In animal studies in several different species, we have produced irreversible nerve damage in the ear with two hours of continuous exposure to noise at 100 to 104 decibels (dB). There is every reason to believe that human ears are just as sensitive. Most daily exposures in our lives do not continue for that long. Nevertheless, it is prudent to avoid unprotected exposure to any sounds in excess of 100 dB.
Many sounds in daily life take us into a danger zone. Concert venues and clubs routinely produce peak levels of 115 dB and average levels in excess of 105 dB. Gas-powered leaf blowers and lawn mowers reach levels at the users' ears between 95 and 105 dB, as do power tools such as circular saws. Frequency of the sounds matters. The more high-pitched whine of a belt sander is more dangerous at the same decibel level than the lower-pitched roar of an undermuffled motorcycle. Jackhammers produce levels of 120 dB even for passersby, and the rapid-fire impulses of the metal rod on concrete produce lots of the dangerous high-pitched sounds.
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