Flushing the toilet with the lid up is not wise and is significantly unhealthy. Water vapor that erupts out of the flushing toilet is polluted by faecal bacteria such as E.coli which are harmful to your health. Additionally, it takes a couple of hours for these particles to settle down somewhere. That means if you have placed your toothbrush near your toilet, there are chances that you brush your teeth with what is in the toilet. So the next time you poop make sure you close the lid before flushing!
