This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and Health, Subject name: Social & Preventive Medicine
Upper limit of exposure to noise up to which there is no damage to hearing? 
Select and explain. 
A. 160 dB, B. 70 dB, C. 85 dB, D. 100 dB
C: 85 dB
Answer: C: 85 dB 
Explanation: 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