Industrial hygienist here, very versed in the OSHA standards.
1. For a phone app, try the NIOSH app. Clearly not as good as a top shelf sound meter, but one we use if we don't have our "good" equipment with us just to screen sound levels. Phone quality obviously plays into this, and/or whether or not your phone's speaker is dirty and caked with dust and grime.
2. 85 dB is the level at which an employer is required to have a Hearing Conservation Program, monitor employees with baseline audiograms and provide hearing protection. Hearing protection in a workplace setting MUST be worn at levels exceeding 85 dB as an 8-hour time-weighted average (which basically means, if your work environment is really loud, right around 85 dB (+/- 2 dB or so) for your entire work shift, you need to be wearing appropriate hearing protection.
In practical terms for home audio, you're probably not listening for 8+ hours at levels above 85 dB. If you are, you're destroying your hearing. Intermittent excursion above 85 dB won't destroy your hearing, but could still do damage. So, glad to report that those who identified 85 dB as the magic number.........all get a blue star for the forehead (for those who remember that kind of grade school stuff).