An increase of 6 dB represents a doubling of the SPL, or energy of the sound wave, and therefore its propensity to cause ear damage. Because human ears hear logarithmically, not linearly, it takes an increase of 10 dB to produce a sound that is perceived to be twice as loud. Ear damage due to noise is proportional to sound intensity, not perceived loudness, so it's misleading to rely on subjective perception of loudness as an indication of the risk to hearing, i.e. it can significantly underestimate the danger.