Health Hazard Evaluation Report 2009-0224-3274 Page 1

Evaluation of Noise Exposures in the Knockoff 

Areas of an Iron Foundry

Report No. 2009-0224-3274

March 2017

Scott E. Brueck, MS, CIH

U.S. Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health
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Contents

Highlights...............................................i

Abbreviations ..................................... iii

Introduction......................................... 1

Methods ............................................... 2

Results and Discussion........................ 3

Conclusions ........................................ 13

Recommendations............................. 14

Appendix A ........................................ 17

References.......................................... 21

Acknowledgements........................... 23

The employer is required to post a copy of this report for 30 days at or near the 

workplace(s) of affected employees. The employer must take steps to ensure 

that the posted report is not altered, defaced, or covered by other material.

The cover photo is a close-up image of sorbent tubes, which are used by the HHE 

Program to measure airborne exposures. This photo is an artistic representation that may 

not be related to this Health Hazard Evaluation. Photo by NIOSH.
Health Hazard Evaluation Report 2009-0224-3274 Page i

We measured sound levels and 

noise exposures in the knockoff 

areas of an iron foundry. All 

employees we monitored had 

noise exposures greater than 

noise exposure limits, and most 

noise exposures were greater 

than 100 decibels, A-weighted. 

We recommended installing 

noise controls and reducing 

noise exposure by replacing 

current equipment with less 

noisy equipment. 

Highlights of this Evaluation

The Health Hazard Evaluation Program received a request from a gray and ductile iron 

foundry. The employer was concerned about high noise levels.

What We Did

● We measured employees’ noise exposures in the knockoff areas of the foundry.

● We measured sound levels and noise frequencies at work equipment and during 

different work activities.

What We Found

● Sound levels in the knockoff areas were very 

high. Most noise was caused by metal-tometal contact, shaker conveyors and other 

equipment, and vibration. 

● All employees working in the knockoff areas 

had noise exposures above occupational 

exposure limits.

● Some employees did not fully insert their 

foam earplugs.

● Some employees in the knockoff areas did not 

wear both insert ear plugs and earmuffs.

What the Employer Can Do

● Install noise controls to reduce noise caused by metal-to-metal contact and vibration 

from equipment and shaker conveyors.

● Consult with equipment makers when purchasing new equipment or replacing 

equipment to get equipment that makes the least amount of noise.

● Make sure employees wear their hearing protection properly.

● Use National Institute for Occupational Safety and Health recommendations for 

evaluating employees’ hearing tests.

What Employees Can Do

● Keep the barriers at the exit of the Didion rollers properly positioned across the opening. 

● Wear hearing protection properly to help prevent hearing loss.

● Tell your doctor that you work in areas with high noise levels and discuss hearing 

problems you have.
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Health Hazard Evaluation Report 2009-0224-3274 Page iii

Abbreviations

AL Action level

CFR Code of Federal Regulations

dB Decibels

dBA Decibels, A-weighted

Hz Hertz

NIOSH National Institute for Occupational Safety and Health

NIHL Noise induced hearing loss

OEL Occupational exposure limit

OSHA Occupational Safety and Health Administration

PEL Permissible exposure limit

REL Recommended exposure limit

TLV® Threshold limit value

TWA Time-weighted average

WEELTM Workplace environmental exposure level
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Health Hazard Evaluation Report 2009-0224-3274 Page 1

Introduction

The Health Hazard Evaluation Program received a request from a ductile iron foundry. The 

employer was concerned about noise exposures in the knockoff areas of the facility. The 

company was also seeking guidance on noise control solutions. The Occupational Safety 

and Health Administration (OSHA) inspected the foundry in 2009 and issued citations for 

excessive noise exposures. OSHA suggested that the company contact the Health Hazard 

Evaluation Program for additional noise evaluation and assistance in identifying noise control 

options. We visited the facility in April 2011 to measure employees’ noise exposures and 

noise levels in the knockoff areas, and assess noise control options. We provided summary 

personal noise measurement results and preliminary noise control recommendations in a 

detailed letter to the employer and employee representatives in May 2011.

The foundry manufactured automotive parts for camshafts, axles, braking systems, and 

hydraulic rams out of ductile iron. The foundry had seven knockoff areas where employees 

separated ductile iron castings from the metal riser and runner gating that remained attached 

following the casting process. After casting, the work pieces moved from the casting area 

to the knockoff areas via Didion® rotary sand/casting separators, which were rotating 

metal tunnel structures that separated foundry sand from the casting. The castings exited 

the rotary separator and dropped onto shaker pan conveyors in the knockoff areas. The 

shaker conveyors were custom-made for the company and had a preset rate of throw that 

slowly moved the castings by mechanical vibration of the entire metal pan. Employees in 

the knockoff areas stood on or next to the shaker pan conveyor and separated the castings by 

manually striking the riser and runner gating with a sledgehammer or against the side rail on 

the conveyor. On some castings, depending on the configuration, employees used a hydraulic 

wedge to separate castings from excess gating (Figure 1). After separation, employees 

manually moved or tossed the riser and runner gating onto a separate conveyor or exit chute 

for recycling. Each knockoff area had one to four employees. Six of the seven knockoff 

areas were located in individual rooms. One of the knockoff areas was open to the foundry. 

The facility operated two work shifts per day. Employees normally worked 8-hour shifts. 

However, employees could work longer shifts, depending on production demands.
Page 2 Health Hazard Evaluation Report 2009-0224-3274

Figure 1. Hydraulic wedge suspended at one of the shaker conveyors. Photo by NIOSH.

Methods

Our objectives included the following: 

● Evaluating knockoff employees’ full-shift time-weighted average (TWA) noise exposures

● Measuring one-third octave band noise frequency levels

● Identifying noise control options

We measured full-shift TWA noise exposures on a representative number of employees in 

all of the knockoff areas. Specifically, we monitored 13 employees during the first shift and 

4 employees during the second shift. We used Larson Davis SparkTM model 706RC 

integrating noise dosimeters. We placed the dosimeter microphone on the top of the 

employee’s shoulder at the midpoint between the neck and edge of the shoulder. The 

dosimeters simultaneously collected data using three different settings to allow comparison 

of noise measurement results with three different noise exposure limits, the National Institute 

for Occupational Safety and Health (NIOSH) recommended exposure limit (REL), the OSHA 

action level (AL), and the OSHA permissible exposure limit (PEL). NIOSH uses a 3-decibel 

(dB) exchange rate, and OSHA uses a 5-dB exchange rate. The exchange rate expresses how 

much the sound level could increase or decrease while keeping the risk of hearing loss the 

same, if the exposure duration was simultaneously decreased or increased. For example, a 

3-dB exchange rate requires that noise exposure time be halved for each 3-dB increase in 

noise levels; likewise, a 5-dB exchange rate requires that exposure time be halved for each 

5-dB increase in noise. NIOSH considers noise measured using the 3-dB exchange rate to 

more accurately relate noise exposure to hearing loss risk. Additional information on noise 

exposure limits and health effects is provided in Appendix A.

We used Larson Davis Model 824 integrating sound level meters and frequency analyzers to 

measure sound levels and one-third octave band noise frequency levels (i.e., measurement 
Health Hazard Evaluation Report 2009-0224-3274 Page 3

of noise levels across 30 different frequencies). For most measurements we positioned the 

instrument, either by using a tripod or holding by hand, at a height of approximately 5 feet 

above the floor and within 3–6 feet of the employees or the primary noise source in the vicinity. 

Results and Discussion

Personal Noise Dosimetry Measurements

Personal noise dosimetry results are provided in Table 1. We compared employees’ noise 

monitoring results to the noise exposure limits set by NIOSH and OSHA. These noise 

exposure limits are meant to be the amount of noise that most employees can be exposed 

to without substantial risk of hearing loss. OSHA and NIOSH measure and calculate noise 

exposures in slightly different ways, as noted above. For an 8-hour work shift, the NIOSH 

REL is 85 decibels, A-weighted (dBA); the OSHA AL is 85 dBA; and the OSHA PEL 

is 90 dBA. For work shifts longer than 8 hours, the NIOSH REL and the OSHA AL are 

adjusted, as detailed in Appendix A. The OSHA PEL is not adjusted for extended work shifts. 

Employers are required to keep noise exposures below OSHA limits. However, NIOSH 

considers its REL to be more protective.

Table 1. Time-weighted average personal noise exposure results (dBA)

Knockoff

location

Shift Duration*

(hours:minutes)

Result based 

on NIOSH REL 

criterion

Result based 

on OSHA AL 

criterion

Result based 

on OSHA PEL 

criterion

Herman 1st 11:42 106.4 105.7 105.6

Herman 1st 9:37 104.7 104.3 104.3

DISA 1 Hot 1st 5:57 107.1 106.4 106.4

DISA 1 Cold 1st 6:02 104.8 102.6 102.5

DISA 1 Cold 1st 6:52 104.1 102.1 102.1

DISA 2 Hot 1st 7:07 100.5 99.5 99.4

DISA 2 Cold 1st 5:57 107.1 106.4 106.4

2071 1st 7:47 100.9 99.8 99.7

2071 1st 8:00 100.3 99.4 99.4

2071 2nd 7:06 101.6 100.9 100.9

2071 2nd 7:06 100.1 99.7 99.7

2072 1st 7:49 105.4 103.8 103.7

2072 1st 7:49 102.7 101.1 101

2072 (Sorter) 1st 7:43 97.7 96.1 95.7

2072 2nd 7:55 107.9 107.5 107.5

2072 2nd 7:00 106.6 106.6 106.6

Noise exposure limits (8-hour work shift) 85 85 90

*Noise exposure measurements of less than 8-hours duration are assumed to represent 8-hour 

time-weighted average exposures, on the basis of employees typical work activities during their 

full work shift.

Note: One measurement during the first shift in the DISA 2 cold knockoff could not be used 

because of a dosimeter malfunction.
Page 4 Health Hazard Evaluation Report 2009-0224-3274

Noise sources in the knockout areas included noise from metal-to-metal contact, mechanical 

noise from equipment, and noise generated by vibration of shaker conveyors. Results showed 

that employees in all the job titles we monitored had full-shift TWA noise exposures greater 

than the NIOSH REL, OSHA AL, and OSHA PEL. Employees normally worked 8-hour 

work shifts, but on the days we monitored for noise employees in the Herman knockoff area 

worked longer shifts. Noise levels in knockoff 2071 and in the DISA 2 hot knockoff were 

slightly less than the other knockoff locations. However, employees in all of the knockout 

areas, except for the sorter station in knockoff 2072, had TWA noise exposures greater 

than 100 dBA. This was similar to previous noise measurements taken by OSHA and the 

company. NIOSH and OSHA recommend dual hearing protection, i.e., the use of both inserttype hearing protectors and earmuffs, when noise exposure is above 100 dBA. The company 

required that all employees in the knockoff areas wear dual hearing protection. However, we 

observed that some employees only used single protection. 

One-third Octave Band Sound Level Measurement 

Results and Noise Controls

Workplace noise is usually broadband noise that is distributed over a wide range of 

frequencies. However, noise from equipment, processes, and work activities can also 

have unique frequency-specific characteristics. For analysis of the frequency distribution 

characteristics of workplace noise, the frequency spectrum is broken into smaller frequency 

bands called bandwidths, the most common being the octave band, which is defined as a 

frequency band where the upper band frequency is twice the lower band-edge frequency. 

The one-third octave band further divides each of the single octave bands into three smaller 

frequency bands to provide even more detailed information about the noise frequency 

distribution characteristics. This information is useful for identifying the dominant 

frequencies of noise sources and determining appropriate engineering controls or other noise 

reduction measures. For example, if low frequency noise is dominant (i.e., the highest sound 

levels occur in frequencies of 500 Hertz [Hz] or less), noise is likely generated by vibration, 

and noise controls that reduce or isolate the vibration from tools or equipment might help 

decrease noise levels. If high frequency noise is dominant (i.e., the highest sound levels occur 

in frequencies of 3,000 Hz or greater), the most effective approach for noise reduction is to 

use noise enclosures, barriers, or sound absorption [Driscoll and Royster 2003]. 

Predominant sources of noise in the knockoff areas included the following: 

● Metal castings or riser and runner gatings being dumped or dropped into metal-sided 

bins or onto flat metal shaker conveyors

● Metal castings, risers, and runners moving and bouncing on shaker pans or in Didion rollers 

● Employees using sledgehammers or striking castings against metal rails of shaker 

conveyors to separate risers and runners from castings

● Mechanical and vibration-related noise from operation of the shaker conveyors

● Noise caused by loose or rattling parts on conveyors or other equipment
Health Hazard Evaluation Report 2009-0224-3274 Page 5

Noise from Metal-to-Metal Contact and Vibration

In several of the knockoff areas, metal casting dropped 12–36 inches from Didion rollers 

onto metal shaker conveyors or from one shaker conveyor onto another shaker conveyor. 

Figure 2 shows the time history sound level profile for two cycles of castings being dropped 

about 18 inches from the hopper just outside the DISA 1 cold room onto the shaker conveyor 

below the hopper. Sound levels were 115–120 dBA from metal impact as castings were 

dumped onto the shaker conveyor. Sound levels decreased to 102–103 dBA when castings 

were not being dumped onto the conveyor. This background noise was from the operation of 

the shaker conveyor and from castings bouncing on the shaker pan. Employees in the work 

area reported that castings can be dumped onto this conveyor up to 150 times per shift. 

Figure 2. Time history sound level profile showing the sound level characteristics for two cycles of 

castings being dropped onto the shaker conveyor in the DISA 1 area.

Octave band measurements at the DISA 1 cold knockoff conveyor (Figure 3) reveal 

differences in octave band noise levels when castings were moving on the shaker conveyor 

versus when casting were not on the conveyor. Overall sound levels were nearly 110 dBA 

when castings were moving on the shaker conveyor and employees were hand-separating the 

castings. Sound levels decreased to about 93 dBA when no castings were bouncing 

on the shaker conveyor. Octave band sound level differences were minimal from 12.5–125 

Hz. Sound at these low frequencies was likely due to noise generated from the vibration 

transmitted by operation of the motor, conveyor drive system, and movement of the shaker 

pan. However, across the frequencies 160–20,000 Hz sound levels were 5–21 dB higher, 

showing the contribution of noise from castings moving and bouncing in the conveyor pan.
Page 6 Health Hazard Evaluation Report 2009-0224-3274

Figure 3. One-third octave noise frequency measurements at the cold knockoff shaker conveyor show 

frequency-dependent noise level differences when castings were moving on the shaker conveyor 

versus when castings were not on the shaker conveyor.

Low frequency noise is readily transmitted from equipment to surrounding surfaces. 

We observed that shaker conveyors were bolted directly to the concrete floor (Figure 4). 

However, vibration isolation pads or springs were not used to reduce vibration transmission 

to the floor. Additionally, some of the bolts had become loose, which could increase overall 

vibration of the shaker conveyor frame structure. Strategies to reduce noise due to vibration 

transmission include placing the shakers on vibration isolation pads and eliminating 

connection of other structures to the shaker frame. For example, the metal floor platform in 

the DISA 1 cold knockoff area was connected directly to the frame under the shaker, which 

caused the floor platform to vibrate. We also noted that the sound levels decreased 5 dB when 

the shaker had been turned off between two separate runs of castings.

Figure 4. Frame of shaker conveyor bolted directly to the concrete floor. Photo by NIOSH.
Health Hazard Evaluation Report 2009-0224-3274 Page 7

At the Herman knockoff area, castings exited a chute and dropped about 3 feet from 

one shaker conveyor onto another shaker conveyor. The time history sound level profile 

measurements in this area (Figure 5) revealed that background sound levels were 105 dBA 

from operation of the conveyor and movement of castings on the conveyor. Sound levels 

increased to about 120 dBA when castings fell from one conveyor onto the other conveyor. 

Figure 5. Time history sound level profile as castings dropped 3 feet from one shaker conveyor to 

another shaker conveyor at the Herman knockoff area.

Figure 6 shows large finished metal castings falling from the end of a shaker conveyor into 

metal bins. Average sound levels when the castings were moving down the shaker conveyor 

and falling into the bin were 107–111 dBA due to noise from the castings bouncing on the 

shaker conveyor, mechanical noise from the shaker conveyor, and impact noise from parts 

falling into the bin. Peak sound levels reached 134 dB when castings fell up to 5 feet into the 

metal bins and struck the bottom, side, or edge of a bin.
Page 8 Health Hazard Evaluation Report 2009-0224-3274

Figure 6. Metal castings dropping from end of shaker conveyor into two metal bins. Photo by NIOSH.

Reducing the speed and force of impacts and reducing vibration and resulting reverberant 

ringing of metal surfaces after impact would help decrease metal-to-metal contact noise. 

Overall noise reduction strategies include reducing the distance that metal castings fall or 

are dropped, modifying processes so castings slide from one level to another, increasing the 

thickness or damping of solid metal surfaces on bins and shaker conveyors, covering metal 

surfaces with durable polymers, and replacing solid surface metal bins with durable plastic 

bins or expanded metal bins. 

We noted that in some work areas loose parts or equipment generated noise because of 

excessive rattling. For example, the metal casting diverter at the DISA 1 hot knockoff 

shaker conveyor did not fit firmly into its two-prong metal holding fittings and therefore 

rattled substantially (Figure 7). Sound levels near the diverter decreased 3–7 dBA when a 

NIOSH investigator put his foot on the diverter to prevent it from rattling. Most of this noise 

reduction occurred in the higher frequencies (Figure 8), whereas noise levels in the lower 

frequencies were caused by mechanical operation and vibration of the shaker conveyor. 
Health Hazard Evaluation Report 2009-0224-3274 Page 9

Figure 7. Casting diverter at the DISA 1 hot knockoff area. Photo by NIOSH.

Figure 8. One-third octave noise frequency measurements showing the difference in sound levels 

when the diverter was rattling versus when the diverter was not rattling.
Page 10 Health Hazard Evaluation Report 2009-0224-3274

Noise Barriers and Acoustic Wall Treatments

The Didion roller in some of the knockoff areas had a metal barrier that employees could 

slide into place at the exit of the Didion to help block noise (Figure 9). Sound level 

measurements at the exit for one of the Didion rollers showed that sound levels were 5 dB 

lower when the barrier was in place compared to when it was not in place. Barriers reduce 

noise primarily in higher frequencies. One-third octave band noise measurements indicated 

that sound level reduction occurred primarily at frequencies above 1,000 Hz (Figure 10). We 

observed that employees did not always move the barriers into place to help block noise at 

the exit of the Didion. Additionally, some of the Didion rollers did not have a barrier.

Figure 9. Barrier at exit of Didion roller. Photo by NIOSH.
Health Hazard Evaluation Report 2009-0224-3274 Page 11

Figure 10. One-third octave noise frequency measurements at the Didion show reduction of sound 

levels when the barrier was in place.

In several knockoff areas we observed that the company had installed thin plastic or 

rubber strips (Figure 11), or textile acoustic cloth to act as a noise barrier and separate 

employees in the knockoff areas from adjacent higher noise areas. However, the 

effectiveness of these noise barriers was limited because they had gaps or only partially 

covered openings. Additionally, the barrier material was thin, which also limited 

attenuation. In general, soft barriers such as these provide less noise attenuation than welldesigned solid barriers.

Figure 11. Employee work area in DISA 1 cold room. Photo by NIOSH.
Page 12 Health Hazard Evaluation Report 2009-0224-3274

The company had also installed quilted acoustic textile sound absorption treatment on the 

interior walls in some of the knockoff rooms to reduce reverberant noise within the rooms. 

However, in some knockoff rooms the acoustic treatment did not fully cover wall and ceiling 

surfaces (Figure 12). Additionally, we observed that in some knockoff rooms the acoustic 

material was damaged. These deficiencies can substantially reduce the effectiveness of 

acoustic treatments. 

Figure 12. Knockoff area showing a corrugated metal wall partially covered with acoustic material. 

Photo by NIOSH.

“Buy Quiet” Noise Control Program

In addition to installing noise engineering controls, noise reduction should be considered as 

part of an overall long-term Buy Quiet strategy. For example, when equipment is replaced, 

the amount of noise generated by the new equipment should be considered as part of the 

purchasing decision. Buy Quiet is a concept by which companies can reduce hazardous noise 

levels through their procurement process. Purchasers can compare noise emission levels for 

differing models of equipment and, whenever possible, choose the quieter equipment. 
Health Hazard Evaluation Report 2009-0224-3274 Page 13

Hearing Conservation Program

The company had a written corporate and site-specific hearing conservation program. All 

employees were included in the program. Audiometric testing had previously been done 

onsite or through a local occupational health provider, but starting in 2010 an external 

contractor came onsite with a mobile test van to provide audiograms. The company also 

began to keep electronic records of audiometric test results in 2010. 

We did not review employees’ audiometric test results, but we reviewed the audiometric 

testing program. The audiometric test provider identified threshold shifts using OSHA 

criteria. NIOSH recommends that threshold shifts are also identified using NIOSH criteria, 

as detailed in Appendix A. The audiometric provider referred employees for medical followup on the basis of a medical condition identified during the testing, excessive threshold 

differences compared to the medical baseline, or excessive threshold differences between 

ears. The company provided each employee with a letter detailing the employee’s hearing test 

results and recommendations for follow-up, if needed. Each audiometric test record included 

the employee’s job title and the employee’s noise exposure. However, the noise exposure 

level reported on all of the audiometric test records showed a “default” noise exposure of 

85 dBA, rather than the actual noise exposure for each employee. 

The company offered five models of earplugs (3MTM E-A-RTM ClassicTM, 3M Classic 

SuperfitTM, Howard Leight Max®, Howard Leight Maxlite®, and Moldex Goin’ Green®) 

and an earmuff (Howard Leight model L3). We observed that all employees in the foundry 

wore hearing protection. However, we saw that some employees in the knockoff areas did not 

wear dual hearing protection, as was company policy. We also saw that some employees in 

the mold area did not fully insert their hearing protection, which can substantially reduce the 

ability of the hearing protectors to attenuate noise. 

Although the manufacturers’ noise reduction ratings for these hearing protectors ranged 

29–33 dBA, hearing protectors do not typically achieve this level of attenuation [NIOSH 

1998]. Noise attenuation of insert-type hearing protection by individual users depends on the 

type of hearing protector, shape of the user’s ear canal, how well the hearing protector fits, 

and proper insertion of the hearing protector. Several hearing protection manufacturers have 

developed methods for fit testing individual employees to determine the attenuation they 

actually receive from the hearing protectors they use.

Conclusions

Employees’ TWA noise exposures in all of the knockoff jobs we monitored exceeded the 

NIOSH REL, OSHA PEL, and OSHA AL. Furthermore, TWA noise exposures in all of the 

knockoff locations were greater than 100 dBA. We observed that most but not all employees 

working in these areas wore dual hearing protection as recommended by NIOSH and OSHA. 

We recommend installing engineering noise controls, improving acoustic barriers and 

treatments, maintaining equipment to eliminate unnecessary rattling from loose parts, and 

implementing a Buy Quiet program to help reduce noise exposures. 
Page 14 Health Hazard Evaluation Report 2009-0224-3274

Recommendations

On the basis of our findings, we recommend the actions listed below. We encourage the 

ductile iron foundry use a labor-management health and safety committee or working group 

to discuss our recommendations and develop an action plan. Those involved in the work can 

best set priorities and assess the feasibility of our recommendations for the specific situation 

at this company. 

Our recommendations are based on an approach known as the hierarchy of controls 

(Appendix A: Occupational Exposure Limits and Health Effects). This approach groups 

actions by their likely effectiveness in reducing or removing hazards. In most cases, the 

preferred approach is to eliminate hazardous materials or processes and install engineering 

controls to reduce exposure or shield employees. Until such controls are in place, or if they 

are not effective or feasible, administrative measures and personal protective equipment may 

be needed. 

Elimination and Substitution

Eliminating or substituting hazardous processes or materials reduces hazards and protects 

employees more effectively than other approaches. Prevention through design, considering 

elimination or substitution when designing or developing a project, reduces the need for 

additional controls in the future.

1. Replace shaker conveyors with belt-driven conveyors, where feasible.

Engineering Controls

Engineering controls reduce employees’ exposures by removing the hazard from the process 

or by placing a barrier between the hazard and the employee. Engineering controls protect 

employees effectively without placing primary responsibility of implementation on the 

employee. No single noise engineering control will likely reduce employees’ noise exposures 

to below the noise exposure limits. However, a combination of multiple engineering controls 

can help reduce overall noise exposures.

1. In general, noise from metal-to-metal contacts can be decreased by reducing the speed 

and force of impacts and by reducing vibration and reverberant ringing of flat metal 

surfaces after impact. Specific actions that can be taken include the following:

a. Reduce the distance that metal castings or metal scrap drop into bins and onto 

 shaker conveyors.

b. Use slide pans with an angle of 45 degrees or less to move castings or scrap 

 pieces from one level to a lower level instead of allowing the pieces to drop 

 straight down. 

c. Increase the thickness of metal surfaces on shaker conveyors and walls of 

 metal bins with thicker metal or constrained layer damped metal. 

 Alternatively, replace metal bins with bins made of durable plastic 

 polymer materials.
Health Hazard Evaluation Report 2009-0224-3274 Page 15

d. Attach resilient plastic polymer material to the surface of flat metal pans and 

 shaker conveyors to reduce noise from metal casting bouncing or moving on 

 these surfaces. 

e. Perform preventive maintenance on equipment to eliminate unnecessary 

 rattling of metal parts. 

2. Place shaker conveyors on vibration isolation springs or pads and, where possible, 

eliminate direct metal-to-metal connection to other structures of the shaker frame. 

3. Place barriers at the exit of all Didions to block noise. 

Administrative Controls

The term administrative controls refers to employer-dictated work practices and policies 

to reduce or prevent hazardous exposures. Their effectiveness depends on employer 

commitment and employee acceptance. Regular monitoring and reinforcement are necessary 

to ensure that policies and procedures are followed consistently.

1. Make sure that employees keep the barriers properly positioned at all times. 

2. Consult with acoustic noise control engineers who have expertise in foundry noise 

reduction approaches for additional guidance on noise controls. The noise control 

engineers should be board certified by the Institute of Noise Control Engineers.

3. Implement a long-term strategy to reduce noise exposures by purchasing new 

equipment that generates less noise. Information on Buy Quiet programs is available at 

http://www.cdc.gov/niosh/topics/buyquiet/. 

4. Refer to OSHA’s Technical Manual for additional information on noise and noise 

control at https://www.osha.gov/dts/osta/otm/new_noise/index.html. NIOSH has noise 

control information at http://www.cdc.gov/niosh/topics/noisecontrol/ and information 

on noise generated by power tools can be found in the NIOSH power tools database at 

http://wwwn.cdc.gov/niosh-sound-vibration/.

5. Conduct noise measurement surveys after noise controls have been installed to 

evaluate whether the controls have adequately reduced noise levels and employees’ 

noise exposures.

6. Use NIOSH criteria in addition to OSHA criteria to identify hearing threshold shifts 

to improve detection of potential hearing loss. Include the 8,000-Hz frequency 

in audiometric tests. Refer to NIOSH audiometric evaluation and monitoring 

recommendations at http://www.cdc.gov/niosh/docs/98-126/ for additional information 

on audiometric testing and hearing loss prevention programs.

7. Include the measured noise exposure levels by job title in each employee’s audiometric 

test record. 

8. Advise employees to report any hearing problems to their healthcare provider and to 

the company.
Page 16 Health Hazard Evaluation Report 2009-0224-3274

Personal Protective Equipment

Personal protective equipment is the least effective means for controlling hazardous 

exposures. Proper use of personal protective equipment requires a comprehensive program 

and a high level of employee involvement and commitment. The right personal protective 

equipment must be chosen for each hazard. Supporting programs such as training, changeout schedules, and medical assessment may be needed. Personal protective equipment should 

not be the sole method for controlling hazardous exposures. Rather, personal protective 

equipment should be used until effective engineering and administrative controls are in place.

1. Make sure that employees working in the knockoff areas always wear dual hearing 

protection and that they properly insert foam insert ear plugs. 

2. Provide additional hands-on training for all employees and supervisors on how to 

insert hearing protectors properly and the importance of proper hearing protector fit. 

3. Perform hearing protector fit testing to determine the noise attenuation of protectors 

worn by foundry employees.
Health Hazard Evaluation Report 2009-0224-3274 Page 17

Appendix A: Occupational Exposure Limits and 

Health Effects

NIOSH investigators refer to mandatory (legally enforceable) and recommended 

occupational exposure limits (OELs) for chemical, physical, and biological agents when 

evaluating workplace hazards. OELs have been developed by federal agencies and safety and 

health organizations to prevent adverse health effects from workplace exposures. Generally, 

OELs suggest levels of exposure that most employees may be exposed to for up to 10 hours 

per day, 40 hours per week, for a working lifetime, without experiencing adverse health 

effects. However, not all employees will be protected if their exposures are maintained below 

these levels. Some may have adverse health effects because of individual susceptibility, a 

pre-existing medical condition, or a hypersensitivity (allergy). In addition, some hazardous 

substances act in combination with other exposures, with the general environment, or with 

medications or personal habits of the employee to produce adverse health effects. Most OELs 

address airborne exposures, but some substances can be absorbed directly through the skin 

and mucous membranes.

Most OELs are expressed as a TWA exposure. A TWA refers to the average exposure during 

a normal 8- to 10-hour workday. Some chemical substances and physical agents have 

recommended short-term exposure limits or ceiling values. Unless otherwise noted, the shortterm exposure limit is a 15-minute TWA exposure. It should not be exceeded at any time 

during a workday. The ceiling limit should not be exceeded at any time.

In the United States, OELs have been established by federal agencies, professional 

organizations, state and local governments, and other entities. Some OELs are legally 

enforceable limits; others are recommendations. 

● The U.S. Department of Labor OSHA PELs (29 CFR 1910 [general industry]; 29 CFR 

1926 [construction industry]; and 29 CFR 1917 [maritime industry]) are legal limits. 

These limits are enforceable in workplaces covered under the Occupational Safety and 

Health Act of 1970. 

● NIOSH RELs are recommendations based on a critical review of the scientific and technical 

information and the adequacy of methods to identify and control the hazard. NIOSH 

RELs are published in the NIOSH Pocket Guide to Chemical Hazards [NIOSH 2010]. 

NIOSH also recommends risk management practices (e.g., engineering controls, safe work 

practices, employee education/training, personal protective equipment, and exposure and 

medical monitoring) to minimize the risk of exposure and adverse health effects.

● Other OELs commonly used and cited in the United States include the threshold limit 

values (TLVs), which are recommended by the ACGIH, a professional organization, 

and the workplace environmental exposure levels (WEELs), which are recommended 

by the American Industrial Hygiene Association, another professional organization. 

The TLVs and WEELs are developed by committee members of these associations 

from a review of the published, peer-reviewed literature. These OELs are not consensus 

standards. TLVs are considered voluntary exposure guidelines for use by industrial 
Page 18 Health Hazard Evaluation Report 2009-0224-3274

hygienists and others trained in this discipline “to assist in the control of health 

hazards” [ACGIH 2016]. WEELs have been established for some chemicals “when no 

other legal or authoritative limits exist” [AIHA 2016].

Outside the United States, OELs have been established by various agencies and organizations 

and include legal and recommended limits. The Institut für Arbeitsschutz der Deutschen 

Gesetzlichen Unfallversicherung (Institute for Occupational Safety and Health of the German 

Social Accident Insurance) maintains a database of international OELs from European Union 

member states, Canada (Québec), Japan, Switzerland, and the United States. The database, 

available at http://www.dguv.de/ifa/GESTIS/GESTIS-Internationale-Grenzwerte-fürchemische-Substanzen-limit-values-for-chemical-agents/index-2.jsp, contains international 

limits for more than 2,000 hazardous substances and is updated periodically. 

OSHA requires an employer to furnish employees a place of employment free from 

recognized hazards that cause or are likely to cause death or serious physical harm 

[Occupational Safety and Health Act of 1970 (Public Law 91–596, sec. 5(a)(1))]. This is 

true in the absence of a specific OEL. It also is important to keep in mind that OELs may not 

reflect current health-based information.

When multiple OELs exist for a substance or agent, NIOSH investigators generally 

encourage employers to use the lowest OEL when making risk assessment and risk 

management decisions. NIOSH investigators also encourage use of the hierarchy of controls 

approach to eliminate or minimize workplace hazards. This includes, in order of preference, 

the use of (1) substitution or elimination of the hazardous agent, (2) engineering controls 

(e.g., local exhaust ventilation, process enclosure, dilution ventilation), (3) administrative 

controls (e.g., limiting time of exposure, employee training, work practice changes, medical 

surveillance), and (4) personal protective equipment (e.g., respiratory protection, gloves, 

eye protection, hearing protection). Control banding, a qualitative risk assessment and risk 

management tool, is a complementary approach to protecting employee health. Control 

banding focuses on how broad categories of risk should be managed. Information on control 

banding is available at http://www.cdc.gov/niosh/topics/ctrlbanding/. This approach can be 

applied in situations where OELs have not been established or can be used to supplement 

existing OELs.

Noise

Noise induced hearing loss (NIHL) is an irreversible condition that progresses with noise 

exposure. It is caused by damage to the nerve cells of the inner ear and, unlike some other 

types of hearing disorders, cannot be treated medically [Berger et al. 2003]. More than 

22 million U.S. workers are estimated to be exposed to workplace noise levels above 

85 dBA [Tak et al. 2009]. NIOSH estimates that workers exposed to an average daily noise 

level of 85 dBA over a 40-year working lifetime have an 8% excess risk of material hearing 

impairment. This excess risk increases to 25% for an average daily noise exposure of 90 dBA 

[NIOSH 1998]. 

Although hearing ability commonly declines with age, exposure to excessive noise can 

increase the rate of hearing loss. In most cases, NIHL develops slowly from repeated 
Health Hazard Evaluation Report 2009-0224-3274 Page 19

exposure to noise over time, but the progression of hearing loss is typically the greatest 

during the first several years of noise exposure. NIHL can also result from short duration 

exposures to high noise levels or even from a single exposure to an impulse noise or a 

continuous noise, depending on the intensity of the noise and the individual’s susceptibility 

to NIHL [Berger et al. 2003]. Noise exposed workers can develop substantial NIHL before it 

is clearly recognized. Even mild hearing losses can impair one’s ability to understand speech 

and hear many important sounds. In addition, some people with NIHL also develop tinnitus. 

This is a condition in which a person perceives hearing sound in one or both ears when 

no external sound is actually present. Persons with tinnitus often describe hearing ringing, 

hissing, buzzing, whistling, clicking, or chirping like crickets. Tinnitus can be intermittent 

or continuous and the perceived volume can range from soft to loud. Currently, no cure for 

tinnitus exists.

The preferred unit for reporting occupational and environmental noise measurements is 

the decibel, A-weighted, abbreviated as dBA. A-weighting is used because it approximates 

the “equal loudness perception characteristics of human hearing for pure tones relative 

to a reference of 40 dB at a frequency of 1,000 Hz” and is considered to provide a better 

estimation of hearing loss risk than using unweighted or other weighting measurements 

[Earshen 2003]. The dB unit is dimensionless, and it represents the logarithmic ratio of the 

measured sound pressure level to an arbitrary reference sound pressure (20 micropascals, 

which is defined as the threshold of normal human hearing at a frequency of 1,000 Hz). 

Decibels are used because of the very large range of sound pressure levels audible to the 

human ear. Because the dB is logarithmic, an increase of 3 dB is a doubling of the sound 

energy, an increase of 10 dB is a 10-fold increase, and an increase of 20 dB is a 100-fold 

increase in sound energy. Noise exposures expressed in dB should not be averaged using the 

arithmetic mean.

Workers exposed to noise should have baseline and yearly hearing tests to evaluate their 

hearing thresholds and determine whether their hearing has changed over time. Hearing testing 

must be done in a quiet location, such as an audiometric test booth, where background noise 

does not interfere with accurate measurement of hearing thresholds. In workplace hearing 

conservation programs, hearing thresholds must be measured at 500 Hz, 1,000 Hz, 2,000 Hz, 

3,000 Hz, 4,000 Hz, and 6,000 Hz. Additionally, NIOSH recommends that 8,000 Hz should 

also be tested [NIOSH 1998]. The OSHA hearing conservation standard requires analysis of 

changes from baseline hearing thresholds to determine if the changes are substantial enough to 

meet OSHA criteria for a standard threshold shift. OSHA defines a standard threshold shift as a 

change in hearing threshold relative to the baseline hearing test of an average of 10 dB or more 

at 2,000 Hz, 3,000 Hz, and 4,000 Hz in either ear [29 CFR 1910.95]. If a standard threshold 

shift occurs, the company must determine if the hearing loss also meets the requirements to be 

recorded on the OSHA Form 300 Log of Work-Related Injuries and Illnesses [29 CFR 1904.1]. 

In contrast to OSHA, NIOSH defines a significant threshold shift as an increase in the hearing 

threshold level of 15 dB or more, relative to the baseline audiogram, at any test frequency in 

either ear measured twice in succession [NIOSH 1998]. 

NIOSH has an REL for noise of 85 dBA, as an 8-hour TWA. For calculating exposure limits, 

NIOSH uses a 3-dB time/intensity trading relationship, or exchange rate. Using this criterion, 
Page 20 Health Hazard Evaluation Report 2009-0224-3274

an employee can be exposed to 88 dBA for no more than 4 hours, 91 dBA for 2 hours, 

94 dBA for 1 hour, 97 dBA for 0.5 hours, etc. Exposure to impulsive noise should never 

exceed 140 dBA. For extended work shifts NIOSH adjusts the REL to 84.5 dBA for a 9-hour 

shift, 84.0 dBA for a 10-hour shift, 83.6 dBA for an 11-hour shift, and 83.2 dBA for a 

12-hour work shift. When noise exposures exceed the REL, NIOSH recommends the use of 

hearing protection and implementation of a hearing loss prevention program [NIOSH 1998]. 

The OSHA noise standard specifies a PEL of 90 dBA and an AL of 85 dBA, both as 8-hour 

TWAs. OSHA uses a less conservative 5-dB exchange rate for calculating the PEL and AL. 

Using the OSHA criterion, an employee may be exposed to noise levels of 95 dBA for no 

more than 4 hours, 100 dBA for 2 hours, 105 dBA for 1 hour, 110 dBA for 0.5 hours, etc. 

Exposure to impulsive or impact noise must not exceed 140 dB peak noise level. OSHA does 

not adjust the PEL for extended work shifts. However, the AL is adjusted to 84.1 dBA for a 

9-hour shift, 83.4 dBA for a 10-hour shift, 82.7 dBA for an 11-hour shift, and 82.1 dBA for a 

12-hour work shift. OSHA requires implementation of a hearing conservation program when 

noise exposures exceed the AL [29 CFR 1910.95].

An employee’s daily noise dose, based on the duration and intensity of noise exposure, can 

be calculated according to the formula: Dose = 100 x (C1/T1 + C2/T2 + ... + Cn/Tn), where 

Cn indicates the total time of exposure at a specific noise level and Tn indicates the reference 

exposure duration for which noise at that level becomes hazardous. A noise dose greater than 

100% exceeds the noise exposure limit. 

To calculate the noise dose using NIOSH criteria, the reference duration (Tn) for each time 

period must be calculated using the following formula: T (minutes) = 480/2(L−85)/3, where 

L = the measured noise exposure level for each time period. To calculate noise dose using 

OSHA criteria, the reference duration (Tn) for each time period must be calculated using 

a slightly different formula: T (minutes) = 480/2(L−90)/5, where L = the measured noise 

exposure level for each time period.
Health Hazard Evaluation Report 2009-0224-3274 Page 21

References

ACGIH [2016]. 2016 TLVs® and BEIs®: threshold limit values for chemical substances and 

physical agents and biological exposure indices. Cincinnati, OH: American Conference of 

Governmental Industrial Hygienists.

AIHA [2016]. AIHA 2016 emergency response planning guidelines (ERPG) & workplace 

environmental exposure levels (WEEL) handbook. Fairfax, VA: American Industrial 

Hygiene Association.

Berger EH, Royster LH, Royster JD, Driscoll DP, Layne M, eds. [2003]. The noise manual. 

5th rev. ed. Fairfax, VA: American Industrial Hygiene Association.

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, 

Office of the Federal Register.

Driscoll DP, Royster LH [2003]. Noise control engineering. In: Berger EH, Royster LH, 

Royster JD, Driscoll DP, Layne M. eds. The noise manual, 5th ed. Akron, OH: American 

Industrial Hygiene Association, pp. 279–378.

Earshen J [2003]. Sound measurement: instrumentation and noise descriptors. In Berger EH, 

Royster LH, Royster JD, Driscoll DP, Layne M, eds. The noise manual. 5th rev. ed. Fairfax, 

VA: American Industrial Hygiene Association, pp. 41–100.

NIOSH [1998]. Criteria for a recommended standard: occupational noise exposure (revised 

criteria 1998). Cincinnati, OH: U.S. Department of Health and Human Services, Centers 

for Disease Control and Prevention, National Institute for Occupational Safety and Health, 

DHHS (NIOSH) Publication No. 98-126, 

https://www.cdc.gov/niosh/docs/98-126/pdfs/98-126.pdf.

NIOSH [2010]. NIOSH pocket guide to chemical hazards. Cincinnati, OH: U.S. Department 

of Health and Human Services, Centers for Disease Control and Prevention, National 

Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-168c, 

http://www.cdc.gov/niosh/npg/.

Tak S, Davis RR, Calvert GM [2009]. Exposure to hazardous workplace noise and use of 

hearing protection devices among U.S. workers—NHANES, 1999–2004. Am J Ind Med 

52(5):358–371, http://dx.doi.org/10.1002/ajim.20690.
Page 22 Health Hazard Evaluation Report 2009-0224-3274

Keywords: North American Industry Classification System 331511 (Iron Foundries); 

Wisconsin; Noise; Impact Noise; Noise Controls; Buy Quiet; Hearing Loss; Hearing 

Protection 
Health Hazard Evaluation Report 2009-0224-3274 Page 23

The Health Hazard Evaluation Program investigates possible health hazards in the workplace 

under the authority of the Occupational Safety and Health Act of 1970 (29 U.S.C. § 669(a)

(6)). The Health Hazard Evaluation Program also provides, upon request, technical assistance 

to federal, state, and local agencies to investigate occupational health hazards and to prevent 

occupational disease or injury. Regulations guiding the Program can be found in Title 42, Code 

of Federal Regulations, Part 85; Requests for Health Hazard Evaluations (42 CFR Part 85).

Disclaimer

The recommendations in this report are made on the basis of the findings at the workplace 

evaluated and may not be applicable to other workplaces.

Mention of any company or product in this report does not constitute endorsement by NIOSH.

Citations to Web sites external to NIOSH do not constitute NIOSH endorsement of the 

sponsoring organizations or their programs or products. NIOSH is not responsible for the 

content of these Web sites. All Web addresses referenced in this document were accessible as of 

the publication date.

Acknowledgments

Desktop Publisher: Shawna Watts 

Editor: Ellen Galloway 

Industrial Hygiene Field Assistance: Lilia Chen

Logistics: Donnie Booher, Karl Feldmann

Availability of Report

Copies of this report have been sent to the employer and employees at the facility. The state and 

local health department and the Occupational Safety and Health Administration Regional Office 

have also received a copy. This report is not copyrighted and may be freely reproduced. 

Recommended citation for this report:

NIOSH [2017]. Evaluation of noise exposures in the knockoff areas of an iron foundry. 

By Brueck SE. Cincinnati, OH: U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute for Occupational Safety 

and Health, NIOSH Health Hazard Evaluation Report 2009-0224-3274, 

http://www.cdc.gov/niosh/hhe/reports/pdfs/2009-0224-3274.pdf.
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