Upper limit of exposure to noise up to which there is no damage to hearing?

Ans. c (85 dB) (Ref. Park, PSM, 22nd/pg. 688)NOISE# A daily exposure up to 85dB is about the limit people can tolerate without substantial damage to their hearing.# An acceptable noise level in classroom is 30-40 dB and in wards 20-35 dB.# The human ear can hear frequencies from about 20 to 20,000 Hz.# The ranges of vibrations below 20 Hz are infra audible and those above 20,000 Hz ultra-sonic.# Effects of noise exposure (auditory effects):EffectLevel of noiseAuditory fatigueDeafnessIt appears in the 90 dB region and greatest at 4000-6,000 Hz.Most temporary hearing loss occurs in frequency range of 4000 -6000 Hz.Permanent hearing lossRepeated or continuous exposure to noise around 100 db.Rupture the tympanic membrane and cause permanent loss of hearing.Exposure to noise above 160 dB. Also remember:Corrected effective temperature deg C1. Pleasant and cool......202. Comfortable and cool......20-253. Comfortable.....25-274. Hot and uncomfortable.....27-295. Extremely hot.....28 +6. Intolerably hot.....30 +Predicted four-hour sweat rate (P4 SR)1. Comfort zone.....1-3 litres2. Just tolerable.....3-4.5 litres3. Intolerable.....4.5 + litres