Creating a negative pressured room?

wow - bummer! a fan in the fireplace would take the smell up to the second floor and wouldn't solve your problem. however, if the fireplaces are not being used as such you might consider a flex tube dropped down from the roof. maybe an eight inch diameter tube with a fan in your area. that would exclude the second floor fireplace opening. you would have to p the tube through the flue behind the firewall on the second floor. with the fan on this would create a laminar flow in your area up the tube and out the chimney. if the fireplace idea doesn't work you could put the tube out the window.