Tests carried out by the spa hotel guide Relax Guide revealed that 64.6 percent of pools were not up to scratch - meaning guests could unwittingly be bathing in a Petri dish of germs.
"For some swimmers, there is an acute risk of infection," Relax Guide publisher Christian Werner warned.
615 pools in Germany and Austria were tested between April and June, and sauna plunge pools were shown to be the most unhygienic.
"None of the 19 we tested met the standards," Werner said. Second worst were hot tubs, where only ten percent met the required clean water standard, but outdoor hot tubs fared better.
However, Werner said that "on average, indoor pools tend to be significantly cleaner than outdoor pools." Both countries had similar results.
Hotel pools proved to be cleanest between 11:30am and 2pm. “Water quality was the worst first thing in the morning - we think either because the chlorine has become depleted or the pool pump has been turned off overnight to save electricity, water and chlorine.”
A strong chemical smell can indicate that the pool's water is exceptionally dirty and should have its chlorine and pH levels tested and adjusted. A healthy pool would be odourless, and if chloramines levels have reached a point where they can be smelled, the pool water may act as an irritant to a swimmer's eyes, skin and nose.
Harmful germs can spread in pools that have insufficient chlorine and too-low pH levels, causing diarrhea, nausea, vomiting, and stomach cramps.