Abstract
This paper reviews the scientific issues related to exposure concentrations and particle sizes used in acute inhalation limit tests. The current United States Environmental Protection Agency (USEPA) recommended exposure concentration for such tests is 5 mg/liter, while this level is very high, it is often achievable. On the other hand, its toxicological relevance is questionable. The USEPA recommendation that 25% of the particle distribution be less than 1 μm is a more difficult issue to address. Physical laws for aerosol particle generation and behavior limit the minimum size of particles in an exposure atmosphere at a concentration of 5 mg/liter. Particle size also influences deposition site in the respiratory tract. Since damage to any region of the respiratory tract can produce lethality, and it is not possible to predict, a priori, the most responsive region of the tract or the most harmful particle size of an untested agent, acute limit testing should employ particles in a size range that deposits throughout the entire rodent respiratory tract. Particles between 1 and 4 μm mass median aerodynamic diameter (MMAD) are well suited for such studies. It is, therefore, recommended that the limit test concentration should be the highest concentration (up to 5 mg/liter) that can be achieved while still maintaining a particle size distribution having an MMAD between 1 and 4 μm.
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