California Code, Labor Code - LAB § 5500.6

Liability for occupational disease or cumulative injury which results from exposure solely during employment as an employee, as defined in subdivision (d) of Section 3351, shall be limited to those employers in whose employment the employee was exposed to the hazards of the occupational disease or cumulative injury during the last day on which the employee was employed in an occupation exposing the employee to the hazards of the disease or injury. In the event that none of the employers of the last day of hazardous employment is insured for workers' compensation liability, that liability, shall be imposed upon the last employer exposing the employee to the hazards of the occupational disease or cumulative injury who has secured workers' compensation insurance coverage or an approved alternative thereto. If, based upon all the evidence presented, the appeals board or the workers' compensation judge finds the existence of cumulative injury or occupational disease, liability for the cumulative injury or occupational disease shall not be apportioned to prior employers. However, in determining liability, evidence of disability due to specific injury, disability due to non-work-related causes, or disability previously compensated for by way of a findings and award or order approving compromise and release, or a voluntary payment of disability, may be admissible for purposes of apportionment.