like i said, it also depends on the herbs that are used
i've got buddies that have seriously messed up their lungs from chronic foil smoking and chronic store-bought blend smoking.
i used to smoke a LOT more of this stuff than i do now.... like 1 to 3 grams of strong blend a day... made it myself using Mullein, which is an herb traditionally used by Native Americans to treat upper respiratory problems.... i never even got so much as a cough.
my personal theory is -- humans have been smoking herbs for thousands of years. our lungs are probably a bit more used to processing burnt plant material than they are used to processing pure vaporized (somewhat caustic) chemicals.
sorta like how 190proof Everclear burns the hell out of your throat, but adding a shot of it to a nice tall glass of mixer doesn't hurt so bad.
that's just my theory, though, and i wouldn't be at all surprised if someone proved me wrong some day.