Health Hazard Assessment
Hazard Category - Acoustic Energy 


STEADY-STATE NOISE

Definition:  A pressure wave variation around the ambient atmospheric pressure exceeding 0.5 seconds in duration.  Can be continuous, intermittent, or fluctuating.

Data Requirements and Initial Recommendations.
(1) Collect steady-state noise data in accordance with Military Standard (MIL-STD) 1474D, Requirement 1 at positions occupied by operators, passengers, crew members or maintenance personnel, at noise sources and around the equipment for each noise-unique operating condition (reference 1).  Provide the data, including a detailed use scenario, to the U.S. Army Public Health Command (Provisional) to support the completion of a definitive HHA.  Special attention must be made to collect appropriate data to determine the 85-decibels, A-weighted (dBA), steady state noise contour.  Contact the Army Hearing Program if assistance is needed with data collection or in establishing noise design goals to minimize adverse health effects.

(2) Eliminate steady-state noise associated with materiel and support design to the maximum extent feasible to reduce the reliance upon hearing protection devices for hearing conservation, reduce aural signatures and reduce the range of detectability.

(3) All personnel exposed to hazardous noise must wear hearing protective devices (HPDs).  The Department of the Army Pamphlet (DA PAM) 40-501 lists HPDs approved for Army use (reference 3).  HPDs must be fitted for size by properly trained personnel (if pre-formed earplugs or helmets); adequately maintained; and properly worn by the wearer.  Double hearing protection consists of:  approved earplugs in combination with a noise muff or noise-attenuating helmet.

Health Effects.
Elevated sound levels can cause trauma to the eardrum, to the bones of the middle ear that amplify sound, or to the hair cells of the inner ear that convert sound energy into a signal that travels to the brain.  Overstimulation can lead to a temporary reduction in hearing or ringing in the ears, and the cells can recover if the exposure is not severe.  Damage to the hair cells from repeated exposure over a period of time will result in permanent noise-induced hearing loss.  Excessive noise can be continuous and not vary with time, intermittent if broken by periods of very low noise levels, or fluctuating if the sound pressure varies over a wide range.

Medical Criteria.
(1) A steady‑state noise level of 85 dBA or greater is considered hazardous (reference 1 and 2).  This limit assumes no more than 8 hours per day of exposure to high noise levels.  Prolonged unprotected exposure to hazardous noise levels will cause loss of hearing. 

(2) For exposures exceeding 8 hours per day, noise levels below 85 dBA may be hazardous (reference 2).  An exposure level above 80 dBA is considered hazardous if the duration of exposure is 24 hours.

(3) For long term continuous (24 hours per day) exposures, levels up to 75 dBA are not considered hazardous.  However, if personnel can be exposed to high levels (above 85 dBA) during the duty day then they should be provided with the means of “resting” their ears for at least 8 hours with at-ear exposure below 65 dBA (reference 4).

References.
(1) Military Standard (MIL-STD) 1474D, Department of Defense Design Criteria Standard: Noise Limits, 12 Feb 97.  

(2) Army Regulation (AR) 40‑5, Preventive Medicine, 25 May 2007.  

(3) Department of the Army Pamphlet (DA PAM) 40-501, Hearing Conservation Program, 10 Dec 98.  

(4) U.S. Department of Health, Education, and Welfare, National Institute for Safety and Heath, subject: Criteria for a recommended standard, occupational exposure to noise, 1972.  

 

IMPULSE NOISE

Definition:  High-level, short duration pressure wave disturbance of less than 0.5 seconds at levels that can immediately cause acoustical trauma.

Data Requirements and Initial Recommendations.
(1) Eliminate impulse noise associated with materiel and support design to the maximum extent feasible to reduce the reliance upon hearing protection devices for hearing conservation, reduce aural signatures and reduce the range of detectability.

(2) Collect impulse noise data in accordance with Military Standard (MIL-STD) 1474D, Requirement 4 at all occupied (operator, crew, passenger, maintenance personnel, and any other user) positions and provide it to this Institute to support the completion of a definitive HHA (reference 1).  Collect appropriate data to determine the 140 dB noise contour.  The MIL-STD-1474D requirements may differ from those described in local Test Operating Procedures.  Contact the HHA Project Officer to resolve discrepancies.

(3)  Provide system operations information to fully characterize daily impulse noise exposures for all affected personnel.  Operational information for components (such as cartridges or ancillary equipment) should include a listing of what weapon systems the materiel is used with and any acoustically significant attributes of their use).  For example, provide model designations for rifles using a new cartridge, and describe pertinent silencer, muzzle brake, and barrel length characteristics for the rifles listed.  Operational information for weapon systems that may be fired in different ways during the course of a day should also be provided.  For example, it should be specified what shooter positions (prone, kneeling, standing, from a foxhole, etc.) are anticipated for a rifle system, and it should be specified what mix of firing conditions (azimuth, elevation, charge) are anticipated for a howitzer or mortar.

(4)  Assessments may sometimes be done by analogy in lieu of obtaining data.  This possibility must be decided on a case by case basis to enable new medical criteria and/or the significance of materiel changes to noise generation to be taken into account.

(5) All personnel exposed to hazardous noise must wear hearing protective devices (HPDs).  Department of the Army Pamphlet (DA PAM) 40-501 lists HPDs approved for Army use (reference 2).  HPDs must be fitted for size by properly trained personnel (if pre-formed earplugs or helmets); adequately maintained; and properly worn by the wearer.

(6) Double hearing protection consisting of approved earplugs in combination with an ear muff, headset, or noise-attenuating helmet may be used to increase firing allowances.

(7)  Additional use restrictions (firing allowances or doctrinal use practices) may apply.

Health Effects.
Elevated sound levels can cause trauma to the hearing mechanism.  The mechanical damage caused by the acoustic trauma may cause permanent hearing loss.  This can also be accompanied by ringing (tinnitus) in the ears.

Medical Criteria.
Impulse noise greater than 140 peak decibels (dBP) is considered hazardous (references 1 and 2).  Repeated, unprotected exposure to hazardous impulse noise will cause permanent hearing loss.

References.
(1) Military Standard (MIL-STD) 1474D, Department of Defense Design Criteria Standard, Noise Limits, 12 Feb 97.  

(2) Department of the Army Pamphlet (DA PAM) 40-501, Hearing Conservation Program, 10 Dec 98.  

 

BLAST OVERPRESSURE

Definition:  An instantaneous change in air pressure, typically emitted by an explosive device when it detonates, that imparts mechanical energy to contacted objects.

Data Requirements and Initial Recommendations.
Conduct blast overpressure testing on the materiel in accordance with the guidance for Blast Overpressure Analysis (reference 1) and Test Operations Procedure (TOP) 4-2-831 (reference 2).  After the test is completed, send properly formatted data and a description of the test using the Blast Overpressure Test Information Form found at the end of Reference 1, to the U.S. Army Public Health Command for analysis.        

Health Effects.
When blast waves encounter human tissue they yield mechanical stresses that can injure if they are of sufficient intensity and/or frequency.  Although many organs are susceptible to injury the eardrum and air containing organs such as the heart, lungs, esophagus, stomach, and intestines are particularly vulnerable.  Possible negative health outcomes include mild, transient cognitive dysfunction, visceral injury, and death.  Although it was previously thought that the absence of eardrum rupture could indicate sparing of damage to the viscera, this notion has recently been challenged.  Blast overpressure injury risk is related to the mechanics of the pressure wave and the physical properties of the tissue contacted.  
  
Medical Criteria.
Extensive study by the U.S. Army Medical Research and Material Command has yielded an algorithm that describes the relationship between the intensity of a blast recorded by a blast test device and lung injury (reference 3).  This algorithm resides within the Blast Overpressure Health Hazard Assessment (BOP-HHA) software used by the U.S. Army Public Health Command to assess the risk of lung injury to soldiers exposed to blast.  Due to the complexity of factors found in the blast environment and variations in the intensity and pulse duration of blast waves it is not possible to express blast overpressure injury risk as a simple numeric threshold value.  However, as a general rule of thumb, risk of blast injury to visceral organs is considered to be minimal when the blast falls under the Z limit described in MIL-STD-1474D (reference 4).

References.
(1) U.S. Army Public Health Command (Provisional) Technical Information Paper No. 88-001-0411, Program Guidance for Blast Overpressure Analysis, 12 Apr 11.

(2)  Test Operations Procedure (TOP) 4-2-831, Use of Blast Test Device (BTD) During Auditory Blast Overpressure Measurement, U.S. Army Developmental Test Command, 2008.

(3)  Stuhmiller J.H., Ho K.H., Vander Vorst M.J., Dodd K.T., Fitzpatrick T., Mayorga M.  A model of blast overpressure injury to the lung.  Journal of Biomechanics.  1996;29(2):227–234.

(4)  Military Standard (MIL-STD) 1474D, Department of Defense Design Criteria Standard: Noise Limits, 12 Feb 97.