When you're indoors, you don't need to get bluetooth through your body. You're getting reflections off of nearby walls and ceilings which allow your bluetooth devices to communicate across your body, but without going through your body.
When you're outdoors, you no longer enjoy the benefit of reflected RF, and the design of the phone and the headset antennas needs to be very good, so the RF can make it through your body.
It is a hard problem, but a lot of headset manufacturers do achieve it. I'm surprised you still haven't found one that's acceptable. Of course, it's body dependent. Petite women will have less issues then large men, as the RF just has less water to travel through with them.
Today's bluetooth is limited to 4dBm max transmit power for class 2. Bluetooth 5 will be 20dBm, which is a lot more power. This is, actually, the same power that class 1 bluetooth devices now have, so I'm unsure why they brag about the higher power of bluetooth 5, but to be sure, most bluetooth devices now are not class 1.
The higher power will make even bad antenna engineers be able to get bluetooth through your body, but more importantly, you'll enjoy larger range when you are at the gym or in your home. Also, if you worry about RF effects on the human body, your worry can increase now as well!
But thanks for the explanation. This is good to know. I'll plan my walk to and from work through alleys instead of open parks. Because thinking back, it always works in alleys.
This is one of those moments when I wonder if most of the things I have learned in HN are bullshit, I'm usually struck with awe at the sapience of the HN hivemind too, but in times like this...
Engineers juggle multiple constraints, cost, size, power, and efficacy, and 2.4 GHZ choice was the result of the size of the magnetron that would be cost effective and physically fit into the oven. But it did, of course, have to be within the range of frequencies best absorbed by water, which 2.4GHz is, and which is why it's difficult to get bluetooth through your body.
The parent comment is exactly the kind of comment I expected and hoped for!
This is the most diplomatic way to say "lose some weight" I have ever heard.