Two generally accepted facts are: 1. Noise can cause hearing loss. 2. The noise which will cause a hearing loss in one individual may not be the cause of a hearing loss in another.These immediately suggest two unsolved problems. 1. Why does a noise cause hearing loss in one person and none in another? 2. How much noise will cause a hearing loss?The only logical answer to the first question must be that there is a variation in the sensitivity of ears to noise. Discovery of reliable tests which will predict which ear will be sensitive to noise and which will not, is an important unsolved problem, but if solved would do much to clear many disputed questions.The answer to the second question, “How much noise?,” involves the following: the intensity of the noise, the kind of noise, the duration of the exposure to noise, the amount and character of the hearing loss which noise creates.Noise in intensities of 120 to 150 decibels are conceded to be injurious to hearing. Noise less than 80 decibels is not a problem. Where between these two limits does noise become injurious to some ears? Noise with predominance of higher frequencies apparently is more injurious to hearing than one with predominance of low frequencies.The length of exposure to noise depends upon so many factors that it is certainly an unsolved problem. The character of the loss is an injury to the nerve of hearing and presents no problem except the differentiation between loss due to fatigue and permanent hearing loss.The paper ends with a plea for all who are interested to join in solving these questions so that noise can be less injurious to hearing.