An aerosol is composed of solid or liquid particles of microscopic size dispersed in a gaseous medium. For a proper assessment of the toxic hazard, the size of the aerosol's particles is important. Particles above 1 micrometer tend to deposit in the upper respiratory tract. Particles less than 1 micrometer in diameter enter the lung. Inflammation of tissue results from concentration far below those needed to cause corrosion.
Irritants can also cause changes in the mechanics of respiration and lung function.
A primary irritant exerts no systemic toxic action because the products formed on the tissue of the respiratory tract are non-toxic or because the irritant action is far in excess of any systemic toxic action. A secondary irritant's effect on mucous membranes is over-shadowed by a systemic effect resulting from absorption. Corrosives are chemicals which may cause visible destruction of or irreversible alterations in living tissue by chemical action at the site of contact. Asphyxiants have the ability to deprive tissue of oxygen. Simple asphyxiants are inert gases that displace oxygen.