Mich. Comp. Laws § 691.1608

Sec. 8. (1) A plaintiff under section 7 may prove that a defendant participated in illegal marketing of the market area controlled substance used by the individual abuser who injured the plaintiff by proving both of the following: (a) The defendant was participating in the illegal marketing of the market area controlled substance at the time the individual abuser obtained or used that market area controlled substance. (b) The individual abuser obtained or used the market area controlled substance, or caused the injury, within the defendant's market area. (2) If a person participated in illegal marketing of a market area controlled substance, the person's market area for that controlled substance is the following: (a) For level 4 participation, each county in which the person participated in illegal marketing. (b) For level 3 participation, each market area described in subdivision (a) plus all counties with a border contiguous to each of those market areas. (c) For level 2 participation, each market area described in subdivision (b) plus all counties with a border contiguous to each of those market areas. (d) For level 1 participation, the state.