Can anyone suggest why after covid I can't do Finnish sauna anymore? Prior to that I used to do 1-2x a week a sequence of 5x(10 minutes in sauna + 5 minutes cold water immersion + 10 minutes rest) which was absolutely great for both stress reduction and blood flow. Now if I do 5 minutes in sauna I feel like my skin was burning and I am about to die, and I need to recover for 1 hour from that to be able to just walk away from sauna.

Shot in the dark, but has your actual stove changed? When have you last changed the stones? Is the circulation of air worse?

If your skin feels hot my guess would be that the steaming effect might be disrupted by the water getting evaporated faster than before, and the circulation of air also affects the skin feel (that’s why a certain seating position can make sauna unbearable). You could also try to just turn it on at the lowest setting and see if it changes anything. Maybe the stones have gotten so old that old heat settings have sneakily turned unbearable.

I'm a big fan of soaking in hot water and have noticed that cardiac function seems to have a massive effect on heat tolerance as measured vs body temperature.

For example, if I've been totally sedentary for the whole day (and my feet are chilly+blue), a body temperature as low as 101F is unbearable. But if I've been actively moving around all day (and my feet are warm and pink), I only start getting uncomfortable at a body temperature around 103.5F-104F.

This also seems to correlate over a longer timespan re: exercise habits, consumption habits, sickness, etc.

No idea. How hot is and was your sauna? Is it possible that it's hotter than it used to be? Maybe try one that's slightly less hot?

I've got the opposite problem: saunas don't seem to be able to make me sweat anymore, so I'm looking for the hottest saunas I can find.

Did it happen suddenly? Or did you go for a long time without using a sauna, and noticed the change only when you resumed? Did anything else about your body change, such as weight loss (perhaps from a GLP-1)?

It's possible that Covid had nothing to do with it, and your body is simply changing with age. It's depressing, but it happens!

anecdotal -but- it took me 6 months after covid for my breathing rate to go back to normal, and to be able to do consistent max our efforts of >190BPM for >5 seconds like previously

Once in a while as I get sick I have to retrain myself to going to sauna (e.g. taking lowest level, even skipping the Aufguss, German infusion where temperature is raised gradually etc.)

Also IMO your body fat/water/lean muscle ratio may play a role. I once lost 5 kg due to Influenza A and all my sport achievements as well as sauna endurance were gone

After covid I've found i cannot stand the cold. A friend of mine can't stand alcohol since.