PURPOSE: The purpose of this rule is to establish the procedures to evaluate hearing impairment, setting forth methods for its measurement and calculation.

1. (1) The Division of Workers’ Compensation makes grateful acknowledgment for scientific advisory assistance in the preparation of this rule to the Central Institute for the Deaf, 818 South Euclid, St. Louis, Missouri, in particular to Dr. Hallowell Davis, its director of research, for his/her counsel and guidance, and to Dr. S. Richard Silverman, its director, who made available his/her own time and help and that of his highly qualified staff.

2. (2) The following are definitions relating to this matter and rule:

   1. (A) Hearing loss—the general condition of reduced auditory sensitivity;
   2. (B) Loss of hearing or threshold shift—a change for the worse in auditory sensitivity;
   3. (C) Threshold—the weakest sound that can be heard;
   4. (D) Decibel—a unit conventionally used to measure the magnitude of sound. In the testing of hearing, it is used to measure the threshold of a listener relative to the standard threshold (U.S. audiometers);
   5. (E) Audiometer—a device for the measurement of the threshold of hearing in decibels relative to a standard;
   6. (F) Hearing level or hearing threshold level—the reading on an audiometer in decibels corresponding to the threshold of hearing of the individual being tested;
   7. (G) Frequency—the number of regular fluctuations made by a sound wave in one (1) second;
   8. (H) Cycle—one (1) of a repeated series of regular fluctuations made by a sound wave;
   9. (I) Audiogram—a chart showing hearing levels at different frequencies;
   10. (J) Hearing impairment or impairment of hearing—a malfunction or abnormality of hearing of sufficient severity to constitute a practical handicap such as would justify compensation; particularly a reduction of efficiency in everyday communication by speech;
   11. (K) Deafness—term reserved to designate very severe or total impairment of hearing; and
   12. (L) Presbycusis—a loss of hearing occasioned by the aging process.
3. (3) Weeks of compensation for hearing loss due to a traumatic incident (that is, a single accident such as an explosion, a blast or a blow on the head) shall be those provided in items 27 and 28 of subsection 1 of section 287.190, RSMo.
4. (4) Weeks of compensation for hearing loss due to prolonged exposure to harmful noise in employment (that is, an occupational disease) shall be those provided in subsection 3 of section 287.197, RSMo.
5. (5) Either traumatic hearing loss(es) due to occupational disease shall be measured as prescribed in section 287.197, RSMo and this rule.
6. (6) When both ears show hearing impairment, the computation of impairment shall be on the basis of binaural loss as provided in subsection 5 of section 287.197, RSMo.
7. (7) Liability for occupational hearing loss occurs only when an employee has been exposed to the hazard of such loss for a period of ninety (90) days or longer and becomes exclusively that of the employer in whose employment such exposure took place (section 287.063-5).
8. (8) Each employer is liable for all of the occupational hearing loss to which his/her employment contributed, subject to the limitations of the measurement of hearing loss provisions, but no employer is liable for hearing loss sustained prior to employment with him/her nor for any hearing loss for which compensation previously was awarded or paid (section 287.197-8).
9. (9) The date of disability of occupational hearing loss is the last day of a six (6)-month period following separation from the employment in which the employee was exposed to harmful noise (section 287.197-7).
10. (10) Claim for compensation for occupational hearing loss, if maintained, must be made within one (1) year of the date of disability, as defined in section (9) of this rule. The provision of medical attention and/or the payment of compensation will toll the statute, as in other workers’ compensation cases (section 287.197-7).
11. (11) Only pure-tone air-condition audiometric instruments that meet the standards set by recognized authorities shall be used to measure hearing levels. The reference zero levels of the audiometer used for measuring hearing levels must be explicitly identified either as ASA-1951 (as given in USASI Standard for General Diagnostic Purposes, Z24.5-1951, United States of America Standards Institute, New York 1951) or as ISO (as given in International Organization of Standardization Recommendation R 389, Standard Reference Zero for the calibration of pure-tone audiometers). The corresponding identification must be attached to every decibel value of a hearing level employed in the evaluation of hearing impairment.
12. (12) In the evaluation of hearing impairment, only the hearing levels at the frequencies of five hundred (500), one thousand (1000) and two thousand (2000) cycles per second shall be considered; provided, however, that if a subject does not hear the test tone at the ninety-five (95) decibel hearing level in any or all of the three (3) frequencies, the value of one hundred (100) decibels shall be used for such frequency(ies) in calculating the average hearing level.
13. (13) Three (3) separate audiograms, each on different days, shall be made including at least the frequencies of five hundred (500), one thousand (1000) and two thousand (2000) cycles per second and the lowest hearing level measured at each of the three (3) frequencies shall be used for the computation of hearing impairment. The lowest hearing level at each of the three (3) frequencies shall be added together and the sum divided by three (3) to determine the average hearing level in decibels. If the audiograms show a lowest hearing level at any of these three (3) frequencies that is greater than one hundred (100) decibels, or else no response at all, the value of one hundred (100) dB shall be used for the level at such frequencies in calculating the average hearing level.
14. (14) In order to allow for the average amount of hearing loss due to nonoccupational causes found in the population at any given age (including presbycusis), there shall be deducted from the average hearing level one-half (1/2) decibel for each year of the employee’s age over forty (40) at the time of his/her last exposure to industrial noise. The result shall be termed the corrected average hearing level.
15. (15) For every decibel that the corrected average hearing level exceeds fifteen (15) decibels based on the ASA-1951 reference levels or twenty-six (26) decibels based on the ISO reference levels, an allowance of one and onehalf percent (1 1/2%) shall be made up to the maximum of one hundred percent (100%) which is reached at eighty-two (82) decibels based on the ASA-1951 reference levels and at ninety-three (93) decibels based on the ISO reference levels. The allowance thus calculated is the monaural percentage impairment of hearing in that ear.
16. (16) Binaural impairment of hearing shall be determined by multiplying the percentage of impairment in the better ear by five (5), to which result is added the percentage of impairment in the poorer ear and dividing the sum of the two (2) by six (6). The result is the evaluation in percentage of binaural hearing impairment.
17. (17) No consideration shall be given to the question of whether or not the ability of an employee to understand speech is improved by the use of a hearing aid.
18. (18) An employee may work in successive employments where s/he is exposed to harmful noise and sustain an accumulated hearing loss, only a part of which may be the liability of the last employer. Section 287.197-8, RSMo provides that an employer is liable only for the hearing loss to which his/her employment contributed, which provision requires a rule for the calculation of such proportional liability. The rule applies only to the first employer in whose employ the employee develops a compensable hearing impairment. Each subsequent employer who hires an individual who already has some hearing impairment is liable only for the additional impairment that develops in his/her employ, subject to the correction according to age.
19. (19) The best level of hearing at each of the three (3) frequencies of five hundred (500), one thousand (1000) and two thousand (2000) cycles per second is determined by selection from all available audiogram(s) made within six (6) months prior to or three (3) months after the date of employment, but in any case prior to work in a noisy environment. Earlier audiogram(s) may be used for this purpose only if none is available that were made during that nine (9)-month period.
20. (20) The pre-employment average hearing level for the three (3) frequencies is calculated for each ear (section (13) of this rule). If the decibel values are based on the ISO reference, zero (0) levels eleven (11) decibels shall be subtracted from the average hearing level to convert it to its ASA-1951 equivalent. The remainder of this section remains as originally written in terms of the ASA-1951 reference levels.
21. (21) The correction for nonoccupational hearing loss (section (14) of this rule) is applied by subtracting from the average hearing level for each ear one-half (1/2) decibel for each year of the employee’s age over forty
22. (40) at the time of his/her employment.
23. (22) Now if the corrected average hearing level of the pre-employment audiogram(s) in either ear exceeds fifteen (15) decibels, the percentage of binaural impairment is calculated as in sections (15) and (16) of this rule. The employer is liable for the difference in percentage of impairment between this value and the percentage of binaural hearing impairment calculated from post-employment hearing tests.
24. (23) But if the corrected average hearing level of the pre-employment audiogram(s) does not exceed fifteen (15) decibels in either ear, the corrected pre-employment averages are subtracted from the corresponding corrected post-employment averages for each ear. The difference (that is, the threshold shift during employment corrected for the age factor) is divided by the corrected post-employment average hearing level for each ear. This fraction represents the employer’s share of liability for the impairment of hearing in that ear at the date of disability.
25. (24) The percentage of impairment of hearing in each ear is multiplied by the fraction calculated for that ear to give the percentages of impairment in each ear for which the employer is liable. The binaural percentage of impairment for which the employer is liable is then calculated according to section (16) of this rule.

AUTHORITY: section 287.650, RSMo 1986.* Original rule filed Sept. 11, 1959, effective Sept. 22, 1959. Amended: Filed Aug. 18, 1967, effective Aug. 29, 1967. *Original authority: 287.650, RSMo 1939, amended 1949, 1961, 1980, 1993, 1995, 1998.