(1) The purpose of this rule is to establish maximum noise exposure levels that represent conditions under which it is believed that nearly all workers may be repeatedly exposed without adverse effect on the ability to hear and understand normal speech.   The public should note that the United States Occupational Safety and Health Act (OSHA) may have pre-empted the application of this rule to some work places.

(2) As used in this rule, the following definitions apply:

(a) "ANSI" means the American National Standards Institute of 1430 Broadway, New York, New York, 10018.

(b) "Continuous noise" means those variations of noise levels that involve maxima at intervals of 1 per second or less.

(c) "Impact or impulsive noise" means those variations of noise levels that involve maxima at intervals of greater than 1 per second.

(3) For purposes of determining compliance with this rule, noise levels shall be determined by a sound level meter which operates on the decibels A-weighting network (dbA) with slow meter response. The department hereby adopts and incorporates herein by reference the specifications for sound level meters adopted by the American National Standards Institute, ANSI S1.4-1971 (R1976). This sets forth nationally recognized standards for sound level meters.   A copy of these specifications may be obtained by writing to ANSI at 1430 Broadway, New York, New York 10018.   Inquiries as to whether a particular meter is satisfactory may be answered by contacting the Department of Environmental Quality, PO Box 200901, Helena, MT 59620-0901, telephone 444-3671.

(4) No person may cause or permit the exposure of any worker employed at any work place to noise levels in excess of the maximum noise exposure levels listed in (6) of this rule.

(5) When any worker employed at any work place is exposed to noise levels exceeding those listed in (6) of this rule, feasible administrative or engineering controls shall be utilized by the employer to reduce the noise levels. If such controls fail to reduce the worker's exposure to noise levels within the maximum permissible noise exposure levels listed in (6) of this rule, personal hearing protective equipment shall be provided by the employer to the worker and used to reduce the noise exposure level to within the maximum permissible noise exposure levels listed in (6) of this rule.

(6) The maximum permissible noise exposure levels to which a worker may be exposed are as shown in the following table:

Continuous or Intermittent Noise Exposures

Duration per day in hours	Noise level (dbA)
8	90
6	92
4	95
3	97
2	100
1 1/2	102
1	105
3/4	107
1/2	110
1/4	115

(a) These values apply to the total time of exposure per working day regardless of whether this is 1 continuous exposure or a number of short-term exposures but does not apply to impact or impulsive type of noises.

(b) When the daily noise exposure of a worker is composed of 2 or more periods of noise exposure of different levels, their combined effect should be considered rather than the individual effect of each. If the sum of the following fractions: C₁/T₁ + C₂/T₂ ... C_n/T_n exceeds unity (1) , then the mixed exposure should be considered to exceed the maximum permissible exposure rate. C_n indicates the total time of exposure at a specified noise level, and T_n indicates the total time of exposure permitted at that level.   Noise exposures of less than 90 dbA do not enter into the above calculations. No worker may be exposed to continuous noise in excess of 115 dbA for any duration.

(c) A worker's exposure to impact or impulse noise shall not exceed 140 decibels (dbA) peak sound pressure level.