Water that appears bubbly, cloudy, or milky is typically safe to drink in most household scenarios. This appearance is caused by harmless dissolved air trapped in the water supply system. The cloudiness is a temporary visual effect known as aeration, which is a physical phenomenon, not chemical contamination. When assessing safety, the most important factor is the source of the bubbles and whether they quickly disappear when the water is allowed to sit.
Why Tap Water Appears Milky or Bubbly
The primary reason tap water looks cloudy is a sudden change in pressure or temperature causing dissolved gases to come out of solution. Municipal water is kept under high pressure in distribution pipes, allowing it to hold more dissolved air (primarily oxygen and nitrogen) than normal. When the water leaves the faucet, the pressure drops instantly, and the excess air forms millions of tiny bubbles. These micro-bubbles scatter light, giving the water a milky appearance.
Temperature also plays a significant role in aeration. Cold water holds more dissolved gas than warm water, so chilled water from underground mains contains a high concentration of air. As this cold water flows into warmer pipes inside a home, the temperature rises, forcing the excess air to be released as bubbles. This effect is often more noticeable in the winter months.
A simple test confirms the cloudiness is harmless air. Fill a clear glass of water from the tap and set it on the counter. If the milky appearance is due to dissolved air, the water will begin to clear from the bottom upward within a minute or two as the bubbles rise and dissipate. If the cloudiness persists beyond two minutes, it indicates the water may contain suspended solids or sediment, signaling a different issue.
Differentiating Carbonation and Other Gases
Not all bubbles are simple air; some are intentionally added, and others signal geological or plumbing issues. Carbonation, found in seltzer or sparkling water, is the intentional infusion of carbon dioxide (\(\text{CO}_2\)) gas under high pressure. When \(\text{CO}_2\) dissolves, it forms a small amount of carbonic acid (\(\text{H}_2\text{CO}_3\)), creating a distinct, slightly tangy taste different from tap water. Unlike the fleeting bubbles of aeration, carbonation produces persistent fizz and continuous effervescence that does not disappear quickly.
In rare cases, especially with private well water, bubbling may indicate naturally occurring gases like methane. Methane is colorless and odorless, making it impossible to detect by sight or smell. While not a direct health hazard when consumed, the gas is flammable and can pose an explosion risk if it accumulates in a confined space. A sputtering faucet or persistent bubbling that does not clear quickly may signal the need for specialized testing for methane.
Indicators That Bubbly Water Is Unsafe
While simple aeration is benign, bubbles or cloudiness accompanied by other physical indicators warrant caution. A strong “rotten egg” smell suggests the presence of hydrogen sulfide (\(\text{H}_2\text{S}\)) gas, produced by sulfur bacteria or chemical reactions in water heaters. Although \(\text{H}_2\text{S}\) is generally considered a nuisance that causes corrosion, its presence can sometimes indicate contamination from sewage or other pollution sources.
A metallic taste is often caused by elevated concentrations of metals like iron, copper, or zinc leaching from corroded plumbing systems. While iron and manganese are usually harmless aesthetic issues, a metallic taste can also be an early warning sign of toxic lead or copper contamination. This taste is often most noticeable after the water has been stagnant in the pipes overnight.
An oily sheen or film floating on the water’s surface requires immediate investigation. If the film breaks apart into small fragments when stirred, it is likely a harmless, naturally occurring film of iron-oxide minerals or organic matter. If the film swirls back together and remains intact, it is a strong indicator of petroleum-based contamination, and the water should not be consumed. Any water that is persistently cloudy or bubbly, along with a foul odor, discoloration, or an unusual taste, should be tested by a certified laboratory before use.