In this chapter: where noise comes from in a wood pellet plant or feed mill, how it is measured, what limits the United States, the European Union, Argentina and Mexico set, which controls work at the source, how to choose and use hearing protection, and what to do about vibration.
Damage that doesn't hurt
Noise is the easiest hazard to ignore in a plant. It doesn't cut, burn or explode. The operator "gets used to it." Over the years, they start asking people to repeat things, turn up the TV, and live with a ringing in the ears that won't go away. By then, the damage is done: noise-induced hearing loss is permanent. No treatment reverses it, but it can be prevented.
The risk grows with the noise level and with the years of exposure. The U.S. National Institute for Occupational Safety and Health (NIOSH) estimated that, over a 40-year working life, daily exposure to 85 dBA carries an excess risk of hearing loss of 8%, and that at 90 dBA that excess rises to 25%1. Five decibels may not sound like much, but they triple the risk.
Where noise comes from in a plant
The main sources in a wood pellet plant or feed mill are:
- Hammer mill. The impact of material against hammers and screens, the high-speed rotor and the air it moves. It is usually the loudest source in the plant.
- Pellet mill. The motor, the drive and the compression work in the die.
- Fans for dust collection, coolers and pneumatic conveying, and air discharges from filters and valves.
- Air compressors.
- Material falling onto metal sheets, hoppers and ducts.
- Screeners, bucket elevators, screw conveyors and belts, especially with worn bearings or loose parts.
A NIOSH study at an agricultural plant that had an alfalfa pellet area used personal dosimeters on 19 workers. The pellet mill operator was one of the two who exceeded the OSHA action level. The investigators noted that the hearing conservation program worked well in another part of the company, but did not exist in the feed manufacturing area2. This is a common pattern: people are protected where the noise is obvious and neglected where they work full shifts next to the machine.
How noise is measured
A few ideas are enough to understand a noise report:
- dB(A). The noise level measured with a filter that mimics the sensitivity of the human ear. It is the unit used in almost all legal limits.
- 8-hour equivalent level. Noise in a plant goes up and down. What matters is the average exposure over the shift, expressed as if it were a constant level for 8 hours.
- Exchange rate. It tells you how many extra decibels cut the allowed time in half. With 3 dB, every 3 dB more means half the time: if the limit is 85 dBA for 8 hours, at 88 dBA the time drops to 4 hours. With 5 dB, the reduction is slower. That is why the same noise can comply in one country and not in another.
- Peak. It measures very short, intense bursts of noise, in dB(C). It has its own limit.
- Dosimeter and sound level meter. The worker wears the dosimeter during the shift, and it measures their actual exposure. The sound level meter measures the level at one point and is used to build a noise map of the plant.
In Argentina, Resolución SRT 85/2012 approved a mandatory protocol for measuring noise in the workplace. It requires, at a minimum, an integrating meter or dosimeter of class or type 2, with a calibration certificate from a laboratory. Measured values are valid for 12 months3.
Limits, country by country
This table summarizes the main values. It does not replace reading your country's regulation, but it shows how different they can be:
| 8-hour limit | Values that require action | Exchange rate | Peak | |
|---|---|---|---|---|
| U.S., OSHA 29 CFR 1910.95 | 90 dBA | 85 dBA (action level) | 5 dB | 140 dB |
| U.S., NIOSH (recommendation) | 85 dBA | — | 3 dB | — |
| European Union, Directive 2003/10/EC | 87 dB(A), counting the hearing protector | 80 and 85 dB(A) | — | 135, 137 and 140 dB(C) |
| Argentina, Res. MTEySS 295/2003 | 85 dBA | — | 3 dB | 140 dB(C) |
| Mexico, NOM-011-STPS-2001 | 90 dB(A) | 80 and 85 dB(A) | 3 dB | — |
Sources: OSHA4, NIOSH1, Directive 2003/10/EC5, Resolución 295/20036 and NOM-011-STPS-20017.
What each regulation requires
- United States. When the 90 dBA limit is exceeded, OSHA first requires engineering or administrative controls; personal protection comes after, if those controls are not enough. From the 85 dBA action level, the employer must have a hearing conservation program: exposure monitoring, a baseline audiogram within 6 months and then annually, hearing protectors at no cost for everyone exposed, mandatory use for those above the limit or who have had a change in their hearing, annual training, and records4.
- European Union. From 80 dB(A), the employer must make hearing protectors available, inform and train workers, and offer preventive audiometric testing when the assessment shows a risk. From 85 dB(A), wearing protectors is mandatory, areas are marked with signs, access is restricted where feasible, a program of technical and organizational measures is applied to reduce exposure, and workers are entitled to have their hearing checked. The 87 dB(A) value must never be exceeded, and for that limit the attenuation of the protector does count5.
- Argentina. Annex V of Decreto 351/79, as amended by Resolución 295/2003, sets 85 dBA for 8 hours with a 3 dB exchange rate and prohibits unprotected exposure above a 140 dB(C) peak. When limit values are reached or exceeded, it requires a complete hearing conservation program that includes audiometric testing6.
- Mexico. NOM-011-STPS-2001 requires identifying and assessing areas from 80 dB(A) and training workers exposed to that level. From 85 dB(A), the employer must provide hearing protection, post signs requiring its use, have a hearing conservation program, and carry out health surveillance with annual exams. It prohibits exposing workers to more than 105 dB(A) without hearing protection7.
Controls at the source: the best return
All the regulations agree on the order: first reduce noise at the machine and in the room, and only then turn to hearing protectors. A protector depends on each person wearing it correctly for the whole shift. A quieter machine protects everyone without depending on anyone.
The usual measures are:
- Buy quieter machines. The European Machinery Regulation requires the manufacturer to reduce noise to the lowest possible level, especially at the source, and to declare noise in the instructions: the sound pressure level at workstations when it exceeds 70 dB(A), the peak when it exceeds 130 dB(C), and the machine's sound power when the level at the workstation exceeds 80 dB(A)8. Ask for that data before you buy, and compare.
- Enclose or separate the source. Acoustic enclosures, barriers, or separate rooms for mills, compressors and fans. A classic case from the NIOSH noise control manual shows the mills of a livestock feed plant enclosed with acoustic curtains and absorbent inner panels: the level at the operator's position dropped to 87 dBA, below the target, while keeping access for maintenance9.
- Silencers on fans, compressed air discharges and vents.
- Damp and isolate vibrating sheets, hoppers and ducts, and set machines on anti-vibration mounts where appropriate.
- Maintain. Worn bearings, loose parts and imbalance increase noise. A rise in noise is also a warning that something is wrong.
- An isolated control room for the operator, with a view of the line, so they don't spend the shift next to the machines.
- Organize the work. Limit time in the loudest areas and rotate tasks, without letting that replace the controls above.
Hearing protection that actually protects
When noise cannot be brought down enough, hearing protectors are mandatory. For them to work:
- Choose based on the measurement, not by guesswork. A protector with too little attenuation doesn't protect, and one with too much cuts the worker off from alarms, warnings and the sound of a failing machine.
- Offer options. Earplugs and earmuffs; each person will wear the one they find comfortable more consistently.
- Teach people how to fit them. A poorly inserted earplug protects much less than expected.
- Wear them for the whole exposure time. Taking them off for a while in the noisy area can cancel out much of the day's protection.
- Run periodic audiometric tests. They are how you know whether the program works: they detect changes before the damage is serious.
Vibration
Vibration is the other effect of machines that is transmitted to the body. There are two types:
- Hand-arm: grinders, pneumatic hammers and other hand tools used in maintenance. Over the years they can damage blood vessels, nerves and joints in the hands.
- Whole-body: drivers of forklifts and wheel loaders that travel over uneven yards, and people who work standing on platforms supported by vibrating machines.
The European Union's Directive 2002/44/EC sets, for an 8-hour day, an action value of 2.5 m/s² and a limit of 5 m/s² for hand-arm vibration, and 0.5 m/s² and 1.15 m/s² for whole-body vibration10. In Argentina, Annex V of Resolución 295/2003 also includes criteria for vibration6. The European Machinery Regulation requires reducing vibration at the source, just like noise8.
The measures are similar: choose tools and vehicles with lower vibration, maintain seats, tires and floors, isolate platforms and limit usage times. And a new vibration in a machine on the line is also a maintenance signal you should not ignore.
Chapter checklist
- Do we have a current noise map of the plant, made by a professional?
- Do we measure the personal exposure of the most exposed positions, such as the mill operator and the pellet mill operator?
- Do we know which limit and which exchange rate apply in our country?
- Are areas above the action level marked with signs?
- Is there a plan to reduce noise at the source, with deadlines and owners?
- Do we ask the supplier for noise data before buying a machine?
- Are hearing protectors available, suitable and worn the whole time?
- Do exposed staff have periodic audiometric tests, and are the results reviewed?
- Do we assess vibration from hand tools and vehicles?
What to leave to a professional
Noise and vibration measurement must be done by a professional with calibrated instruments, following your country's protocol. Designing acoustic enclosures and treatments requires a specialist: a poorly built enclosure may fail to reduce noise, make maintenance harder, or cause temperature and dust problems. Audiometric testing and health surveillance are the job of the occupational health service.
At Meelko, we take noise into account when laying out the line: where the mill, fans and compressors go, and where the operator works. If you want to review your project or your plant, tell us about your project.
Chapter sources
- NIOSH. Criteria for a Recommended Standard: Occupational Noise Exposure, Revised Criteria 1998, DHHS (NIOSH) Publication No. 98-126. https://webarchive.library.unt.edu/web/20170214041730/https://www.cdc.gov/niosh/docs/98-126 — Current NIOSH page on the recommended limit: https://www.cdc.gov/niosh/noise/prevent/understand.html ↑
- Achutan, C. and Tubbs, R. L. Noise exposures during potato processing and manufacture of animal feed. Journal of Agricultural Safety and Health, 13(4): 367-374, 2007. https://stacks.cdc.gov/view/cdc/194447 ↑
- Superintendencia de Riesgos del Trabajo (Argentina). Resolución SRT 85/2012, Protocolo para la medición del nivel de ruido en el ambiente laboral. Boletín Oficial, January 30, 2012. https://www.argentina.gob.ar/normativa/nacional/193617/texto ↑
- OSHA. 29 CFR 1910.95, Occupational noise exposure. Paragraphs (b)(1), (c), (d), (g), (i), (k) and (m), and Table G-16. https://www.law.cornell.edu/cfr/text/29/1910.95 ↑
- Directive 2003/10/EC of the European Parliament and of the Council of February 6, 2003, on the exposure of workers to noise. OJ L 42, February 15, 2003. Articles 3 to 10. https://www.boe.es/doue/2003/042/L00038-00044.pdf ↑
- Argentina. Resolución MTEySS 295/2003, which replaces Annex V of Decreto 351/79 (noise, ultrasound, infrasound and vibration). https://www.argentina.gob.ar/normativa/recurso/32030/dto351-1979-anexo5/htm ↑
- Mexico. NOM-011-STPS-2001, Condiciones de seguridad e higiene en los centros de trabajo donde se genere ruido, DOF April 17, 2002. Sections 5.2 to 5.7, 8.2, 8.5, 8.6, 8.7 and Table A.1. https://dof.gob.mx/nota_detalle.php?codigo=734536&fecha=17/04/2002 ↑
- Regulation (EU) 2023/1230 on machinery, Annex III, sections 1.5.8, 1.5.9 and 1.7.4.2, point (u). https://www.boe.es/doue/2023/165/L00001-00102.pdf ↑
- NIOSH. Industrial Noise Control Manual, revised edition, Publication 79-117, 1979. Case of a livestock feed mill. https://www.cdc.gov/niosh/engcontrols/ecd/detail116.html ↑
- Directive 2002/44/EC of the European Parliament and of the Council of June 25, 2002, on the exposure of workers to vibration. OJ L 177, July 6, 2002. Article 3. https://www.boe.es/doue/2002/177/L00013-00019.pdf ↑
