Randomization
We randomly assigned
participants to travel in one of two minivans on each study day. One
vehicle was randomly selected to employ cabin air filtration using
an electrostatic air filter while the other had a placebo air filter
(i.e., unfiltered air). Following a 1-week washout period participants
“crossed over” to the other air supply condition, traveling
along the same driving route. Thus, each participant was exposed once
to filtered air and once to unfiltered air over two visits. Drivers
and study participants were blinded to air supply conditions (filtered
or unfiltered) during all commutes. Participants traveled in the same
vehicle and sat in the same seat during each trip. Up to three participants
rode in the vehicle, and participants were asked to rest quietly and
not to engage in discussion during the exposure route. Vehicle ventilation
settings were the same for all trips: medium airflow, recirculation
off, dashboard vent, and temperature set to 22 °C. Minivans were
used to fit multiple participants comfortably and to accommodate the
exposure equipment.