Meaning of these health warning signs on properties for sale?

[Mark Hu]

On occasion I have seen rental properties with signs posted on them like this:
'Warning, this site is known to have chemicals known to cause cancer'
The sign may or may not list specifics, such as asbestos, lead, ect.
Are owners forced to put up these signs by the city or county? Or do the owners put them up as protection from some sort of liability?
My location is Orange County, California. Thanks