Partition Coefficient see Octanol/water partition coefficient. Passive Transfer The movement across a membrane by simple diffusion. Pathology The branch of medicine that involves the functional and structural changes in tissues and organs which are caused by disease. PEL Permissible Exposure Level.  The standard stipulated by OSHA for the highest safe level of exposure to a chemical in the workplace. Percutaneous Movement through the skin. Pharmacokinetics Quantitation of the time course of chemical absorption, distribution, metabolism, and elimination. Pharmacology The science that deals with the origin, nature, chemistry, effects and uses of drugs. Phototoxic The enhanced toxicity of a substance in or on the skin due to exposure to light (usually ultraviolet light). Physiologically-Based Pharmacokinetic Model (PB-PK) A risk assessment model that quantitates risk using biological data on the absorption of a foreign substance, its distribution, metabolism, storage is tissues, and elimination. Picogram (pg) A unit of weight consisting of one quadrillionth of a gram (1 x 10-12 g). Poison A substance capable of causing toxicity when absorbed into the body in a relatively small quantity. Polyploidy An increase in the normal number of chromosomes. Power of the Study The statistical ability of a study to detect an effect. PPB Parts per billion.  The number of units of a substance in a billion units.  PPB is a common concentration unit for dilute samples of dissolved substances or airborne substances. PPM Parts per million - the number of units of a substance in a million units.  PPM is a common concentration unit for dilute samples of dissolved substances or airborne substances. Primary Dermal Irritation Test A test with laboratory animals (usually rabbits) that determines dermal toxicity of a substance when applied to the skin.  It is manifest mainly by erythema, edema, and eschars. Probit Model A risk assessment model that assumes log normal distribution for tolerances of an exposed population.  It is generally considered inappropriate for the assessment of cancer risk. Prospective Cohort Study An epidemiology study in which cohorts are identified according to current exposures.  The cohort is followed over time for the development of specific effects, such as cancer.