Interesting findings in this ABC Science piece:
Washing the hands, and then using paper towels or continuous-loop cotton towels reduced the bacterial count by about 45–60 per cent. But washing, and then using a warm-air dryer actually increased the bacterial count by an average of 255 per cent.
When they went looking further, the researchers found out how. The bacteria were already inside the warm-air dryers, thanks to the warm moist environment. Every single warm-air dryer they tested had high bacterial counts on the air inlet, while 97 per cent had them on the outlet nozzle surfaces as well.