"How do I prove it?"

What actually carries it

The mechanism, described specifically. What you were doing, with what, how much weight, how high, how often, for how long. "I hurt my back" is not a mechanism. "Lifting 60-pound cases from floor level onto a shoulder-height rack, about 200 times a shift, for six years" is.

Contemporaneous documentation. A text, an email, an incident report, a first-aid log entry, a message to a supervisor. Same-day beats same-week beats same-month.

The first medical record, saying it happened at work with the date.

Your job duties in writing — a job description, a posting, or your own specific account with weights and frequencies.

Your pre-injury records showing no prior complaint. The absence of complaint is affirmative evidence, and almost nobody produces it.

And a physician's opinion connecting the two. In a disputed case this is what decides it — which is why the medical-legal evaluation matters more than any argument. How.

In an exposure or cumulative case

Name the substance, not the category. Request the Safety Data Sheets in writing — your employer must maintain them and retain exposure records for 30 years.

Request the monitoring results — air sampling, hearing conservation audiograms, medical surveillance.

And check the Cal/OSHA citation history, which is public and free. What to ask for.

What the other side will do

Records review. A recorded statement. Sometimes surveillance. And a search for a prior claim, a prior injury to the same body part, or a non-industrial explanation.

None of that requires you to prove more than a contributing cause. More.

And two provisions that shift the ground in your favor

§ 5402(b): if liability was not rejected within 90 days of your claim form, the injury is presumed compensable — rebuttable only by evidence discovered after the 90 days.

§ 3202: the workers' compensation statutes are to be "liberally construed by the courts with the purpose of extending their benefits for the protection of persons injured in the course of their employment."