risk personally just ask the simple question “can I walk up two flights of stairs without having to stop to catch my breath”? If the answer is no, then you are at higher risk. On the other hand if you can run a mile without stopping you won’t have a problem with a little hypoxia or hypercapnia while snorkeling.
If you can’t do a couple of flights of stairs at a normal pace without having to stop you shouldn’t be snorkeling.