humidifier trouble beware
A humidifier works by drawing air through it, and removing the moisture from the air as it passes through. The air that's pushed through has to come from somewhere, and go somewhere else. If the floors are open to allow air to pass, then air certainly might pass between the floors. 

If you believe the air currents are causing exhaust fumes to be drawn to you living space, you should have the air tested for byproducts of burning fuels.  This will prove definitively that there's a problem.