What health threat can swimmers face in inadequately treated pool water?

Swimmers can face various health threats in inadequately treated pool water, with the risk of infectious diseases being the most critical concern. Pools that are not properly disinfected or maintained can harbor harmful microorganisms such as bacteria, viruses, and parasites. These pathogens can be introduced through fecal matter, skin, or respiratory secretions, leading to illnesses like gastrointestinal infections, skin rashes, or respiratory issues.

The other potential threats, while significant, are often secondary to the immediate health risks posed by infectious diseases. For example, dehydration can occur due to prolonged exposure to hot pool environments, but it's not directly caused by water quality. Similarly, exposure to harmful chemicals can happen when water treatment chemicals are mismanaged, yet the likelihood of an infectious disease outbreak often presents a more urgent and common immediate risk in swimming pools with inadequate water treatment. Thus, while dehydration and chemical exposure are relevant, the prevalence and potential severity of infectious diseases make them the primary health threat swimmers face in such environments.