Maryland Code, Labor and Employment § 9-651

(a) Except as provided in subsection (b) of this section, an employer is liable for the full extent of the occupational deafness of a covered employee if:

(b) An employer is liable only for the part of the deafness attributable to the employment by the employer if the employer establishes by competent evidence, including the results of a professionally controlled hearing test, the extent of the deafness of the covered employee that existed before exposure to harmful noise in the employment of the employer.