Inhaled Insoluble Particles are more than a Nuisance

Many years ago I did my graduate research on the measurement of airborne insoluble particles.  These often  occur from the generation (e.g., spraying or cutting), transport or disturbance of liquids, powders or dusts.  Because of all the work and study I did with these aerosols, I have maintained an interest in this subject. 

Of course, airborne particles are a lot different than vapors.  Compared to vapor molecules they are a lot bigger.  Indeed, they have a lot more mass and instead of being mixed in with the other gaseous molecules in air, they are more or less suspended into the air.  Depending on their size they tend to settle downward either quickly or slowly.   Indeed, knowing the size of the particles is important for other reasons because size determines whether a particle is inhaled and, once inhaled, its chance of making it into the deep lung where gas exchange takes place.

The population of insoluble airborne particles from any source is almost always of variable size which makes the characterization of that size somewhat challenging.   The sizing of a population of aerosol particles will be the subject of next week's blog.

A logical and somewhat convenient way of “sizing” any individual airborne particle is by its aerodynamic diameter (AD).   This convention compares the settling velocity of the particle of interest to that of a unit density sphere.   Thus particles of any density or shape can be characterized for how they will react in the air using this method.   It is interesting to note that I once measured the aerodynamic diameter (AD) of one of my kid’s balloons at about 150 microns (AD).   That is a foot long balloon fell to the ground at about the same rate as a unit density sphere with a diameter of 150 microns (0.015 mm).    On the other hand, 1 micron sphere of mercury will have a much larger AD than 1 micron but you get the idea.

Indeed, the science of aerosols is quite interesting.   Very small particles “see” or experience the air as a very viscous medium compared to their larger brothers.   The more viscous the air appears, the more the particles tend to move with the airstream and not charge through it.   An analogy for us would be that we are very free to move (at least horizontally) through air but we will be much more “stuck” and restricted if we were immersed up to our necks in molasses.  In that case if the molasses moved we would move with it.

As mentioned above the AD size of any particle determines where in the body it might deposit.   Three size measurement classification or “bins” of AD have been defined by the ACGIH going from large to small sizes:

•      INHALABLE: any particle that penetrates/deposits past the nose and mouth.
•     THORACIC: particles that penetrate/deposit anywhere within the lung airways and the gas-exchange region
•     RESPIRABLE: particles that penetrate/deposit exclusively into the gas-exchange region or pulmonary region of the deep lung.

The above classifications describe the proportion of any particular AD size that will fit into that classification or bin.    For example a particle that is 10 microns AD will be considered 77% inhalable,  73% Thoracic and 1% respirable if I read the ACGIH criteria algorithms correctly.   That is, this particular particle will have a 77% probability of being inhaled, a 73% probability of making it to the thoracic region of the respiratory tract and a 1% chance of getting to the alveolar or pulmonary region of the lung.

If you are interested in studying this further I have a set of Power Point slides that goes into a lot more detail including the algorithms which I will be happy to send to whoever ask me at mjayjock@gmail.com  

So what does this mean for the inhalation of insoluble particulate?   When the airborne particle size is respirable (less than 10 microns AD) a good portion of it makes it to the alveolar region of the lung.   The clearance mechanism for insoluble particles in this region of the lung is quite slow.   As such a daily exposure to respirable particles tend to accumulate and potentially overwhelm the clearance mechanism.   This has been termed particulate overload and can lead to irritation and irreversible toxic effects such as Chronic Obstructive Pulmonary Disease (COPD).   

I am not a toxicologist but for many years I have been of the opinion that the OEL for insoluble “Nuisance Particulate” or the more modern term: Particles No Otherwise Characterized (PNOC) should be around 1 mg/m³ (respirable) which is considerably lower than it has been.   I base this view on my review of some of the earlier toxicological studies and conclusions.   Some of these references, data and conclusions are included in the above mentioned Power Point slide deck.