In general, chemicals in the gas phase are at their most hazardous from the point of view of acute toxicity since the lungs have evolved to facilitate uptake of oxygen and absorb most gases readily. In addition, absorbed gases enter the general blood circulation directly, unlike substances absorbed from the intestine, which are often transformed by the liver before entering the general circulation in the form of derivatives. Some transformation to derivatives may occur in the lung and, as with transformations in the liver, the derivatives produced may be either more toxic or less toxic. Assessment of these transformations must be done on a case by case basis. Vapour Vapours are the gaseous form of substances which are normally in solid or liquid form at the existing temperature and pressure.
Vapours are in equilibrium with the solids or liquids from which they originate.
The equilibrium changes with changes in temperature and pressure. Vaporization increases with increase in temperature or with decrease in pressure. Usually the size of the particles in an aerosol is sufficiently small to allow them to remain suspended for long enough to be widely dispersed. Amongst aerosols, a distinction is made between dusts, fumes, smokes, and mists and fogs. The nature of any dust is highly dependent on the process that produced it.