Industrial Hygiene

Industrial Hygiene 3



levels in dB. Many SLMs do not have the ability to measure time-weighted average (TWA) noise exposures using data-logging instruments nor are they convenient for an employee to wear during a work shift. Therefore, the most common use of SLMs is to evaluate areas for the need to perform personal noise monitoring and to identify noise sources. OSHA does allow the use of an SLM to evaluate personal exposures for jobs with low mobility, if the data can be shown to be representative of workers’ actual exposures. However, the majority of personal noise evaluations are performed using noise dosimeters. Octave band analyzers are filters that can be added to an SLM to provide additional data about sound pressure levels at different frequencies. Some SLMs already have the octave band analyzers added during manufacturing. A common octave band analyzer will provide sound pressure level readings at 11 different bands ranging from 16 Hz to 16 KHz. See the OSHA technical manual in your Required Reading for photographs and further discussion of octave band analyzers. The results of an octave band analysis will become clearer when we discuss control methods for reducing noise exposure in the next two units. In most occupational settings, it is important to be able to evaluate personal exposures to noise. To get results that represent the worker’s exposure, workers must wear data-logging instruments. These devices, called noise dosimeters, are smaller devices designed to be worn by a worker and use data-logging capabilities to record a time-weighted average (TWA) noise exposure. Noise dosimeters used to consist of a small box with a microphone connected by a wire. The box was placed at the worker’s waist, and the microphone was placed on the collar. The traditional style noise dosimeter is still available, but there are now several models that incorporate the entire dosimeter into a small device that sits entirely on the worker’s shoulder. The placement of the noise dosimeter has been a point of discussion for many years. OSHA states that the device must be placed in the hearing zone of the employee. In OSHA’s technical manual, compliance officers are instructed to place the microphone pointing straight up in the hearing zone, which is defined as a 2-foot- wide sphere surrounding the employee’s ear (OSHA, 2013). Another topic concerning the evaluation of noise exposures that has been discussed over the years is the setting to be used for the evaluation. First, OSHA specifies that readings will be taken using the A scale (dBA) and the slow setting. The responses of an SLM or noise dosimeter can be modified to read based on weighting of different frequencies (OSHA, 2013). The three most common frequency weighting networks are the A scale (dBA), the B scale (dBB), and the C scale (dBC). The human ear does not respond equally to noise at all frequencies, meaning it is more sensitive to noise at some frequencies than others. OSHA uses the A scale because it approximates the way the human ear responds to noise at different frequencies better than the B scale and C scale (OSHA, 2013). The slow response setting is used because the meter fluctuates less and is therefore easier to read. OSHA initially established an 8-hour TWA PEL of 90 dBA for general industry based on ACGIH TLVs in existence at the time. This OSHA PEL remains unchanged to date, even though both NIOSH and ACGIH currently recommend an exposure limit of 85 dBA. OSHA also established an 8-hour TWA action level of 85 dBA for 8-hour work shifts, or an equivalent exposure of 50%. This means that the action level changes with extended shift lengths. The OSHA technical manual (p. 59) shows examples of how to calculate action levels for extended work shifts (83.4 dBA for a 10-hour work shift, 82.1 dBA for a 12-hour work shift) (OSHA, n.d.). Exceeding the action level requires the employer to take specific actions, including implementation of an effective hearing conservation program (OSHA, 1970). Read about the requirements for an effective hearing conservation program in 29 CFR 1910.95. As discussed in OSHA technical manual: Noise, one of the most debated topics in noise evaluation concerns the exchange rate used for noise. The exchange rate represents the change in exposure where the sound pressure doubles or halves, and the allowable exposure time also doubles or halves. OSHA has always maintained a 5 dBA exchange rate. OSHA states that it uses the 5 dBA exchange rate to account for the time when workers are not exposed to noise during the workday (e.g., breaks or time in offices). The ACGIH, NIOSH, and most foreign countries currently use a 3 dBA exchange rate (OSHA, 2013). The 3 dBA exchange rate is believed to be more representative because it has been shown that sound pressure actually doubles every 3 dBA, not every 5 dB. For example, under OSHA, exposure at 90 dBA is allowed for eight hours, but exposure is only allowed for four hours at 95 dBA (an increase of 5 dBA). If a 3 dBA exchange rate were

OSH 4301, Industrial Hygiene 4