The intensity of sound that does not cause ear damage-

**Core Concept**
The intensity of sound that does not cause ear damage is measured in decibels (dB) and is related to the threshold of hearing damage. Prolonged exposure to sounds above a certain threshold can cause permanent damage to the hair cells in the cochlea, leading to hearing loss.

**Why the Correct Answer is Right**
The threshold for hearing damage is generally considered to be 85 dB, as sounds above this level can cause permanent damage to the hair cells in the cochlea. This is because prolonged exposure to sounds above 85 dB can cause the hair cells to become overstimulated and eventually die, leading to permanent hearing loss. The National Institute for Occupational Safety and Health (NIOSH) recommends that workers be exposed to no more than 85 dB of sound over an 8-hour period.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because there is no established safe threshold for sound intensity that is lower than 85 dB. While sounds below 85 dB may not cause permanent damage, they can still cause temporary threshold shifts and discomfort.

**Option B:** This option is incorrect because there is no established safe threshold for sound intensity that is higher than 85 dB. Sounds above 85 dB can cause permanent damage to the hair cells in the cochlea, leading to hearing loss.

**Option C:** This option is incorrect because the intensity of sound that does not cause ear damage is not related to the frequency of the sound. While some frequencies may be more damaging than others, the intensity of the sound is the primary factor in determining whether it will cause hearing damage.

**Clinical Pearl / High-Yield Fact**
The "85 dB rule" is a widely accepted guideline for safe sound intensity levels, and it is essential for healthcare professionals to educate patients and workers about the risks of prolonged exposure to loud sounds.

**Correct Answer: D. 85 dB**