Is there some amount of cigarettes that is beneficial too?
So long as the line is going up and to the right, it's specific shape only matters if you want to debate the micromort value, not whether the concept exists.
If you go up to the top of this thread, I think you will find that we are bickering about specific micromort values listed on the Wikipedia page.
Some of these things plausibly can be modeled as LNT. Radiation possibly can, perhaps airplane rides too. Some of them cannot be, like alcohol consumption.
So long as the line is going up and to the right, it's specific shape only matters if you want to debate the micromort value, not whether the concept exists.