Aerodynamic diameter (dae) refers to the size of particles. It is the diameter of a sphere of unit density that behaves aerodynamically (has the same settling velocity in air) as the particle of the test substance. It is used to compare particles of different size, shape, and density, and to predict where in the respiratory tract such particles may be primarily deposited.
Exposure response is the relationship between the exposure concentration and the measured toxic response, whether expressed as a group mean ±standard deviation) in the case of a continuous variable or as incidence in the case of a quantal variable. This definition should not preclude the exploration of other dose metrics in establishing this relationship.
Geometric standard deviation (GSD) is a dimensionless number equal to the ratio between the mass median aerodynamic diameter (MMAD) and either 84% or 16% of the diameter size distribution (e.g., MMAD = 2 [MICRO]m; 84% = 4 [MICRO]m; GSD = 4/2 = 2.0.) The MMAD, together with the GSD, describe the particle size distribution of an aerosol. Use of the GSD may not be valid for non-lognormally distributed aerosols. (If the size distribution deviates from the lognormal, it shall be noted).
Inhalability is the ratio of the number concentration of particles of a certain aerodynamic diameter, dae, that are inspired through the nose or mouth to the number concentration of the same dae present in the inspired volume of ambient air. In humans, inhalability can exceed 15 [MICRO]m dae, whereas inhalability dramatically decreases for particles above 4 [MICRO]m dae in small laboratory animals.
Lower respiratory tract consists of those structures of the respiratory tract below the larynx.
Mass geometric mean aerodynamic diameter or the mass median aerodynamic diameter (MMAD) is the calculated aerodynamic diameter that divides the particles of an aerosol (a gaseous suspension of fine liquid or solid particles) in half, based on the weight of the particles. By weight, 50% of the particles will be larger than the MMAD and 50% of the particles will be smaller than the MMAD.
Particle regional deposition is the fraction of inhaled particles that deposits in the specific region of the respiratory tract. The major mechanisms of particle deposition in the respiratory tract include impaction, sedimentation, diffusion, interception, and electrostatic precipitation. The deposition mechanism that is dominant for a given region depends on the respiratory tract architecture and ventilation rate of the species and the aerosol particle size and distribution. The respiratory tract in both humans and various experimental mammals can be divided into three regions on the basis of structure, size, and function:
Respiratory effects are any adverse effects on the structure or functions of the respiratory system related to exposure to a chemical substance.
Target organ is any organ found to be a target of toxicity in the 4-hour (hr) high concentration group as a result of:
Toxic effects are any adverse changes (a change that is statistically and biologically significant) in the structure or function of an experimental animal as a result of exposure to a chemical substance.
Upper respiratory tract consists of those structures of the respiratory tract above and including the larynx.
40 C.F.R. §799.9135