The water and the way you use one could be harmful.

According to WTEN, the Food and Drug Administration has announced a warning to people about the use of nasal irrigation systems like the popular brand, Neti Pot, which many people use to flush out clogged nasal passages when they are sick.

The FDA said that using any nasal irrigation system can increase the risk of infection and that using tap water isn't safe for a nasal rinse because it's not filtered or treated.

They said, "Some tap water contains low levels of organisms - such as bacteria and protozoa, including amoebas - that may be safe to swallow because stomach acid kills them. But in your nose, these organisms can stay alive in nasal passages and cause potentially serious infections. They can even be fatal in some rare cases."

The FDA also said that these are the waters you should use if you choose to Neti Pot:

Distilled or sterile water.
Boiled and cooled tap water that is boiled for 3 to 5 minutes, then cooled until it is lukewarm.
Water passed through a filter designed to trap potentially infectious organisms.

You can learn more on how to safely use most nasal irrigation systems through the FDA website.