The two filters were connected to a flow meter to monitor the air
flow (L/min) passing through the apparatus, which was maintained be-
tween 6 and 8 L/min. The flow meter was attached with Tygon tubing to
an electric switch box driving solenoid valves to activate or interrupt the
vacuum when the switch was flipped. Two boxes were set up, each to
control two hookah devices. Tygon tubing connected the switch box
to the vacuum line. These smoking systems were contained in a hood
for vapor extraction during the experiment.