'Cancer-causing' fumes from the gunsmoke? That's interesting. I've only been asked once if the smoke can do any harm. I responded by saying that it was all entirely natural in content, rather like the smoke from a barbecue is natural. Some of us find both to be equally pleasant. Walking through a field full of cows will likely do you more physical damage - not from the actual cows per se - but from inhaling the gas that comes out from under their tails.
There can be no doubt in anybody's mind that any kind of gallumphing across the landscape, carrying all the doo-dads needed for a hunt, and moving uphill and down dale in order to catch and honourably kill a meal MUST be better for your long-life prospects than getting into the car and buying it from your local Fred Myers or Market of Choice. Exercise is good - crashing out on a settee eating/sucking down a box of lil' Debbies is decidedly not.
However, I realise that not every BP shooter is minded to do the first, although I have evidence that many here in UK do the second - or rather, the British equivalent.
Let's address the physical benefits of muzzleloading - on a range, that is.
Lots of arm stretching and compression from the actions of loading - great for muscle tone. More so with a long rifle than with a carbine, obviously. It has to be said, though, that shooting thirty or more shots in a long rifle on a range day, and the tone you are most likely to detect might be an anguished moan of pain from strain..
Next, the mental exercise of the complex loading procedure, compared with that of the unmentionables, many of which require nothing more strenuous than a gentle trigger squeeze, can only be helpful to maintaining an active and questioning mind, even if the question is 'did I put the powder in first?'
IOW, practicing of the mental agility necessary for insuring that a number of very important actions all happen in the right order, and the concentration and patience needed for it to happen all over again. And again.
In the end, the self-gratification that accrues from getting it ALL right, and in the right order AND hitting what you are aiming at.
Add up the mental and physical benefits of shooting a muzzleloader of any kind, and you can easily see why one medical person recommended it as a curative.
Let's sum it all up, then, making it as simple as possible.
ALL guns go bang to some degree. This is fun. Some make a loud and very sharp bang, like most unmentionables do. However, that nastiness can be mitigated somewhat by the addition to the gun of a device that renders the sharpness of the report less sharp.
The device that can do the same thing with a muzzleloader has yet to be developed. Even if it were to be invented, I dare opine that the likelihood of any rifleman of our ilk adding a foot-long tube to his already five-foot long rifle is slim indeed. Plus, it is definitely NOT HC.
Smaller calibre patch and ball rifles CAN make a sharp crack. but it's not the explosion of the charge that makes it, unlike the 'other' kind of gun - it's the sound of the ball as it busts the sound barrier. It is usually indicative of what we call 'a good load'.
Bigger calibre muzzleloaders tend to be somewhat slower - note I wrote 'tend'. We all know that you CAN make a .58cal ball beat the barrier, but on a range? So bigger calibres tend to go 'Boom'. VERY big calibres, like a Brown Bess or similar might even go 'KA-boom'.
However - we are on a range, and we are required by range regulations to wear some kind of hearing protection, even if, like me, it's only window dressing. A long career in uniformed noise generation has ensured that I need not necessarily waste too much money on hearing protection. You only need to protect something you HAVE, not something you HAD.
What else?
Eye protection? Seems sensible to me, I do. Most people should, I reckon, and I base that opinion on having the experience of tweezing bits of a musket cap out of the eyelid of a fellow shooter who scorned the use of a pair of safety glasses. Half an inch lower and he would have had to change over his shooting style to that of a left-hander.
Lastly, recoil. I'm not sure why this has been a made a point of contention, since the way we feel recoil is personal and mostly indescribable. Do I feel it? Yes, of course I do - recoil is a fact of physics. How MUCH do I feel? Is it different to an unmentionable? Well, the answer is yes, and for the reasons already mentioned here by other posters. A charitable person might describe me as being well-built, in the manner of a Peterbilt truck, so it stands to reason that I'm not going to be laid on my butt shooting most guns that you can put to your shoulder. Somebody built like Stan Laurel, on the other hand, might well be more moved by the experience, particularly shooting off a bench, as many folks do.
As the shooter progresses from smaller to larger calibre, and accommodates/acclimatizes their body to shooting, so their tolerance to shooting increasingly heavy-recoiling guns will inure them to the effects of doing so. To show how some people respond to shooting a well-known hard kicker, and with apologies to the management - this is a little movie of a the son of a friend of mine shooting a 500gr bullet over a compressed load of 68gr of 2Fg in one of my rifles - this is his VERY first shot, EVER, with a real firearm - you can see just how upset HE was -
Apologies for long resplone. I'm sure that Zonie will either pull it, or not.
