Is it healthy to use Onsen during summer?

Last summer (July-Aug) I went around Japan visiting a few onsen, most outdoor located. Since the outdoor temperature was around 30 degrees I think, bathing in 30-40 degree (or more?) water felt quite odd, and I have had discussions with friends regarding this. Still, the other visitors (mostly locals or domestic travellers) seemed rather unaffected.

Isn't it rather dangerous bathing in such warm water during such warm days?

It's not really an issue, though to be mindful of your overall feeling. If you get dehydrated or already feeling very hot, just be weary of that as you don't pass out.

Of course it's more common for onsen to be used during slightly cooler weather, you shouldn't have an issue in the summer.

Only you know when it�fs too hot. If you feel overheated, get out and cool off (some Onsen have cooling bath, or cooling shower). Most Onsen water temp is somewhere between 38 deg C to 43 deg C (there are hotter ones) and YOU have to know what temperature YOU like. And some Onsen have hot and not-so-hot tubs. My body likes 42 deg C, but I cannot stay in too long or I will overheat. For long duration bathing, I choose cooler (39 to 40 deg) bath.

Many Onsen water is claimed to have health-beneficial effects, like skin cleaning, good for rheumatism related things, muscle easing, menstruation problems and such.

those positive effects might outdo the problems for feeling a little "too hot" in summer for many people.

as it refers me, there is no "too hot". i love to boil myself even if outside hat 38 degrees :-)

There is a lot of difference in a degree or even half a degree once you get up around 40. 37 is pretty cool, 40 is pretty hot, 41 is pushing it and over that you feel like your foot is burning when you put it in the bath. You can spend 10 minutes plus when it is 40, but at 41 or 42 your heart starts thumping away after a few minutes (or at least it does for me).

I like the hot baths in summer too, as long as there is somewhere to cool off.

Maybe my post yesterday was deleted, but the point is still relevant - when you are submerged in water, the air temperature has little bearing on the water temperature. You will be affected by the water, not the air so 40C in summer or winter will still be 40C. An air temperature of 0C in winter or 40C in summer will have very little impact.

That is very true, but if you get your core temp up from a long soak in summer when it is 30 plus outside the tub, you'll still be sweating a few hours later! That's when it is nice to have a dip in something cool after the soak in the onsen.