This diagram provides a simplified illustration of the human respiratory tract.
Outside air enters the nose and/or mouth during breathing,
flowing quickly through the nasal and mouth openings and into the nasal/pharyngeal/laryngeal,
or extra-thoracic, region of the respiratory tract.
This region includes the airways of the nose and mouth,
the pharynx or throat, and the larynx or voice box.
After this, the air enters the lungs, the thoracic region of the respiratory tract.
The tracheobronchial region is the upper part of the lung, the conducting airways that move air to the deep lung.
In this region, air moves first into the trachea or windpipe
and then branches repeatedly into a series of airways referred to as bronchi and bronchioles.
The air moves more slowly as it passes deeper and deeper into the lung.
The air finally reaches the alveolar region of the lung, the deep lung.
Air is almost stationary in the alveoli, providing time for oxygen to diffuse through the alveolar wall
and enter the blood and for carbon dioxide to leave the blood and enter the air that will subsequently be exhaled.
If particles are present in air that is inhaled, it is possible that they may not move with the air into the human respiratory tract.
Particle deposition in the human respiratory tract relates particle exposures to applied doses.
A particle's size is the most important factor determining if and where a particle may deposit.
Before we can even think about a particle depositing in the respiratory tract,
we must know if a particle can even be inhaled into the tract.
Because the smallest particles have little inertia,
all of them move along with the air as it is inhaled to enter the respiratory tract through the nose or mouth.
As we will see, most of these particles will deposit in the respiratory tract with only a small percentage being breathed back out.
Medium-sized particles also tend to move along with the air and are inhaled into the respiratory tract.
However, the majority of these particles fail to deposit in the respiratory tract and are exhaled back out.
Large particles, which have a lot of inertia, may not follow air streamlines as the air turns to be inhaled into the nose or mouth.
Some of these particles will fail to be inhaled at all.
Most of those that are inhaled will deposit within the respiratory tract, but some will leave with the exhaled air.
This figure will show the percentage of particles inhaled, on the vertical axis, as a function of particle diameter, on the horizontal axis,
assuming that the particles are spheres with a density of 1 gram per cubic centimeter, the density of water.
The diameter is presented on a logarithmic scale to cover several orders of magnitude.
Let's assume that the red particles from the animation are 10 nanometers, or 0.01 micrometer, in diameter.
Almost all of these particles, essentially 100%, are inhaled, and we can plot this value on the figure.
Next, let's say that the orange particles are 300 nanometers, 0.3 micrometer, in diameter.
Again, nearly all of these particles, about 99%, are inhaled, and we can plot this on the figure.
Finally, let's assume that the green particles are 10 micrometers in diameter.
A little more than 77% of these particles are inhaled, and once again we show this data point on the figure.
By international agreement, a relationship has been established for inhalation percentages as a function of particle size
for use in particle sampling protocols.
The relationship is shown by this purple curve, and is referred to as the inhalable sampling criterion.
The curve shows that particles larger than about 300 nanometers, or 0.3 micrometer,
are inhaled at lower and lower efficiencies as particle size increases, due to greater particle inertia and gravitational settling
that keep them from being breathed into the nose or mouth.
For particles that are inhaled with the air, they may or may not deposit in the respiratory tract.
If not, they will be breathed back out with exhaled air.
In the nasal/pharyngeal/laryngeal region of the respiratory tract, inhaled air moves through the nasal cavity with turbulent motion,
but with a relatively long residence time.
The air speeds up as it passes through the bends of the pharynx and larynx.
As the air is exhaled, it retraces its path through the larynx, pharynx, and nasal cavity.
When very small particles are inhaled, most pass through the nasal cavity and then the pharynx and larynx.
However, the residence time in the nasal cavity is long enough for some of them to deposit there.
Many of these particles will deposit deeper in the lungs, but some will be exhaled.
Few medium-sized particles diffuse enough to deposit in the nasal cavity,
nor do they have enough inertia to deposit in the pharynx or larynx.
Most will pass through to the lungs upon inhalation,
and the majority of those particles will eventually leave with the exhaled air
rather than depositing anywhere in the respiratory tract.
Large particles have a significant amount of inertia.
Some of these particles will deposit in the pharynx or larynx as air is inhaled.
In addition, some will deposit there as air is exhaled.
With the inhalable sampling criterion already plotted on the figure,
we can look at the percentage of particles deposited in the nasal/pharyngeal/laryngeal region as a function of particle size.
These values were derived by the International Commission on Radiological Protection, or ICRP.
Roughly 20% of the red 10-nanometer particles are deposited in the nasal/pharyngeal/laryngeal region,
whereas only about 4% of the orange 300-nanometer particles will deposit there.
Nearly all of the green 10-micrometer particles that are inhaled end up depositing in the nasal/pharyngeal/laryngeal region,
about 75% of the particles that were originally in the air that is inhaled.
The light blue curve shows the percent of particles deposited in the nasal/pharyngeal/laryngeal region for all particle diameters.
Notably, almost all particles larger than 10 micrometers deposit in this region if they are inhaled due to their inertia.
In addition, a large fraction of the nanoparticles smaller than 10 nanometers
deposit in the nasal/pharyngeal/laryngeal region due to rapid diffusion.
Past the larynx is the tracheobronchial region of the respiratory tract, the upper part of the lungs.
The diaphragm and other muscles cause the lungs to expand and contract.
When the diaphragm tightens and moves downward, space in the chest is created,
allowing for the lungs to expand and air to be inhaled.
Then, when the diaphragm relaxes and moves upward, the space in the chest is reduced,
forcing the lungs to become smaller and air to be exhaled.
The rate of inhalation changes in response to the body's need for oxygen.
Looking more closely, we can see that a large fraction of very small particles that enter the tracheobronchial region
are likely to be collected on the walls of the bronchi or bronchioles due to diffusion caused by Brownian motion.
This may occur both during inhalation and exhalation.
In addition, some of these particles will be able to pass through the tracheobronchial region deeper into the lungs.
Because they diffuse less but have relatively low inertia, most medium-sized particles will not deposit in the tracheobronchial region.
Some will penetrate deeper into the lungs, but most will be exhaled.
Similarly, many larger particles will not have sufficient inertia
or Brownian motion to deposit in the tracheobronchial region and will be breathed back out.
However, a few of these particles will have sufficient inertia to be collected near bifurcations
as larger bronchi split into smaller ones.
A few of these large particles will deposit due to gravitational settling.
Overall deposition in the tracheobronchial region is relatively low.
While about 25% of 10-nanometer particles deposit here,
less than 1% of 300-nanometer particles and only about 1.5% of 10-micrometer particles do.
The medium blue curve from the International Commission on Radiological Protection
shows the percent of particles deposited in the tracheobronchial region for all particle diameters.
Deposition for particles larger than 3 micrometers and smaller than 10 nanometers
would be greater if many of these particles were not already depositing in the nasal/pharyngeal/laryngeal region.
The alveolar region is the deepest part of the lungs where oxygen is supplied to the bloodstream while carbon dioxide is removed.
The alveoli at the ends of the smallest bronchioles in the lung are a little like leaves on the branches of a tree.
Healthy human adults have about 500 million alveoli.
The surface area of all of the alveoli in a human adult is a little larger than a basketball court.
Looking more closely, alveoli appear in clusters.
Inside, air enters these clusters through an alveolar duct into the alveolar sac.
Air passes back and forth from the sac to the individual alveoli through small holes.
Many of the very small particles that penetrate to the alveolar region are able to diffuse into the alveoli and deposit there.
Because the residence time of air in the alveolar region is relatively long
and the dimensions of the alveolar sac and the individual alveoli are relatively small,
very few of the small particles that enter the alveolar region will leave with the exhaled air.
Similarly, some medium-sized particles will be able to diffuse into the alveoli and deposit there.
However, some of these particles will be able to leave the alveolar region with the exhaled air
because they diffuse less than smaller particles.
Very few large particles are able to reach the alveolar region
because they have already deposited in the nasal/pharyngeal/laryngeal or tracheobronchial regions.
However, the few large particles that reach the alveolar sac may have sufficient time to deposit due to gravitational settling.
We can compare deposition in the alveolar region to deposition in the other regions.
About 42% of the red 10-nanometer particles deposit in the alveolar region, primarily due to diffusion.
Unsurprisingly, deposition is lower for larger particles.
A little less than 6% of the orange 300-nanometer particles and about 2% of green 10-micrometer particles
deposit in the alveolar region.
Deposition of the 10-micrometer particles is so low, in part,
because so few of these particles remain by the time inhaled air reaches the alveoli.
The dark blue curve shows the percent of particles deposited in the alveolar region for all particle diameters.
Deposition of nanoparticles smaller than about 20 nanometers declines as diameter decreases
because these particles deposit in the nasal/pharyngeal/laryngeal and tracheobronchial regions.
The curves for the three regions from the ICRP can be summed to plot a total deposition curve in gray.
When compared with the inhalation curve, the total deposition curve indicates that almost all particles larger than 5 micrometers
will deposit in the respiratory tract if they are inhaled in the first place.
In addition, more than 75% of particles smaller than 20 nanometers will deposit.
However, in between these sizes, deposition is considerably lower.
Fewer than 15% of 300-nanometer particles will deposit in the respiratory tract,
because the mechanisms of gravitational settling, inertial impaction, and diffusion do not influence these particles substantially.
For particles that deposit, where they deposit in the respiratory tract helps to determine the health consequences of particle exposures.
Depending on where particles deposit, they may be cleared from the respiratory tract by alveolar macrophages
and the mucociliary escalator, often through the gastrointestinal tract, without causing harm;
or they may cause direct injury to the respiratory tract where they deposit;
or they may translocate to other parts of the body via blood or lymphatic circulation or along sensory neurons,
potentially leading to damage of organs or tissue outside of the respiratory tract.