max safe dose
many substances have a maximum safe dosage. after that they become dangerous in a more/less linear fashion. like say water. you can drink lots of water. but at some point you exceed the msd. and it can do harm. the further past this limit you go the more harm it will do. in extreme cases drinking too much pure water while say running a marathon can kill you. some substances have a maximum safe dosage of zero. that means the chance of harm is proportional to the exposure. the proportion can be pretty small. that means you can tolerate quite a bit before the harm rises above the background level. examples would be: radiation and cigarette smoke. so when people say second hand smoke will kill you they mean there's no safe dosage. when they say it won't kill you they mean the risk from typical exposure won't exceed normal background levels.