I’ve seen these Proposition 65 warning signs around town, but this one was posted just below the buzzers for my friend’s apartment building. So welcoming! I just did a quick search to find out what exactly is so dangerous, and came across this explanatory website. I’m guessing the problem in this case is because the building has a parking garage, but it could also warn of chemicals used to clean the hot tub, “combustion sources” like gas stoves, or carpeting that contains formaldehyde, among other things. I’d prefer a fill-in-the-blank type of sign that warned about pool chemicals, pesticides, or whatever the specific problem is. That way, at least you’d know which part of the building might be killing you.