When water flows backward through the water supply system, it is called backsiphonage or backflow. When that water is accidentally mixed with hazardous chemicals or bacteria, it is called dangerous.
The danger comes when a hose is connected to a harmful substance. If the pressure in a water main drops while your hose is submerged in contaminated water, then the water and hazardous pollutants could be sucked back into your pipes and drinking water supply. Water pressure drops are not uncommon; and can happen when firefighters battle a nearby blaze, or before utility crews repair a broken main.
Some harmful substances you should be wary of are the chemicals used to
fertilize your grass or the weed killer used on your lawn. The cleanser
used on your kitchen sink could be hazardous if swallowed, as could the
bacteria in the water from your wading pool or water bed.
Fortunately, keeping your water safe from these contaminants is easy. Take the following precautions to protect your drinking water:
Never submerge hoses in buckets, pools, tubs, or sinks.
Always keep the end of the hose clear of possible contaminants.
Do not use spray attachments without a backflow prevention device. The chemicals used on your lawn are toxic and can be fatal if ingested.
Do buy and install inexpensive backflow devices for all threaded faucets around your home. They are available at hardware stores and home improvement centers.