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 the hose, any hose, is connected to a harmful substance. If the pressure in a water main drops while your hose is submerged in polluted or contaminated water, then the water (and whatever is in it) could be sucked back into your pipes and your drinking water supply. Water pressure drops are not uncommon. They 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.
not use spray attachments without a backflow prevention device. The
chemicals used on your lawn are toxic and can be fatal if ingested.
buy and install inexpensive backflow devices for all threaded faucets
around your home. They are available at hardware stores and home
A man sprays commercial weed killer containing an arsenic compound on
his lawn using a hose attachment. After he finishes, he disconnects the
applicator. It is a hot day so he takes a refreshing drink of water from
the hose. A short time later he dies from arsenic poisoning.
At some time while the man was spraying weed killer, the water pressure
dropped, which resulted in the poison being sucked back into the hose.
Later, when he drank from the hose, the poison inside was released with
the water. He unknowingly poisoned himself.