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."
When two or more causes combine to cause an injury, the defendant is a cause in fact if the defendant's conduct was a substantial factor in causing the injury.
When a group of defendants produce a substantial share of the market share of a drug, they may be held liable for harms resulting therefrom for their approximate portion of market share when the specific manufacturer is unknown. Sindell.