There is no form of inhaled combustion that, consumed regularly or intensely, won't harm the body. None. None whatsoever. Tobacco, wood smoke, car exhaust, Marijuana... none. The human body is fantastic at protecting itself from minor exposures, but long-term or intense exposures will always damage the body.
But maybe we can learn something from another vice. There is no alcohol consumed regularly or intensely that won't harm the body. None. None whatsoever.
And yet most of the world believes drinking red wine is beneficial.
They're not entirely wrong.1 When you ignore the alcohol, red wine appears to have benefits and an argument could be made that weighing the pros and cons results in a net positive.
So, the question becomes:
What benefits can come from smoking anything that would, at least to the degree of suspension-of-disbelief, have a net positive benefit?
Proponents of Marijuana will tell you the benefits far outweigh the consequences. Science appears to be proving that the beneficial aspects of Marijuana can be had without any of the negatives (it needn't be smoked), but let's ignore that. The problem with something like Marijuana is that it alters perception and mental acuity more than tobacco does. Is there something that doesn't?
There might be. From this website we learn that there may be benefits from smoking herbs such as:
- Red Raspberry Leaf
If nothing else, your smoker will be unique compared to all those tobacco smoking characters.
Legal Disclaimer #1: Smoking anything is harmful. It doesn't matter what benefits may exist. Your lungs weren't designed to absorb ash or, worse, any combusting material. This answer is for the purposes of a work of fiction and should not be construed as an endorsment of any kind. Though there are many who will completely disagree with me, smoking anything is simply foolish.
Legal Disclaimer #2: Writing a story that includes a reference such as is suggested by this answer (i.e., a rationalization that smoking anything could have a net-positive) could have a consequence among readers of your story and distributors of your story in the Real World. At least here in the U.S., we've been on an anti-smoking kick for a long time and the existing subcultures that embrace smoking won't change the fact that (e.g.) Barnes and Noble might not carry your story because a vocal organization like the Truth Initiative threatens to boycott them over the matter. Probably won't happen, but fair warning.
1 Just to make a point, the world is full of false news, false beliefs, and false hope. An example of this is an article titled, "7 Science-Backed Health Benefits of Drinking Red Wine," which, after making the audacious claim that science has proven red wine to be beneficial, thereafter preceeds every claim with words like "potentially," "can be," and "may be." When humanity is brought to the point of a tack, the truth is we'll convince ourselves that any vice has a benefit.