Noise Monitoring and HCP at Darajat Geothermal Field to Prevent NIHL

Noise Monitoring and Hearing Conservation Program at Darajat Geothermal Field to Prevent Noise-Induced Hearing Loss

Author: Muhyidin, SKM

Star Energy Geothermal Darajat II, Limited
Sentral Senayan II, 25^th Floor. Jl.Asia Afrika No.8, Jakarta 10270. Indonesia

Keywords: Noise monitoring, hearing conservation program, Darajat geothermal, noise-induced hearing loss

At Darajat geothermal field, during normal operation, the highest noise is at turbine floor power plant area that reached 92 dBA. During routine activity, the noise at well pad area is 58.9 – 70.1 dBA, but during non-routine activity such as steam blow activity reached 97.5 dBA and vertical discharge test activity reached 133.1 dBA at the 1-meter distance from the source of noise. Recently, the vertical discharge test is avoided and prohibited by authority due to highly noise hazard, environmental and community issues.

Annual noise dose sampling conducted to operator personnel, maintenance personnel, resource management personnel, and construction personnel. During period 2018-2020, the highest noise dose of 8-hour time-weighted average from similar exposure group are: Operation personnel is 90.1 dBA (415.4 % of noise dose), Maintenance personnel is 86.5 dBA (134% of noise dose), Resource Management personnel is 88.2 dBA (210.5% of noise dose), Construction personnel is 87.5% (178.6 % of noise dose) and Drilling personnel is 81.8 dBA (48.7% of noise dose).

Noise control, annual audiometric test, use of hearing protection devices, employee training, and program evaluation are conducted as part of hearing conservation program. This program is aim not only to prevent noise-induced hearing loss (NIHL), but also boost employee morale, and reduce stress levels among exposed employees.

1. INTRODUCTION

There are 466 million people or over 5% of the world’s population has disabling hearing loss (432 million adults and 34 million children). It is estimated that by 2050 over 900 million people (one in every ten people) will have disabling hearing loss. World Health Organization (WHO) estimates that unaddressed hearing loss poses an annual global cost of US$ 750 billion which includes health sector costs, costs of educational support, loss of productivity, and societal costs (WHO, 2020).

The prevalence of noise exposure at work has been reported to be approximately 15% in Canada (Feder et al., 2017),c 20% in the European Union (Eurostat, 2004), and 20.1% in Australia (Williams, 2013). Noise exposure at work is responsible for an estimated 16% of disabling hearing loss in adults worldwide (Nelson et al., 2005). Noise at the workplace is increasing in many developing countries as economies shift from an agricultural to a more industrial base, and indicates occupational exposure to noise may be slowly decreasing in parts of the developed countries (Fuente and Hickson, 2011).

The top three highest prevalence of NIHL among noise-exposed workers is from Mining sector (61%), Construction sector (51%) and Manufacturing sector (47%). At Mining sector, 23% report having hearing difficulty, 25% have a material hearing impairment, and 13% of noise-exposed workers not wearing hearing protection (Tak et al., 2009). The Construction sector has the second highest prevalence of noise exposure, 14% of all workers report hearing difficulty, 25% have a material hearing impairment and the incidence is reported to be 9% between 2006 and 2010 (Kerns et al., 2018). While at Manufacturing sector,18% report having hearing difficulty, 20% have a material hearing impairment and incident is estimated at 7% between 2006 and 2010 (Masterson et al., 2015).

According to the European Survey on Working Conditions (ESWC) about 20% of European workers are exposed, half or more of their working time, to noise so loud that they would have raise their voice to talk to other people. Exposure to noise is especially common in the manufacturing and construction sectors, where about 40% of the workers are exposed. The prevalence of noise exposure to women is less than men. It usually takes a long time to develop noise-induced hearing loss, and about half of the incident cases were seen among those aged 55 years or more. In many sectors, the workforce with high noise exposure is predominantly male. About 97% of the cases were reported in men (Eurostat, 2004).

Workers with hearing loss face challenges to their personal safety, show higher rates of absenteeism, may be at increased risk (probability) of work-related injuries, and are more likely to be underemployed or unemployed (Neitzel et al., 2017). Worker who has hearing loss and left untreated can lead to communication difficulty, fatigue and stress (see review by Themann et al., 2013).

Every day, we hear sound from television, radio, smart phone, household appliances, music and traffic. These sounds can be harmful when they are too loud, even for a brief time and can damage the inner ear and cause NIHL. NIHL can be caused by one-time exposure to an intense impulse sound such as an explosion or by continuous exposure to loud sound over an extended period of time. Recreational activities that can put at risk for NIHL such as target shooting and hunting, listening music at high volume through earbuds or headphones, playing in a band, and attending loud concerts. Tinnitus with a buzzing, ringing, or roaring in the ears can be caused by high noise exposure. NIHL is the only type of hearing loss that is completely preventable (NIDCD, 2014).

There are some noise exposure limits standards & requirement such as from OSHA (Occupational Safety and Health Administration), NIOSH (National Institute for Occupational Safety and Health), ACGIH (American Conference of Governmental Industrial Hygienists) and MoM RI (Ministry of Manpower Republic of Indonesia). OSHA, NIOSH, and ACGIH are international standards which based at United States (US). OSHA is part of Department of Labor at US. NIOSH is part of Centers for Disease Control and Prevention (CDC) in Department of Health and Human Services at US. While ACGIH is a professional association of industrial hygienist and practitioners of related profession.

MoM RI has released noise exposure limit regulation under Permenaker No.8/2018. This regulation has the same exposure limit and 3-dB exchange rate with 2 other international standards under NIOSH and ACGIH. Threshold limit value (TLV) for an 8-hour time-weighted average (TWA) sound level is 85 dBA among those regulations. The difference of these regulations are described in Table 1.

Table 1. Duration of Noise Exposure Allowed without Wearing Hearing Protection Under the Criteria OSHA, NIOSH, ACGIH and MoM RI

PEL = permissible exposure level. REL = recommended exposure limit. TLV = threshold limit value. NAB = nilai ambang batas

Based on OSHA standard 29 CFR 1910.95, hearing conservation program (HCP) shall be implemented whenever employee noise exposures equal or exceed an 8-hour time-weighted average sound level (TWA) of 85 dBA equivalent to a dose of 50%. The % ADD (Allowable Daily Dose): An indication of employee exposures to varying noise levels over a length of time compared to the maximum unprotected dose permitted by OSHA (i.e., 100% ADD). An 8-hour TWA of 85 decibels or 50% ADD referred as the action level. In addition, hearing protection is mandatory above 50% ADD for employees who have experienced a Standard Threshold Shift (STS).

The Action Level is equivalent to an 8-hour time weighted average of 85 dBA; a 16-hour time weighted average of 85 dBA; and so on (see Table 1 on OSHA Criteria). If refer to NIOSH, ACGIH and MoM RI, the action level is at 82 dBA. Doses at or above the Action Level require that employees be included in a Hearing Conservation Program. In addition, hearing protection is mandatory above 50% ADD for employees who have experienced a STS.

STS is the reduction of hearing sufficient to require follow up action. It is defined by OSHA as an average decrease in hearing ability of 10 dBA or more in either ear at frequencies of 2000, 3000 and 4000 Hz when compared to the baseline audiogram. The STS of hearing loss audiogram result can be seen at Figure 1. Among other requirements described in the OSHA standard, refitting and retraining on hearing protectors are required for an employee who experiences a persistent STS, and use of protectors is mandatory for exposures above 50% ADD. The STS data is taken from annual audiometry test during Medical Check Up (MCU). An STS is calculated by subtracting the baseline audiogram results from the most recent audiogram at 2000, 3000 and 4000 Hz for each ear and taking the average of these values.

Figure 1. Typical audiogram patterns for people with conductive and sensorineural hearing loss.

Source: OSHA, 2013

Based on health risk assessment (HRA) result of Star Energy Geothermal Darajat II, Limited, there are similar exposure group (SEG) who have potential exposure to noise hazard as described at Table 2.

Table 2. Summary HRA on Noise Exposure Based on SEG

Source: SEGD, 2019

2. METHOD

This research is descriptive quantitative method to describe and explain the noise monitoring, noise mapping and hearing conservation program at Darajat geothermal field by data collection method (direct reading by internal company and external party), review of literature, interview & discussion. The rationale of this quantitative approach was to explore and describe the experiences and best practice of industrial hygiene (IH) program especially in handling noise hazard in geothermal operation.

Primary data from direct reading is taken from IH report from period 2018-2020 such as noise mapping and noise dose measurement to some employees and contractors at Darajat geothermal field during that period. Secondary data are taken from journal, textbook and laboratory result from external party. Annual IH monitoring is conducted to meet regulatory requirement. Indonesia regulatory requirement is based on Ministry of Manpower regulation (Permenaker No.5 / 2018) regarding Occupational Safety and Health at the Workplace.

Noise intensity measurement conducted by using noise intensity direct reading instrument, that is a sound level meter (SLM). Noise intensity measurement, conducted by setting the SLM at the height of human ear area and/or at the height of ± 1.5 m from working floor. Design criterion = 65 dBA, and exchange rate (ER) = 3 dBA to meet requirement at SNI 7231-2009 Measurement Methods for Noise Intensity at Workplace.  The noise dose measurement conducted by using noise dose meter installed on at the breathing zone of employees during their normal activity. The noise dose sampling is taken on 8-hour TWA.

This study will contribute to the literature on noise hazard at Darajat geothermal operation, the mitigation, and best practice HCP program. By sharing this paper, we hope noise hazard at geothermal operation can be handled safely so it will improve worker morale & productivity.

3. RESULT & DISCUSSION

Noise mapping & noise dose monitoring is conducted annually at Darajat geothermal field. The noise monitoring is conducted both internally by SEGD or externally by third party. The noise mapping at power plant & well pad area during period 2018-2020, the result quite similar due to there is no significant changes in the process. The noise mapping result shown on this paper is only 2020 to represent the current condition of noise hazard at the Darajat.

During normal operation, the highest noise is at turbine floor of power plant area that reach 92 dBA. The control room area which located at the same floor, where the operators are working there, the noise level 50.9 – 62 dBA. There is a wall barrier between turbine area and control room area so the noise is not entering the control room area. So, if the operator or anyone exit the control room area, they shall wear hearing protection device (earplug or earmuff) due to the noise exceed 85 dBA. At turbine floor are, the highest noise point is between steam turbine unit 2 and steam turbine unit 3. The noise mapping result can be seen at Figure 2.

Figure 2. Noise Mapping at Turbine Floor Area

Source: SEGD, 2020

At the ground floor of power plant building, the noise mapping showed that the highest noise is 88.5 dBA. The highest noise point is between the 2 condensers on each unit 2 and unit 3. Between ground floor and maintenance office there is a wall barrier. So the maintenance personnel can do their administrative activity at the safe noise level 47.3 – 56.4 dBA. The maintenance office is enclosed with the solid wall and gypsum so the noise will not enter into the office. While maintenance workshop at the ground floor has a noise level 60.1 – 76.3 dBA. The maintenance workshop and ground floor of power plant only separate with the rolling door. The noise level is still within the safe limit below 85 dBA and the hearing protective device is not required to be worn at this area. The noise mapping at ground floor area is shown at Figure 3.

Figure 3. Noise Mapping at Ground Floor Area including Maintenance Workshop

Source: SEGD, 2020

The noise mapping result at power plant area consist of power plant building including maintenance workshop, cooling tower, and operation office are can been seen at Figure 4. The area inside Figure 4 including security post (Bravo 4), laboratory, water treatment, warehouse 2, and fire station. The highest noise point is between cooling tower 2 and unit 3 that reached 83.9 dBA. The other area has a safe noise level less than 85 dBA, so hearing protection device is not mandatory worn. At Figure 4, the power plant building with no noise contour color due to it has been covered in Figure 2 and Figure 3. While the other area at warehouse and water treatment is not measured due to the areas were locked during the measurement.

When entering the facility at power plant area to their office at operation office or maintenance office, employees and contractors going through via Bravo 4 with no hearing protection device required as it is at safe level to walk there without protection and the noise level is less than 85 dBA.

Figure 4. Noise Mapping at Power Plant, Cooling Tower and Operation Office Area

Source: SEGD, 2020

At Figure 5, it is the noise mapping at Well Pad 9, Porta Camp, Mosque, Clinic, Canteen and Facility Engineering (FE) Office area with noise level less than 85 dBA. During normal operation, at Well Pad 9, the noise level is low 58.9 – 70.1 dBA. This area also covering the Mosque, contractor offices area at porta camp. Employees and contractors can walk through at this area safely without hearing protection device.

Figure 5. Noise Mapping at Well Pad 9, Porta Camp, Mosque, Clinic, Canteen and FE Office Area

Source: SEGD, 2020

During non-routine activity, there is drilling activity at Darajat in 2020 period. Noise mapping is conducted to get baseline data of noise level at drilling area. There are many equipment and machines operate in this area with noise level 66.4 – 104.8 dBA. The highest noise point is at machine area such as generators, compressors and pumps. When entering the drilling area, everyone shall wear hearing protection devices to protect their hearing from noise hazard. They can pull out their earmuff or earplug at the safe area such as inside their temporary office at porta camp. The noise mapping result shown at Figure 6.

Figure 6. Noise Mapping at Drilling Project at Well Pad 20 Area

Source: SEGD, 2020

At Darajat geothermal field, there are some routine activity where the noise exceeded 85 dBA which are during steam blow and vertical discharge activity. Steam blow activity on 3 August 2018 at Helipad Area, the maximum noise reached 97.5 dBA. Flow test vertical discharge test (VDT) activity on 15 August 2018 at Pad SF, the maximum noise reached 133.1 dBA. Those maximum noise data are taken at the 1-meter distance from the source of noise. When the distance further from the source, the noise level is getting lower. The noise measurement during non-routine activity conducted internally to take mitigation plan, so the activity can be run safely. Currently, the VDT is avoided and changed to other method, as it is prohibited by authority due to highly noise hazard and have environmental issue as well as community issue.

The other activity that has high noise hazard such as cutting or grinding process that usually performed by maintenance support or construction personnel. The use of equipment such as mobile diesel generator, compressor and operating of heavy equipment or heavy equipment are resulting in high noise exposure hazard. Employees and contractors expose with noise hazard more than 85 dBA shall wear earmuff or earplug to protect their hearing.

The noise dose monitoring also conducted on annual basis to monitor if any significant changes on the noise exposures to employees or contractors working at the noisy environment. The noise dose sampling result during period 2018-2020 shown at Figure 7. The noise dose sampling based on the SEG at Table 2.

The noise dose sampling result showed at period 2018, the noise dose is below 85 dBA while at period 2019 and 2020 some noise dose sampling showed there are some personnel exceed 85 dBA. During period 2019-2020 there are some big project at Darajat field that exposed to high noise hazard, especially construction activities to prepare drilling project such as piping installation, well pad opening, road opening, soil cut & fill, etc. The drilling activity itself, started from May 2019 until first quarter 2020. Many heavy vehicles and heavy equipments involved with high noise hazard during the activity.

Figure 7. Noise Dose Sampling Result Period 2018-2020

Source: SEGD, 2018-2020

Tabel 3 show the noise dose sampling result, % dose and summary whether the HCP (hearing conservation program) is required or not. The action level is based on 50% ADD which is 82 dBA as per Permenaker No.8/2018. The % dose figure the 8-hour TWA using the following equation:

TWA = 16.61 Log10 (D/100) + 85

Where:

TWA      = the 8-hour Time Weighted Average Sound Level
D            = the noise exposure dose. Dose % as calculated belowe (or measured with a dosimeter)
Log10     = the Logarithm to base 10

The noise dose is calculated as:

Dose = 100 x (C1/T1 + C2/T2 + … + Cn/Tn)

Where:

C_n           = total duration of exposure at specific noise level
T_n           = total duration of exposure permitted at the level

Table 3. The % Dose Result of Noise Dose Monitoring

Source: SEGD, 2020

The % noise dose is not considering the wearing of hearing protection devices such as earmuff or earplug. The earmuff or earplug have different value of noise reduction rating (NRR). NRR is a single number that represents the overall average noise reduction, in decibels, that a hearing protector can provide in an environment with a known C-weighted noise level. When working with A-weighted noise levels, subtract 7 dB from the labeled NRR to obtain an estimate of the A-weighted noise level under the protector.

Calculated attenuation values reflect workplace values only if protectors are properly fitted and worn. The NRR value for the protectors is based on controlled laboratory tests in which experimenters fit hearing protectors on trained listeners. Published studies indicate that the NRR obtained in these tests is often not achieved in the workplace. OSHA has instructed its compliance officers to derate (reduce) the NRR by 50% in enforcing the engineering control provision of the OSHA noise standard. For example, operator working at turbine floor with highest noise is 92 dBA and he wear earmuff with NRR 27 dB, the calculation of estimated sound level at the operator’s ear when wearing earmuff as follow:

L_prot        = L – (NRR-7)/2

= 92 – (27—7)/2

= 82 dBA

Where:

L            = the measured level or TWA

NRR       = the noise reduction rating provided by the hearing protector manufacturer

L_prot        = the estimated sound level at the ear when waring the hearing protection

So, the estimated sound level received by operator’s ear at the turbine floor is 82 dBA. If the the 8-hour TWA from Operator 2 as described at Table 3, the TWA is 90.1 dBA then the estimated sound level at ear’s operator 2 is 80.1 dBA when wearing earmuff with NRR 27 dB.

Based on summary at Table 3 above, the similar exposure group that need to be included in the HCP are: RM Technician, Construction Worker, Operator, Contractor Overhaul, Mechanic, and Instrument personnel. The similar exposure group that do not required HCP, hierarchy of control on noise hazard is also implemented such as engineering control, administrative control, and wear of personal protective equipment (PPE). The scope of HCP is for Star Energy Geothermal Darajat II, Limited (SEGD) employees, while HCP for contractors conducted by their internal company. SEGD monitor the HCP implementation through the contract requirements such as annual MCU include audiometric test and the use of PPE (earmuff or earplug) provided by contractor. The HCP training conducted by SEGD also include the contractors’ personnel.

The HCP is conducted to prevent noise-induced hearing loss (NIHL). As per Presidential Regulation No.7/2019, NIHL is part of noise related disease and it shall be reported to Ministry of Manpower and other related institution such as BPJS Ketenagakerjaan. If any confirmed NIHL case, it will become work-related incident and the employees can claim the insurance. HCP continuously implemented at SEGD. There are 5 elements of HCP at SEGD which are: exposure monitoring; noise controls; annual audiometric tests/hearing evaluations; hearing protection; and annual employee training.

1. Exposure Monitoring

The noise exposure monitoring conducted annually at Darajat geothermal field. The noise mapping & noise dose monitoring are part of exposure monitoring. Noise mapping has a purpose to capture the noise hazard area at Darajat, whether any significant noise hazard changes at field.

Noise dosimeter monitoring has 2 purposes, first, it identifies which employees receive noise doses that meet or exceed the action level and therefore must be enrolled in HCP. Second, it enables proper selection and evaluation of hearing protection devices. Noise exposures measured by noise dosimeters that individual employees wear during their activity. Noise monitoring is required for every similar exposure group that is likely to exceed dose of 50% ADD.

It must be repeated when changes in the job, equipment, process or control methods increase exposures to the point that additional employees may need to be included in the Program or the attenuation of hearing protectors may be inadequate. Spot checks at least every 1-2 years are recommended, even if no changes occur, to identify any unexpected changes in noise doses. Noise dosimeter is conducted by third party under the contract. The third party shall meet or exceed qualification in the contract include meet the regulatory requirement.

2. Noise Control

The noise exposures need to be reduced using feasible engineering or administrative controls whenever employee exposures exceed 100% ADD. Engineering control include eliminating or modifying existing machinery and placing noise limit specifications on new machinery. Barrier between the noise source and the workplace where the employee usually work at that area is part of engineering control. Administrative controls involve reducing exposure duration through job rotation or scheduling to control the overall dose. It engineering control and administrative control fail to reduce noise doses below 50% ADD, then PPE shall be worn. SEGD will provide the PPE to employees and change it if broken or not in good condition anymore.

The highest hazard at Darajat geothermal field is at turbine floor area. The control room where the operators work in that area are working at the turbine floor area. There is concrete block between control room and turbine area so that the noise will not enter into the control room. During the noise measurement. The control room noise is between 50.9 – 62 dBA. So, operators no need to wear earmuff or earplug inside the control room as it is safe noise level.

3. Annual Audiometric Test

Audiometric test conducted during annual medical checkup (MCU) to those employees whose noise exposure equals or exceeds 50% ADD. Usually the MCU is conducted at operation office Darajat field. The audiometric test shall be preceded by at least 14 hours without workplace noise exposure. Hearing protectors can be worn as a substitute if this requirement cannot be met. The employees also be advised to avoid off the job noise for 14 hours prior to the test.

All audiometric tests must be repeated annually (i.e., within 12 months) for employees who continue to be exposed to noise levels equal to or above 50% ADD. When exposure drops below 50% ADD, employees may be removed from the program. Doctor on duty and Medical Department will review all audiograms in the HCP. The annual audiogram is compared to the current baseline test to detect and review if any standard threshold shift (STS). Medical team will inform the employees if any STS and if required, the re-test to be performed to determine if the shift is persistent or temporary.

If any employee has STS, Medical will review and advise the employee personally at Darajat clinic. Medical will revise the employee’s baseline audiogram when an STS is confirmed with a second audiogram. Medical may also revise the baseline audiogram if a significant hearing improvement compared to the current baseline is confirmed.

4. Hearing Protection

Hearing protection is only effective when it fits properly and is worn correctly. After proper training, workers must be allowed to select their hearing protectors from a variety of types provided by the employer so that each individual can find something comfortable with adequate noise attenuation properties.

There are 3 types of hearing protection devices that is used at Darajat geothermal field, they are formable plugs, pre-molded plugs, and earmuff. Employees can choose base on their comfortable when using it and adequate noise attenuation properties. In certain cases, such as during non-routine activity like steam blow, due to the noise is very high, double protection (earplug and earmuff) shall be worn together to reduce noise exposure to employees.

5. Employee Training

Employees training is conducted for employees included in the HCP minimum every 2 years, and annual as a refresher training. At Darajat, the training conducted together by Medical and SHE (safety, health and environment) personnel. The training material consist of effect of noise on hearing, hearing protectors (purpose, advantage, disadvantage, attenuation of various types, selection fitting, use, and care), audiometric testing and the noise mapping result.

Communication of noise hazard also conducted during daily morning safety talk (MST). The topic of MST is changed on weekly basis. One of the MST topic is about hearing conservation program. All employees and contractors are briefed on this topic at different location of MST. The HCP topic during MST is discussed to increase awareness.

Warning sign of noise hazards are posted at some areas so all employees, contractors or visitors will aware if any hearing protection devise shall be worn at that area. The noise mapping also posted at high noise hazard such as at power plant building area.

4. CONCLUSION

During routine activity at Darajat geothermal field, the area that has noise hazard above 85 dBA continuously is at turbine floor area. Other areas such well pad, office, warehouse, clinic has noise level less than 85 dBA. Some of non-routine activity which have high noise exposure hazard above 85 dBA are steam blow, flow test, drilling and construction activities.

Based on noise dosimeter monitoring in 2020, similar exposure group from RM Technician, Construction Worker, Operator, Contractor Overhaul, Mechanic, and Instrument personnel are included in the HCP. By implementing 5 elements of HCP (exposure monitoring, noise control, annual audiometric test, hearing protection usage, and employee training), it will prevent NIHL and improve employee morale, and reduce stress levels among exposed employees.

5. REFERENCES

Eurostat: Work and Health in the EU: A Statistical Portrait, Luxemborg Office for Official Publications of the European Communities, (2004). Available at: http://edz.bib.uni-mannheim.de/www-edz/pdf/eurostat/04/KS-57-04-807-EN-N-EN.pdf  Accessed on 14 April 2020.

Feder, K., Michaud, D., McNamee, J., Fitzpatrick, E., Davies, H., and Leroux, T.: Prevalence of hazardous occupational noise exposure, hearing loss, and hearing protection usage among a representative sample of working Canadians, J. Occup. Environ. Med. 59(1), (2017), 92–113.

Fuente, A., and Hickson, L: Noise-induced hearing loss in Asia, Int. J. Audiol. 50, (2011), S3–S10.

Kerns, E., Masterson, E. A., Themann, C. L., and Calvert, G. M.: Cardiovascular conditions, hearing difficulty, and occupational noise exposure within U.S. industries and occupations, Am. J. Ind. Med. 61, (2018), 477–491.

Masterson, E. A., Deddens, J. A., Themann, C. L., Bertke, S., and Calvert, G. M.:Trends in worker hearing loss by industry sector, 1981-2010, Am. J. Ind. Med. 58, (2015), 392–401.

National Institute on Deafness and Other Communication Disorders (NIDCD): Noise-Induced Hearing Loss, (2014). Available at: https://www.nidcd.nih.gov/health/noise-induced-hearing-loss. Accessed on 18 April 2020

Nelson, D. I., Nelson, R. Y., Concha-Barrientos, M., and Fingerhut, M.: The global burden of occupational noise-induced hearing loss, Am. J. Ind. Med. 48(6), (2005), 446–458.

Neitzel, R. L., Swinburn, T. K., Hammer, M. S., and Eisenberg, D.:Economic Impact of Hearing Loss and Reduction of Noise-Induced Hearing Loss in the United States, J. Speech Lang. Hear. Res. 60(1), (2017), 182–189.

Occupational Safety and Health Administration (OSHA): OSHA Technical Manual, Section III Chapter 5: Noise, (2013). Available at: https://www.osha.gov/dts/osta/otm/new_noise/. Accessed on 21 May 2020

Occupational Safety and Health Administration (OSHA): 1910.95-Occupational Noise Exposure, (1995). Available at: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.95. Accessed on 21 May 2020

Star Energy Geothermal Darajat II, Limited (SEGD): Industrial Hygiene Monitoring Report, (2018), 14-35

Star Energy Geothermal Darajat II, Limited (SEGD): Industrial Hygiene Monitoring Report, (2019), 11-29

Star Energy Geothermal Darajat II, Limited (SEGD): Industrial Hygiene Monitoring Report, (2020), 15-40

Star Energy Geothermal Darajat II, Limited (SEGD): Health Risk Assessment, (2019), 21-24

Tak, S., Davis, R. R., and Calvert, G. M.:Exposure to hazardous workplace noise and use of hearing protection devices among US workers—HANES, 1999–2004, Am. J. Ind. Med. 52(5), (2009), 358–371.

Themann, C. L., Suter, A. H., and Stephenson, M. R.: National research agenda for the prevention of occupational hearing loss—Part 1, 34(3), (2013), 145–207.

Williams, W.: The epidemiology of noise exposure in the Australian workforce, Noise Health 15(66), (2013), 326–331.

World Health Organization (WHO): Deafness and Hearing Loss, (2020), 1-3. Available at: https://www.who.int/news-room/fact-sheets/detail/deafness-and-hearing-loss. Accessed on 14 April 2020

Note: the video presentation can be seen at the Youtube as below