If I start putting some stuff into your tapwater, or into the air around your house, and you feel convinced it's going to cause you harm, but I insist that it's perfectly safe, what standard of proof do you think you should have to put together to stop me from doing that? And what would need to constitute harm?

If it didn't hurt humans but just insects that you're fond of, or just the ozone layer that protects you from radiation, or just the climate that you've come to enjoy, what standard of proof is needed?