Several unions have joined in opposition to a California bill that would stop hunters from using lead bullets, and have asked Democratic Gov. Jerry Brown not to sign the legislation.

The Democratic-led Legislature passed the bill earlier this month, based in part on studies that appear to show animals ingest lead fragments found in the soil and the dead prey they eat, which can contaminate the human food supply.

Critics say the studies have not made that clear link between the bullets and the food, and that retooling guns and switching to bullets made of copper or other metals will be an expensive job-killer for firearms and ammunition makers.

The group Californians for Conservation has released a list of nine union leaders and labor groups that think the measure is a bad idea, according to The Los Angeles Times.