Under the "substantial factor" test, causation is determined by whether or not the defendant's act was a "substantial factor" in causing the harm.
A substantial factor in causing harm is a factor that a reasonable person would consider to have contributed to the harm. It must be more than a remote or trivial factor. It does not have to be the only cause of the harm.
For hazardous chemical cases, to be a substantial factor requires "evidence of exposure to a specific product on a regular basis over some extended period of time in proximity to where the plaintiff actually worked."