There is an article on ill effects of noise and about hearing protection measures. But, today I learnt that though all of us know to reduce the noise levels to below 85 dBA, it is also not advisable to reduce it below to 70 dBA as it can cause hearing problems to operators regarding instructions, warning alarms, etc.
There is also a calculator in the above link to calculate the effectiveness of hearing protection based on the results of noise survey in your shopfloor.
Though Factories Act says 90 dBA is the noise limit for 8 hour exposure, it is a good practice to subject all employees to audiometry test, if they are exposed to 85 dBA even once in a year. Though supervisors will be reluctant to send the employees to audiometry tests to avoid production loss, considering the ill effects of noise induced hearing loss (NIHL) and in extreme case may lead to loss of life if the employee is unable to hear warning alarms.