Today, while driving a car I had a little quarrel witch my brother about the loundness of music in my headphones. He says that if he can hear my music when I take headphones off my head, I listen too loud and it can damage my hearing. I disagree.
I’m driving M40x from my phone.
I need other people opinion to show him I am right.
Sounds at or below 70 dBA are generally considered safe. Any sound at or above 85 dBA is more likely to damage your hearing over time. Researchers have found that people who are exposed over long periods of time to noise levels at 85 dBA** or higher are at a much greater risk for hearing loss.
So if you listen music with normal converstation volumes, no issues. The risk factor goes up after that, if the levels are “people yelling” loud. You listen too loud already and hearing damage might happen.
I have the same reaction, but that’s when I can hear someone’s music while they’re wearing their headphones. If it can be heard only when you take them off it’s probably fine, though there’s also the confounding issue of the car noise.
If you want to settle it, just install something like https://play.google.com/store/apps/details?id=kr.sira.sound (pretty accurate, I’ve compared it to a dedicated sound meter and it does well) and measure how loud your music is in your headphones. Ideally you should recreate the closed space inside the cup that exists when you’re wearing it. So, for example, put your phone flat against the side of your head and cover the microphone end with the headphone cup while music is playing. The app is continuously graphing the values so you will be able to see them after you remove the “setup” from your head. 
Then let your brother search online for decibel (dBA) levels that are known to cause hearing loss.
(Keep in mind, though, that not everyone is the same, and vulnerability to hearing loss is not constant even in the same person over time. You can have temporarily increased sensitivity to damage when taking some ototoxic painkillers like ibuprofen or paracetamol/acetaminophen for example.)
It only means his ears are still working… everyone can hear that…
This is dangerous information in a hifi forum. 
I measured about 75db when music was loud even for me, so I’m fine.
Besides, mu brother is weirdo - he listens everything at whisper level no matter what it is.
Maybe your brother is an introvert who has sensitivity to stimuli more than you do.
