Noise pollution is measured in decibels (dB). Sound becomes hazardous in noise…

Concept — The decibel (dB) is a logarithmic unit used to express sound-intensity level: each rise of 10 dB stands for a tenfold rise in sound intensity. Because the scale is logarithmic and not descriptive, noise-pollution standards do not speak of “loud” or “very loud” sound; they fix numbered points on the dB scale. The standard scale marks three such graded points: an ordinary audible level, a level above which continued exposure becomes hazardous to health, and a much higher level at which sound becomes physically painful.

Application — In the classification used for noise pollution, sound up to about 80 dB is borne by the human ear without lasting injury. Beyond 80 dB, continued exposure starts to damage the hair cells of the inner ear and sets off the physiological stress responses associated with noise pollution — raised blood pressure, disturbed sleep and loss of concentration. The hazard threshold the question asks for is therefore 80 dB.

Cross-check — Placing every offered level on the same scale shows why only one of them is the hazard mark:

• 30 dB — a soft whisper or the hush of a reading room. This is far below any injurious level; it is near the ambient limit prescribed for silence zones at night, not a hazard.

• 80 dB — heavy street traffic or a kitchen blender. This is the point at which the standard scale begins to class noise as hazardous to health.

• 100 dB — a petrol lawn mower or a motorcycle engine at the rider’s ear. Sound here is already well inside the damaging range, so it is not the level at which the hazard begins.

• 120 dB — a jet aircraft at take-off or a thunderclap overhead. This is the threshold of pain, a separate and far higher mark on the same scale, often confused with the hazard mark.

Hence sound becomes hazardous in noise pollution above 80 dB.