Find out on further reading that I’m obviously in error on the first point. dB SPL is dB SPL no matter what the sound frequency and a measure of the sound pressure outside your ear. It turns out that all the contours are related to the acoustics of the human ear and the bio-mechanics of sound transfer to the inner ear - and high frequency sound is better conducted to the inner ear and more damaging once there than lower frequency sound or much higher frequency sound (the rising part of the curves on the right that I’m referencing in my post above with the contour “equal loudness” curves).
The dBA variable that’s depicted changing in the curve is not a direct measure of damage potential. It’s just measure of dB after the sound is passed through a particular type of filter (A filter ) (once) thought to mimic human hearing sensitivity closely. The A-filter was designed based on the sensitivity of human hearing over a frequency range to pure tones compared to the loudness perceived relative to a 1 KHz standard tone. Sound measured through a A-filter, since it mimics the human range of hearing sensitivity, has been found in extensive hearing damage studies to reasonably reflect the potential for sound damage as a function of frequency. But the problem is is that noise is usually not a pure tone but more like white noise so the A-filter-based standard, an American standard, is considered inadequate by some and in Europe, a different standard based on more general types of noise is used as the basis of filtering to determine damaging noise levels. Another variable is the tests were done with headphones on, I believe, or at least the sound coming from the side, and the more usual assault of noise is from the front so modern day standards have been modified to take that into account. The dBA standard is popular in part because of its historical acceptance and history and dBA-based sound meters are cheap and plentiful.
Sorry for errors in my explanation. I think that if I were a Ph.D. in audiology, I would understand a bit better than I do currently!