For me, this is obvious. I avoid municipal water when possible, because it often has chlorine in it. Even for showers, a shower filter can make sense to reduce the amount of chlorine absorbed through your skin.
regular exercise reduces cardiovascular risk by a factor of two or three. But the extended vigorous exercise performed during a marathon raises cardiac risk by seven-fold!
it puts an extraordinary stress on your heart, one that your body was not designed for.
A related article talks about how older marathoners develop scar tissue in their hearts.
But half of the older lifelong athletes showed some heart muscle scarring [fibrosis]. The affected men were, in each case, those who’d trained the longest and hardest. Spending more years exercising strenuously or completing more marathon or ultramarathon races was, in this study, associated with a greater likelihood of heart damage.
I believe that marathons are not the healthiest thing in the world, and I believe in doing traditional aerobic exercise and high intensity training as I posted about here.