Absorbed Dose The amount of a substance that actually enters into the body, usually expressed as milligrams of substance per kilogram of body weight (mg/kg). Absorption The process by which toxicants cross body membranes and enter the blood stream or lymphatic system.  Absorption frequently refers to the movement of a chemical from outside the body across membrane barriers to gain entrance into the body. Acetylcholine An important chemical in the body having physiological functions, including the neurotransmission of electrical impulses across synapses of nerve endings. Acetylcholinesterase An enzyme present in nervous tissue, muscle, and red blood cells that catalyzes the hydrolysis of acetylcholine to choline and acetic acid. Acid A substance with one or more hydrogen atoms that are readily replaceable by electropositive atoms.  It is a donator of protons.  In aqueous solution, it will undergo dissociation with the formation of hydrogen ions.  It has a pH of less than 7.0. Active Transport The movement of a substance across a membrane requiring energy.  The substance moves against a concentration gradient, from a less concentrated region to a more concentrated region. Acute Effect An effect that occurs almost immediately (hours/days) after a single or brief exposure to a toxic agent.  Generally, acute effects will be evident within 14 days. Aerosols Aerosols are suspensions of very small airborne particles of a liquid or solid in a gas. Albumin A simple protein soluble in water and distributed throughout body tissues.  It is the most abundant plasma protein. Allergy An immune hypersensitivity reaction of body tissues to allergens that can affect the skin (urticaria), respiratory tract (asthma), gastrointestinal tract (vomiting and nausea) or produce a systemic circulatory response (anaphylactic response). Alveoli The air sacs at the ends of the tracheo-bronchial tree in which gases are exchanged between inhaled air and the pulmonary capillary blood. Anemia A condition in which there is reduced or impaired red blood cells or hemoglobin resulting in an inadequate capacity of the blood to transport oxygen to body tissues. Anoxia An insufficient (below normal) supply of oxygen in body tissues. Antagonism An interaction between two chemicals in which one decreases the expected toxic effect of the other. Antibody An antibody is a protein molecule (immunoglobulin with a unique amino acid sequence) that only interacts with a specific or closely related foreign substances (antigen).  The antibody is induced (a response of the immune system) as a result of prior exposure to the antigen. Anticholinergic Effects Neurological effects resulting from the blockage of acetylcholine, which transmits impulses across nerve junctions. Antidote A remedy for counteracting a poison. Aqueous Of a watery nature.  Prepared with water. Asphyxiant A substance, that in high concentrations in air, replaces or reduces the oxygen level such that a person inhaling the air mixture suffers hypoxia.