Occasional tap water may develop a slight smell of rotten eggs that makes you reluctant to drink it or bathe in it. In some regions this smell can be more pronounced than it is in others. You may even called your local water department to ask what's going on. After all, you pay for what's expected to be clean, fresh water, and you expect a better product than the stinky stuff flowing from your tap. In this instance, there is little cause for concern. What you're smelling is sulfur which can occur naturally in groundwater and which is dangerous dangerous.
Actually, the smell as well as a bad taste in the water can be caused by a larger-than-normal concentration of hydrogen sulfide gas. There are several reasons why this gas can build up: it can be produced by sulfur bacteria, come from pollutants that are introduced into the water, or can be produced by a chemical reaction which took place in your water heater. It's important to know exactly what is causing the problem, because there are different methods of treating it depending on the way it has been generated.
Even though the foul-smelling water may not seem like something you want to put into your body, it generally is not harmful. If you use well water in your home, you should have the well tested for coliform bacteria and nitrate. Hydrogen sulfide gas is generated when organic matter decays or from the water's contact with soil that contains sulfur. A small level of this gas is not going to hurt anything. If the gas collects in large volumes, though, it does need to be either removed from the water or vented into the atmosphere.
In order to make a check of where the smell is coming from, you need to perform a test after you've been away from home for several hours. First pour yourself a glass of cold water and smell it; then do the same with a glass of hot water. If only the hot water smells, it's likely that your problem is in your water heater. In cases where the water starts running with a strong odor which historically lessens, then the culprit is most likely your city's water distribution center or your well. Once you've isolated the reason for the sulfur smell, you can take the proper steps to have the problem taken care of, such as calling the city, getting your well checked, or replacing your water heater.