Many doing critical cleaning have used aerosol dispensed cleaning agents to reach certain portions of valuable surfaces so that the cleaning agent could do its “magic” and be removed with soil by a person using a fabric wiper. In this column, I want to address the chronic health hazard to the respiratory system associated with doing that work, and in the subsequent column the acute flammability hazard to staff of the same. While this may be old business for some, others may not be aware of either hazard because aerosol-dispensed cleaners are common and have been used for so long.

AEROSOLS DEFINED
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.