You might think that a pool is clean because you can smell the chlorine, but this can actually be a sign that its simply reacting with the bacteria in the water. A study on holiday swimming pools has shown that some are no cleaner than a toilet with one in four being identified as a health hazard. Pools often contain sweat, urine, mucas, saliva, dead skin and faecal matter and failure to clean them properly can cause stomach upsets and ear and eye infections. However, children or the elderly these mild infections can be more serious. You just might want to think twice before jumping into that pool.