: . . .You might try a
: simple test (seated, on dry land) by rebreathing into a balloon to see
: what happens.

If you want to test your lung capacity, breathe into plastic bags, not balloons. The common balloons are coated with a fine powder, frequently talc, which is not a good thing to inhale. Besides, the balloons requre you to exert more pressure to get them to expand, and they push the air into you as they deflate. You can buy gallon and 2 gallon food storage bags at a grocery store and see how many breaths it takes to fill them. Then you can practice rebreathing from them to see how long you go before the air goes bad. A gallon i abot 1/8th of a cubic foot. If you can breathe for a minute from a gallon bag, then you might think that you can go for about 8 minutes from a cubic foot -- but that is not so. With the gallon of air, the last breath you take will have very low oxygen content, but then you set it aside. with the cubic foot of air your oxygen content gets very low, and after it is 1/2 or 3/4 used up you would think that you would have more breaths, but the last breaths would have VERY low oxygen content.

PGJ