The chlorine dioxide gas is a potential health hazard.  You should avoid direct contact with the tablets.  You should also avoid any contact with the gas which can be extremely irritating and potentially deadly.  The chemical reaction starts immediately upon the addition of water.  Therefore, it is critical that you get out of the car immediately upon the addition of water and close the door.  This will avoid personal exposure to the gas and help ensure maximum effectiveness of the kit by containing the gas inside the vehicle.