I worked for a chemical company in Oxnard years ago and came into contact with the air folks after Prop 65 was passed. I asked the person in charge how they went about determining when companies were required to put up the signs. Her response was something along the order of : if one person breathes nothing but the air coming out of a plants theoretical dome (put the plant in a baggie and have only one exhaust point) for 70 years, and that person develops one extra cancer, it meets the requirement for a warning sign. At the time, they were debating outlawing gas driven mowers and lighter fluid used for backyard BBQs. Same with charcoal soaked briquettes.
Needless to say, we had to put up fence line monitors and warning signs.
Risk that you take is fine and dandy. Risk that someone else throws on you is usually a big deal.