An analysis of health effects and risk factors related to Standard Threshold Shift (STS) in motor compressor workers

Introduction
Standard threshold shift (STS) is an early indicator to identify deteriorating hearing or a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2, 3 or 4 kHz in either ear. Risk factors related to noise induced hearing loss have been studied but the change in hearing level due to the effects of noise and STS has not been studied yet. Our objective was to investigate risk factors and health effects related to STS in motor compressor workers.

Methods
A cross sectional study of 500 motor compressor workers was conducted over 2 years (2010 and 2011) investigating hearing health by audiogram examination in a motor compressor factory. Participants answered questionnaires about their employment and their physical health history. The noise level in the workplace was monitored.

Results
There were more men 80.6% (aged range 31-40 years old) than women working for the company. The average continuous noise level in the workplace was 78.42 ±10.90 dB(A) (mean ±SD), and the highest was 97 dB(A). The study showed that the number of working years in the factory (p =0.013) and the use of personal protective equipment while working in a noisy environment (p=0.002) resulted in a significant change of STS.

Discussion
Workers exposed to continuous noise in the motor compressor industry were at risk of STS. Hearing tests should be performed on all new employees to develop a standard baseline for further STS study. To control noise in the workplace is important for hearing protection.