The diy retailer pleads guilty to a breach of the Health and Safety at Work Act.
As a result of the breach at its Longwell Green store, in April 2005, the company has been fined £20,000.
The breach occurred when the Bristol-based store failed to follow company procedure by filling a display hot tub with water.
The bubbling water created an aerosol of contaminated airborne water particles to which customers were exposed.
"When B&Q was made aware of the situation all stores were re-issued with a mandatory notice banning the display of working hot tubs," a spokesperson said.
They added: "We are not aware of any sickness reported by customers or employees as a result of this breach."