(a) The employer must consider an injury or illness to be work-related if an event or exposure in the work environment either caused or contributed to the resulting condition or significantly aggravated a pre-existing injury or illness. Work-relatedness is presumed for injuries and illnesses resulting from events or exposures occurring in the work environment, subject to certain restrictions set forth in section 901.5 of SH 901 Instructions for Recording and Reporting Public Employees' Occupational Injuries and Illnesses and accompanying forms SH 900, SH 900.1 and SH 900.2.