior, deposition, and effects of particles into the human respiratory system are determined by the chemical nature and size of the particle. Aerosol size fractions that relate to the region of the respiratory tract where they penetrate and deposit can be defined by three fractions:

  • Inhalable fraction: airborne particles that enter the nose and mouth during breathing and therefore can deposit anywhere in the respiratory tract

  • Thoracic fraction: inhaled airborne particles that penetrate beyond the larynx

- It has been shown that approximately 50% of the particles in the air with an

aerodynamic diameter of 10 microns belong to the thoracic fraction

- Thoracic sized particles can come from natural sources like volcanoes and dust

storms to man-made sources like power plants, industrial processes, vehicular

traffic, coal burning, and fireplaces

  • Respirable fraction: inhaled airborne particles that penetrate to the lower gas exchange region (nonciliated airways) of the lung

- It has been shown that 50% of the particles with an aerodynamic diameter of 4

microns will be in the respirable fraction

Examples of particles in certain size groups

For size reference, human hair is 50-180 microns in size and a grain of salt is 60 microns in size. The limit of what the naked eye can see is 10-40 micron-sized particles. Particles under 10 microns in size fall under the respirable fraction:

2.5-10 microns: pollen, desert dust, mold spores, pet dander (coarse particles)

  • Impacts the upper respiratory tract

  • Mold spores range from 1-30 microns in size

  • Pet dander range from 5-10 microns in size

  • Dust particles range from 2.5-10 microns in size

<2.5 microns: bacteria, fungi, mold spores, pollen (fine particles)

  • Impact the lower respiratory tract

  • Bacterium range from 1-3 microns in size

<1 micron: viruses, exhaust gases, wildfire smoke, light dust particles

(inhalable particles)

  • Impacts the alveoli

  • Light dust particles range from 0.5-1 micron in size

  • Wildfire smoke can persist in the air for days to months and range from 0.4-0.7 micron in size

<0.1 micron: nanoparticles (ultrafine particles)

  • Impacts the bloodstream/whole body

  • Tobacco smoke particles range from 0.1-4 microns in size

  • The coronavirus is 0.1-0.5 micron in size

  • Other viruses like the zika virus range from 0.005-0.3 microns

HEPA filters are the most recommended to eliminate the smallest of particles such as viruses, mold spores, pet dander, and bacteria and because it captures at least 99.97% of airborne particles that measure .3 microns and larger.

How do air particles enter the human body?

As air is inhaled, it is drawn through the nose or mouth into the upper respiratory system, consisting of nasal passages, the trachea, and the airways conducting the flow down to the lungs comprising the bronchi and bronchioles. The inhaled air becomes moist and makes countless twists and turns through the nasal passages and branching airways.

Our natural defense helps us cough or sneeze out some coarse particles from our bodies but fine and ultrafine particles pose the risk of entering your body and getting trapped in the lungs, passing into the bloodstream, and leading to severely negative health effects.

A majority of fine particles may deposit deep in the lungs because of reduced inertia when you take deep breaths or inhale slowly. They penetrate deep into the acinus, passing through the nasal and conducting airways without depositing.

There are 3 main ways particles deposit once entering the human body:

  1. Impaction- coarse particles primarily deposit in the nasal, pharyngeal, and laryngeal passages, trachea, and bronchi.

  2. Sedimentation -- fine particles primarily deposit in the respiratory bronchioles and alveoli.

  3. Diffusion -- ultrafine particles diffuse to respiratory surfaces and deposit.

Children under 10 have a faster breathing rate compared to older teens and adults which results in the inhalation of higher quantities of air and depending on the air quality, higher quantities of particulate matter exposure.

Why does particle size matter when it comes to inhalation?

While the hazard of airborne particles depends on airborne concentration (how much is deposited) and chemical composition (the fate and biological responses to the presence of particles), particle size is also an important factor because it determines how the particle penetrates and deposits in the human body.

Fine and ultrafine particles are the greatest factor for adverse health effects for they are small enough to pass through the lung tissue and into the bloodstream, while coarse particles have lower influence due to their relatively large size:

  • Particles 10 to 100 microns in size are unable to make the turns and impact on nasal hairs and mucus. Soluble particles trapped will dissolve while insoluble particles are transported up the conducting airways by the ciliated epithelium and swallowed or cast out from the body through coughing or nose blowing.

  • Particles < 10 microns in size generally travel into the pulmonary part of the lungs where respiration occurs (respirable particles). If deposited, they are typically removed by two mechanisms: mucociliary clearance and phagocytosis

    • Mucociliary clearance: primary defense mechanism of the lung; propels pathogens and inhaled particles trapped in the mucous layer out of the airways

    • Phagocytosis: phagocytes ingest or engulf foreign cells, bacteria, and particles

What are the effects of each of the fractions of particulate matter?

  • Inhalable particles: do not stay in the body and pose no health risk

  • Thoracic fraction: can enter the sinuses and airways of the lungs. 10-30 micron particles can be trapped by the body’s natural defenses then breathed, coughed, or sneezed out

  • Respirable fraction: small enough particles to penetrate the natural defense of the nose, throat, and lungs. Can cause irritation, inflammation, silicosis, and lung cancer

Tips to reduce the effects of poor air quality and to breathe easier:

  • Avoid outdoor activities in the afternoons on warm days (the risk of air pollution is often high during this time)

  • Check your regions’ air quality index using to see if you should avoid strenuous outdoor activities

  • Increase ventilation through the opening of windows and maintaining HVAC systems as well as avoiding smoking, burning candles/incense, lighting a fireplace, and wet wiping dusty areas

  • A HEPA filtration system may clean up dust and reduce inhalation (see our blog about air purifiers)

How can BNF Consulting help with airborne particles?

To increase the protection of workers and families from intakes of airborne dust and particles, airborne particle sampling plays an important role in the survey of the worker’s exposure and for industrial hygiene.

BNF Consulting offers many services to ensure a healthier, safe environment including:

  • Testing and evaluation of indoor air quality

  • Air monitoring, bulk sampling, radiation monitoring

  • Site safety surveys

  • Air duct cleaning for water damage, smoke/fire, mold

  • Making timely, cost-effective, and site-specific recommendations

If you are concerned for your and your family’s safety or curious about air pollution in your home, schedule a free call with us today at 914-297-8335 to learn more about our services. We also offer a free phone/video consultation.


About Authors:

Esther Hur: Computer science in Binghamton University, BNF Consulting internship program participant.

Justin H. Joe, PhD, CIH, CSP, CPE, is a Certified Industrial Hygienist and a principal consultant of BNF Consulting, Inc. Dr. Joe graduated from the University of Illinois at Urbana-Champaign with PhD and MS degrees in nuclear engineering. Dr. Joe has provided industrial hygiene consulting as a core function with his diverse background of experience and education. _________________________________________________________________________


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