Bottled mineral water, generally considered purer than tap water, is often contaminated with bacteria and fungi, a Dutch researcher warned Monday.

“Hospital water is increasingly accepted as a source for pathogens,” Dr. Rocus R. Klont commented to Reuters Health. Consequently, patients with compromised immune systems “often receive bottled mineral water under the assumption that it is safer than tap water,” the investigator added at a meeting of the American Society for Microbiology in Washington, DC.

To evaluate the risk of infection from bottled water, Klont, from the University Medical Center Nijmegen, and colleagues looked for bacterial and fungal contamination in 68 commercial mineral waters, one tap water, and one water sample from a natural well. The samples came from nine European and seven non-European countries.

“We found high levels of bacterial contamination in commercially bottled mineral water,” Klont told Reuters Health.

Overall, 40 percent of all samples showed evidence of contamination with either bacteria or fungi. Bacteria could be grown in lab cultures from 21 samples.

“These findings indicate that the general perception that bottled water is safe and clean is not true,” Klont noted.

“The risk of disease to healthy individuals may be limited, but immunocompromised patients are generally more susceptible to infection and therefore might be at higher risk of becoming infected,” he added.