Potential for a substance to come in contact with a living organism and then interact with it. This may lead to absorption.

Note: A substance trapped inside an insoluble particle is not bio-accessible although substances on the surface of the same particle are accessible and may also be bio-available. Bio-accessibility, like bio-availability, is a function of both chemical speciation and biological properties. Even surface-bound substances may not be accessible to organisms which require the substances to be in solution.