# Noise Sources and Control, and Exposure Groups in Chemical Manufacturing Plants
## 1 Introduction  
The manufacturing sector inclusive of chemical manufacturing transforms materials, chemicals and components to value-added consumer and commercial goods using power-driven and material-handling machinery in installations commonly referred as plants, factories or mills [1, 2]. Chemical manufacturing plants use natural gas or refinery by-products such as ethylene, hydrogen rich gas, tail gas and many more as feedstock to produce chemical products and many consumer goods in downstream chemical processes [3, 4]. Plastic and paper manufacturing, petroleum refining and textile mills are but some of the industries classified under the manufacturing sector employing millions of workers worldwide [1, 2].  
Noise exposure in the manufacturing sector inclusive of the chemical manufacturing plants is widespread and amongst the loudest [5]. Cited reasons for the noise problem in existing installations include the inadequate knowledge of its mechanism of generation and abatement, lack of proper consideration during plant design phase and installation [6].  
The noise sources operating in chemical manufacturing plants emits noise through acoustical radiation and aerodynamic turbulence [7, 8]. The noise is then transmitted through pathways suchas turbulence, shock and pulsation, cavitation, impact and tooth meshing [8; 9]. Noise regulations require the mandatory of implementation of hearing conservation programmes (HCPs) for exposure prevention and control in workplaces emitting noise exceeding the universal noise rating limits, 85 or 90 dBA [10; 11; 12]. However, HCPs have not eliminated excessive noise levels in some chemical industries and have tended to shift focus away from control of noise at the source towards the use of hearing protection devices (HPDs).  
Exposure to occupational noise may lead to auditory and non-auditory health effects. Examples of auditory effects includes noise induced temporary threshold shift, acoustic trauma, tinnitus and noise-induced hearing loss (NIHL). Examples of the non-auditory effects include interference with speech and telephone communication, annoyance and job performance. The auditory effects of noise on humans result in a loss of hearing sensitivity whereas the non-auditory effects of noise affect the psycho-social wellbeing of humans and are mainly subjective [13; 14].  
In the case of compensable auditory effects, the illness affects other stakeholders such as the immediate family, co-workers, medical practitioners and insurers. In addition, there are costs associated with noise-induced illnesses relating to general economic costs, social costs, employer and employee costs. The understanding of the overall impact of noise sensities all affected stakeholders to be aware of the true impact of noise-related illnesses and a need to plan for the implementation of required preventive actions [15; 16]. Emerging issues related to noise exposure includes its effect on blood pressure, its effect on reproduction, its association with ototoxic chemicals and ototoxic medicines further highlighting the need for exposure prevention.  
This review provides an appraisal of prominent noise sources and control, noise levels and exposure groups in chemical manufacturing plants.