A liquid is converted to an aerosol when it is expanded under pressure through an orifice and sheared by the associated frictional forces from a continuum into small (or tiny droplets). Most commercial aerosol cans produce fluid droplets whose size is barely in the visible range (50 to 60 microns) down to the submicroscopic level.
DROP SIZE DETERMINES ALL
The hazard presented by an aerosol droplet to the respiratory system first depends upon the site within the respiratory system where it is deposited, and secondly upon its inherent toxicity at the point of deposition. The path to that site is determined by its aerodynamics in the air stream of breath, and aerodynamics is all about droplet size.
Droplets of a respirable aerosol greater than around 30 microns (possibly barely visible to the eye) aren’t well entrained in incoming air. It is likely many settle (because their inertia outweighs the buoyant force of the moving breath) outside human bodies, though some are trapped in the nose and mouth on breathing. This is called an inertial separation mechanism.
Droplets sized between around 10 to 30 microns (not visible to the eye) penetrate into the curving torturous path that is the throat (pharynx) but impact on and stick to wet tissue surfaces. So they become deposited in the airways of the head.
SOLVENT AEROSOL VS. SOLVENT VAPOR
Solvent in the vapor phase would penetrate to this position as well. But the crucial difference, what makes exposure limits of solvent aerosols so low, is essentially density of contact.
The mass of liquid solvent held in a droplet of less than 1 micron diameter contains huge numbers of solvent molecules which are all deposited where that droplet becomes trapped on wet lung tissue, while vapor also contains huge numbers of solvent molecules which condense over the more than fifty square meters of wet lung tissue.1
The effect of inhaling aerosols is to damage one’s lungs by administering a large dose of whatever toxic harm the solvent presents to a myriad of sites where oxygen transfer with the blood is accomplished. Aerosols thus become an amplifier for application of toxic damage to the lungs—the applied dose is exaggerated vs. exposure to solvent vapor. And solvents which are less toxic, and have higher exposure limits, must be treated as if they were more toxic, and have lower exposure limits.
EXPOSURE LIMITS (GASP!)
TLVs® for solvent aerosols are typically two orders of magnitude below those for solvent vapors. Yes, that’s two orders of magnitude because of the concentration effect noted above.2
RELIEF FROM THE HOSPITAL
The hazards described above can be overcome, as can most, involved with the use of solvents. The approach is simple: obtain and use personal protective equipment. An operator applying solvents to surfaces via aerosol delivery should at a minimum wear an N95 hospital breathing mask (and possibly appropriate gloves) during and after use; at maximum, wear a self-contained respiration system.
Proven specifications are that it will remove more than 95% of particles (droplets) whose sizes are above 0.3 microns. Every hospital supply store dispenses them at a reasonable cost.
FIRE NEXT TIME
In next month’s column we’ll cover the other hazard of using aerosol-dispensed cleaning solvents, flammability, and present that in a graphic manner which may be new to some.
- Luttrell, W. E. Stull, K. R., and Jederberg, W.W. Toxicology Principles for the Industrial Hygienist, American Industrial Hygiene Association, 2008, ISBN 1931504881.
- Poppendorf, W., Industrial Hygiene Control of Airborne Chemical Hazards, CRC Press, 2006, ISBN 0849395283, Chapter 4, Table 4.1 and Figure 4.2, pages 80 to 82. This analysis is somewhat flawed by there being only a small number of solvent aerosols for which there is epidemiological data and so an exposure limit.