How does an employer determine if an event or exposure in the work environment “significantly aggravated” a preexisting injury or illness?

OSHA says that a preexisting injury or illness has been significantly aggravated when an event or exposure in the work environment results in any of the following:

• Death provided that the preexisting injury or illness would likely not have resulted in death but for the occupational event or exposure.
• Loss of consciousness provided that the preexisting injury or illness would likely not have resulted in loss of consciousness but for the occupational event or exposure.
• One or more days away from work, or days of restricted work, or days of job transfer that otherwise would not have occurred but for the occupational event or exposure.
• Medical treatment in a case where no medical treatment was needed for the injury or illness before the workplace event or exposure, or a change in medical treatment was necessitated by the workplace event or exposure.