A Critical Flow Device for Accurate PM₁₀ Sampling and Correct Indication of PM₁₀ Dosage to the Thoracic Region of the Respiratory Tract

Abstract

A state-of-the-art flow controller is presented that maintains constant volume flow at ambient conditions. It utilizes the concept of critical flow and is extremely simple in design. It is insensitive to pressure changes; the flow varies only with the square root of ambient absolute temperature, a very small correction that, except under extreme circumstances, can probably be ignored. Both theory and field data are presented.

Included is a discussion of the present practice in use by the U.S. EPA to convert the sampled ambient volume to EPA reference conditions (298 K, 101.3 kP_a). The intent of size selective ambient monitoring is to mimic tracheobronchial deposition to provide an indicator of particulate matter dosage to the thoracic region of the respiratory tract. Below 2400 m humans breathe at essentially constant volume, where this volume is expressed at ambient pressure and body temperature (37°C). Thus, to simulate dosage to the respiratory tract with an air sampler, we must sample at constant volumetric flow rates at ambient pressure and temperature conditions and, if necessary, correct for the difference between ambient and body temperature. It is inappropriate from both the standpoint of accurate PM₁₀ sampling and normal behavior of the respiratory tract to operate a PM₁₀ sampler using mass flow control and then to calculate mass concentration values using a reference mass of air, rather than the actual volume of air that existed and passed through the sampler during normal operation.