The light bulbs containing mercury being referred to here are compact fluorescent light bulbs, and ilmdamaily provides a more accurate, albeit hyperbolic comment than Aruthur Fontes.
The amount of mercury in CFL is around 4mg to 5mg. That amount by itself is not toxic to anyone. Nonetheless, the problem with mercury is that it is a cumulative toxin; that means it does not dissipate or go away with time.
Still, compare 5mg to the amount of mercury that was in old mercury thermometers in nearly every home. Those old thermometers contained on average 500mg of mercury, more than 100 times the amount in a compact fluorescent light bulb.
That's why ilmdamaily isn't all that far off the mark when he mentions breaking 1,000 bulbs all at once and bathing in the mercury dust; it would take a great many CFLs breaking at the same time to pose any immediate risk to anyone.
Now, if you're referring to LED light bulbs, they are completely innocuous and contain no toxic materials; thus, they are completely harmless to humans. Unless of course someone uses an especially large LED light bulb as a bludgeon. In that case I would repeat what ilmdamaily said: "But hey, I don't know what you're into..."