i think you guys are missing the point. the OP is asking if there is a scale with definitions.
the answer to that is no there is not. because there are too many variables. how long your session lasts, what coals you use, how much carbon monoxide your coals put off, how much vaporizing vs how much burning is being done in the bowl, how many hits you take, how deep you inhale, how long you hold the smoke in your lungs, as well as countless other factors (those i listed are just the ones that are obvious variables to me) which change how toxic a session is to your body. and then that is all multiplied by how many sessions you smoke. its really just too variable. its not a matter of subjectivity or relativity to your lifestyle. i mean yes it is relative to your lifestyle, i mean if you sit on your ass and eat shit food and have HIV, you probably shouldnt be smoking, especially not with other people. thats the relativity aspect of it. but theres just so many variables going on in a single session to calculate what the OP is asking.
i know that wont bring you any closure, but atleast maybe it will enable you to stop analyzing it and just do what makes you happy. life is short. do what makes you happy.